RE: [PATCH v3] RISC-V: Implement ASID allocator

[Gary Guo]

This patch absolutely does not work with a hardware that implements ASID feature. Aside from many corner cases, it does not even boot on a hardware with ASID implemented. It only works because QEMU does not use ASID at all. I have already pointed out many issues multiple times in the thread but most of them are not addressed.
 
> -----Original Message-----
> From: Anup Patel <anup@xxxxxxxxxxxxxx>
> Sent: Thursday, April 25, 2019 03:04
> To: Palmer Dabbelt <palmer@xxxxxxxxxx>
> Cc: Anup Patel <Anup.Patel@xxxxxxx>; Albert Ou <aou@xxxxxxxxxxxxxxxxx>;
> Gary Guo <gary@xxxxxxxxxxx>; Atish Patra <Atish.Patra@xxxxxxx>; Christoph
> Hellwig <hch@xxxxxxxxxxxxx>; Paul Walmsley <paul.walmsley@xxxxxxxxxx>;
> Mike Rapoport <rppt@xxxxxxxxxxxxx>; linux-riscv@xxxxxxxxxxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx List <linux-kernel@xxxxxxxxxxxxxxx>
> Subject: Re: [PATCH v3] RISC-V: Implement ASID allocator
> 
> On Thu, Apr 25, 2019 at 5:06 AM Palmer Dabbelt <palmer@xxxxxxxxxx> wrote:
> >
> > On Thu, 28 Mar 2019 21:51:38 PDT (-0700), Anup.Patel@xxxxxxx wrote:
> > > Currently, we do local TLB flush on every MM switch. This is very harsh on
> > > performance because we are forcing page table walks after every MM
> switch.
> > >
> > > This patch implements ASID allocator for assigning an ASID to a MM context.
> > > The number of ASIDs are limited in HW so we create a logical entity named
> > > CONTEXTID for assigning to MM context. The lower bits of CONTEXTID are
> ASID
> > > and upper bits are VERSION number. The number of usable ASID bits
> supported
> > > by HW are detected at boot-time by writing 1s to ASID bits in SATP CSR.
> > >
> > > We allocate new CONTEXTID on first MM switch for a MM context where
> the
> > > ASID is allocated from an ASID bitmap and VERSION is provide by an atomic
> > > counter. At time of allocating new CONTEXTID, if we run out of available
> > > ASIDs then:
> > > 1. We flush the ASID bitmap
> > > 2. Increment current VERSION atomic counter
> > > 3. Re-allocate ASID from ASID bitmap
> > > 4. Flush TLB on all CPUs
> > > 5. Try CONTEXTID re-assignment on all CPUs
> > >
> > > Please note that we don't use ASID #0 because it is used at boot-time by
> > > all CPUs for initial MM context. Also, newly created context is always
> > > assigned CONTEXTID #0 (i.e. VERSION #0 and ASID #0) which is an invalid
> > > context in our implementation.
> > >
> > > Using above approach, we have virtually infinite CONTEXTIDs on-top-of
> > > limited number of HW ASIDs. This approach is inspired from ASID allocator
> > > used for Linux ARM/ARM64 but we have adapted it for RISC-V. Overall, this
> > > ASID allocator helps us reduce rate of local TLB flushes on every CPU
> > > thereby increasing performance.
> > >
> > > This patch is tested on QEMU/virt machine and SiFive Unleashed board. On
> > > QEMU/virt machine, we see 10% (approx) performance improvement with
> SW
> > > emulated TLBs provided by QEMU. Unfortunately, ASID bits of SATP CSR are
> > > not implemented on SiFive Unleashed board so we don't see any change in
> > > performance.
> >
> > My worry here is the testing: I don't trust QEMU to be a good enough test of
> > ASID handling to shake out the bugs in this sort of code -- unless I'm missing
> > something, we're currently ignoring ASIDs in QEMU entirely.  As a result I'd
> > consider this to be essentially untestable until someone comes up with an
> > implementation that takes advantage of ASIDs.  Given that bugs here would be
> > super hard to find, I'd prefer to avoid merging this until we're sure it's
> > solid.
> 
> This patch is tested on both QEMU and SiFive Unleashed board so it works
> perfectly fine on existing HW out there at this moment.
> 
> For future HW having ASID bits, it will certainly save development efforts for
> the RISC-V HW vendors if we merge it now.
> 
> Irrespective to whether we merge this patch now or not, the debugging and
> development complexity of ASID allocator will not change for whoever does it.
> 
> Not merging it now is a bigger problem because RISC-V HW vendor not
> following
> RISC-V LKML might re-implement it from scratch without realizing that there is
> already an implementation sitting on RISC-V LKML.
> 
> In ARM world, any new ARM feature is first implemented and merged in Linux
> kernel by ARM Ltd folks (tested on their ARM fast models). Later some ARM
> SOC vendor, sends patch fixes (if required) after trying out latest
> Linux on their
> HW having this new ARM feature.
> 
> IMHO, we should merge this patch for Linux-5.2 so that RISC-V SOC vendors
> can try it on their HW without re-implementing it from scratch. The "debugging
> complexity" is integral part of kernel development and this should not be a
> reason to defer critical changes as long as changes are tested on existing HW.
> 
> Linux kernel is all about "organic development". We start with a reasonable
> implementation of any thing and perfect it incrementally over time. That's how
> we get a solid implementation.
> 
> Regards,
> Anup
> 
> >
> > > Co-developed-by:: Gary Guo <gary@xxxxxxxxxxx>
> > > Signed-off-by: Anup Patel <anup.patel@xxxxxxx>
> > > ---
> > > Changes since v2:
> > > - Move to lazy TLB flushing because we get slow path warnings if we
> > >   use flush_tlb_all()
> > > - Don't set ASID bits to all 1s in head.s. Instead just do it on
> > >   boot CPU calling asids_init() for determining number of HW ASID bits
> > > - Make CONTEXT version comparison more readable in set_mm_asid()
> > > - Fix typo in __flush_context()
> > >
> > > Changes since v1:
> > > - We adapt good aspects from Gary Guo's ASID allocator implementation
> > >   and provide due credit to him by adding his SoB.
> > > - Track ASIDs active during context flush and mark them as reserved
> > > - Set ASID bits to all 1s to simplify number of ASID bit detection
> > > - Use atomic_long_t instead of atomic64_t for being 32bit friendly
> > > - Use unsigned long instead of u64 for being 32bit friendly
> > > - Use flush_tlb_all() instead of lazy local_tlb_flush_all() at time
> > >   of context flush
> > >
> > > This patch is based on Linux-5.1-rc2 and TLB flush cleanup patches v4
> > > from Gary Guo. It can be also found in riscv_asid_allocator_v3 branch
> > > of https://github.com/avpatel/linux.git
> > > ---
> > >  arch/riscv/include/asm/csr.h         |   6 +
> > >  arch/riscv/include/asm/mmu.h         |   1 +
> > >  arch/riscv/include/asm/mmu_context.h |   1 +
> > >  arch/riscv/mm/context.c              | 261 ++++++++++++++++++++++++++-
> > >  4 files changed, 259 insertions(+), 10 deletions(-)
> > >
> > > diff --git a/arch/riscv/include/asm/csr.h b/arch/riscv/include/asm/csr.h
> > > index 28a0d1cb374c..ce18ab8f53ed 100644
> > > --- a/arch/riscv/include/asm/csr.h
> > > +++ b/arch/riscv/include/asm/csr.h
> > > @@ -45,10 +45,16 @@
> > >  #define SATP_PPN     _AC(0x003FFFFF, UL)
> > >  #define SATP_MODE_32 _AC(0x80000000, UL)
> > >  #define SATP_MODE    SATP_MODE_32
> > > +#define SATP_ASID_BITS       9
> > > +#define SATP_ASID_SHIFT      22
> > > +#define SATP_ASID_MASK       _AC(0x1FF, UL)
> > >  #else
> > >  #define SATP_PPN     _AC(0x00000FFFFFFFFFFF, UL)
> > >  #define SATP_MODE_39 _AC(0x8000000000000000, UL)
> > >  #define SATP_MODE    SATP_MODE_39
> > > +#define SATP_ASID_BITS       16
> > > +#define SATP_ASID_SHIFT      44
> > > +#define SATP_ASID_MASK       _AC(0xFFFF, UL)
> > >  #endif
> > >
> > >  /* Interrupt Enable and Interrupt Pending flags */
> > > diff --git a/arch/riscv/include/asm/mmu.h b/arch/riscv/include/asm/mmu.h
> > > index 5df2dccdba12..42a9ca0fe1fb 100644
> > > --- a/arch/riscv/include/asm/mmu.h
> > > +++ b/arch/riscv/include/asm/mmu.h
> > > @@ -18,6 +18,7 @@
> > >  #ifndef __ASSEMBLY__
> > >
> > >  typedef struct {
> > > +     atomic_long_t id;
> > >       void *vdso;
> > >  #ifdef CONFIG_SMP
> > >       /* A local icache flush is needed before user execution can resume. */
> > > diff --git a/arch/riscv/include/asm/mmu_context.h
> b/arch/riscv/include/asm/mmu_context.h
> > > index bf4f097a9051..bd271c6b0e5e 100644
> > > --- a/arch/riscv/include/asm/mmu_context.h
> > > +++ b/arch/riscv/include/asm/mmu_context.h
> > > @@ -30,6 +30,7 @@ static inline void enter_lazy_tlb(struct mm_struct *mm,
> > >  static inline int init_new_context(struct task_struct *task,
> > >       struct mm_struct *mm)
> > >  {
> > > +     atomic_long_set(&mm->context.id, 0);
> > >       return 0;
> > >  }
> > >
> > > diff --git a/arch/riscv/mm/context.c b/arch/riscv/mm/context.c
> > > index 0f787bcd3a7a..863b6926d6d9 100644
> > > --- a/arch/riscv/mm/context.c
> > > +++ b/arch/riscv/mm/context.c
> > > @@ -2,13 +2,213 @@
> > >  /*
> > >   * Copyright (C) 2012 Regents of the University of California
> > >   * Copyright (C) 2017 SiFive
> > > + * Copyright (C) 2019 Western Digital Corporation or its affiliates.
> > >   */
> > >
> > > +#include <linux/bitops.h>
> > >  #include <linux/mm.h>
> > > +#include <linux/slab.h>
> > >
> > >  #include <asm/tlbflush.h>
> > >  #include <asm/cacheflush.h>
> > >
> > > +static bool use_asid_allocator;
> > > +static unsigned long asid_bits;
> > > +static unsigned long num_asids;
> > > +static unsigned long asid_mask;
> > > +
> > > +static atomic_long_t current_version;
> > > +
> > > +static DEFINE_RAW_SPINLOCK(context_lock);
> > > +static cpumask_t context_tlb_flush_pending;
> > > +static unsigned long *context_asid_map;
> > > +
> > > +static DEFINE_PER_CPU(atomic_long_t, active_context);
> > > +static DEFINE_PER_CPU(unsigned long, reserved_context);
> > > +
> > > +static bool check_update_reserved_context(unsigned long cntx,
> > > +                                       unsigned long newcntx)
> > > +{
> > > +     int cpu;
> > > +     bool hit = false;
> > > +
> > > +     /*
> > > +      * Iterate over the set of reserved CONTEXT looking for a match.
> > > +      * If we find one, then we can update our mm to use new CONTEXT
> > > +      * (i.e. the same CONTEXT in the current_version) but we can't
> > > +      * exit the loop early, since we need to ensure that all copies
> > > +      * of the old CONTEXT are updated to reflect the mm. Failure to do
> > > +      * so could result in us missing the reserved CONTEXT in a future
> > > +      * version.
> > > +      */
> > > +     for_each_possible_cpu(cpu) {
> > > +             if (per_cpu(reserved_context, cpu) == cntx) {
> > > +                     hit = true;
> > > +                     per_cpu(reserved_context, cpu) = newcntx;
> > > +             }
> > > +     }
> > > +
> > > +     return hit;
> > > +}
> > > +
> > > +/* Note: must be called with context_lock held */
> > > +static void __flush_context(void)
> > > +{
> > > +     int i;
> > > +     unsigned long cntx;
> > > +
> > > +     /* Update the list of reserved ASIDs and the ASID bitmap. */
> > > +     bitmap_clear(context_asid_map, 0, num_asids);
> > > +
> > > +     /* Mark already acitve ASIDs as used */
> > > +     for_each_possible_cpu(i) {
> > > +             cntx = atomic_long_xchg_relaxed(&per_cpu(active_context, i), 0);
> > > +             /*
> > > +              * If this CPU has already been through a rollover, but
> > > +              * hasn't run another task in the meantime, we must preserve
> > > +              * its reserved CONTEXT, as this is the only trace we have of
> > > +              * the process it is still running.
> > > +              */
> > > +             if (cntx == 0)
> > > +                     cntx = per_cpu(reserved_context, i);
> > > +
> > > +             __set_bit(cntx & asid_mask, context_asid_map);
> > > +             per_cpu(reserved_context, i) = cntx;
> > > +     }
> > > +
> > > +     /* Mark ASID #0 as used because it is used at boot-time */
> > > +     __set_bit(0, context_asid_map);
> > > +
> > > +     /* Queue a TLB invalidation for each CPU on next context-switch */
> > > +     cpumask_setall(&context_tlb_flush_pending);
> > > +}
> > > +
> > > +/* Note: must be called with context_lock held */
> > > +static unsigned long __new_context(struct mm_struct *mm)
> > > +{
> > > +     static u32 cur_idx = 1;
> > > +     unsigned long cntx = atomic_long_read(&mm->context.id);
> > > +     unsigned long asid, ver = atomic_long_read(&current_version);
> > > +
> > > +     if (cntx != 0) {
> > > +             unsigned long newcntx = ver | (cntx & asid_mask);
> > > +
> > > +             /*
> > > +              * If our current CONTEXT was active during a rollover, we
> > > +              * can continue to use it and this was just a false alarm.
> > > +              */
> > > +             if (check_update_reserved_context(cntx, newcntx))
> > > +                     return newcntx;
> > > +
> > > +             /*
> > > +              * We had a valid CONTEXT in a previous life, so try to
> > > +              * re-use it if possible.
> > > +              */
> > > +             if (!__test_and_set_bit(cntx & asid_mask, context_asid_map))
> > > +                     return newcntx;
> > > +     }
> > > +
> > > +     /*
> > > +      * Allocate a free ASID. If we can't find one then increment
> > > +      * current_version and flush all ASIDs.
> > > +      */
> > > +     asid = find_next_zero_bit(context_asid_map, num_asids, cur_idx);
> > > +     if (asid != num_asids)
> > > +             goto set_asid;
> > > +
> > > +     /* We're out of ASIDs, so increment current_version */
> > > +     ver = atomic_long_add_return_relaxed(num_asids, &current_version);
> > > +
> > > +     /* Flush everything  */
> > > +     __flush_context();
> > > +
> > > +     /* We have more ASIDs than CPUs, so this will always succeed */
> > > +     asid = find_next_zero_bit(context_asid_map, num_asids, 1);
> > > +
> > > +set_asid:
> > > +     __set_bit(asid, context_asid_map);
> > > +     cur_idx = asid;
> > > +     return asid | ver;
> > > +}
> > > +
> > > +static void set_mm_asid(struct mm_struct *mm, unsigned int cpu)
> > > +{
> > > +     unsigned long flags;
> > > +     bool need_flush_tlb = false;
> > > +     unsigned long cntx, old_active_cntx;
> > > +
> > > +     cntx = atomic_long_read(&mm->context.id);
> > > +
> > > +     /*
> > > +      * If our active_context is non-zero and the context matches the
> > > +      * current_version, then we update the active_context entry with a
> > > +      * relaxed cmpxchg.
> > > +      *
> > > +      * Following is how we handle racing with a concurrent rollover:
> > > +      *
> > > +      * - We get a zero back from the cmpxchg and end up waiting on the
> > > +      *   lock. Taking the lock synchronises with the rollover and so
> > > +      *   we are forced to see the updated verion.
> > > +      *
> > > +      * - We get a valid context back from the cmpxchg then we continue
> > > +      *   using old ASID because __flush_context() would have marked ASID
> > > +      *   of active_context as used and next context switch we will allocate
> > > +      *   new context.
> > > +      */
> > > +     old_active_cntx = atomic_long_read(&per_cpu(active_context, cpu));
> > > +     if (old_active_cntx &&
> > > +         ((cntx & ~asid_mask) == atomic_long_read(&current_version)) &&
> > > +         atomic_long_cmpxchg_relaxed(&per_cpu(active_context, cpu),
> > > +                                     old_active_cntx, cntx))
> > > +             goto switch_mm_fast;
> > > +
> > > +     raw_spin_lock_irqsave(&context_lock, flags);
> > > +
> > > +     /* Check that our ASID belongs to the current_version. */
> > > +     cntx = atomic_long_read(&mm->context.id);
> > > +     if ((cntx & ~asid_mask) != atomic_long_read(&current_version)) {
> > > +             cntx = __new_context(mm);
> > > +             atomic_long_set(&mm->context.id, cntx);
> > > +     }
> > > +
> > > +     if (cpumask_test_and_clear_cpu(cpu, &context_tlb_flush_pending))
> > > +             need_flush_tlb = true;
> > > +
> > > +     atomic_long_set(&per_cpu(active_context, cpu), cntx);
> > > +
> > > +     raw_spin_unlock_irqrestore(&context_lock, flags);
> > > +
> > > +switch_mm_fast:
> > > +     /*
> > > +      * Use the old spbtr name instead of using the current satp
> > > +      * name to support binutils 2.29 which doesn't know about the
> > > +      * privileged ISA 1.10 yet.
> > > +      */
> > > +     csr_write(sptbr, virt_to_pfn(mm->pgd) |
> > > +               ((cntx & asid_mask) << SATP_ASID_SHIFT) |
> > > +               SATP_MODE);
> > > +
> > > +     if (need_flush_tlb)
> > > +             local_flush_tlb_all();
> > > +}
> > > +
> > > +static void set_mm_noasid(struct mm_struct *mm)
> > > +{
> > > +     /*
> > > +      * Use the old spbtr name instead of using the current satp
> > > +      * name to support binutils 2.29 which doesn't know about the
> > > +      * privileged ISA 1.10 yet.
> > > +      */
> > > +     csr_write(sptbr, virt_to_pfn(mm->pgd) | SATP_MODE);
> > > +
> > > +     /*
> > > +      * sfence.vma after SATP write. We call it on MM context instead of
> > > +      * calling local_flush_tlb_all to prevent global mappings from being
> > > +      * affected.
> > > +      */
> > > +     local_flush_tlb_mm(mm);
> > > +}
> > > +
> > >  /*
> > >   * When necessary, performs a deferred icache flush for the given MM
> context,
> > >   * on the local CPU.  RISC-V has no direct mechanism for instruction cache
> > > @@ -58,20 +258,61 @@ void switch_mm(struct mm_struct *prev, struct
> mm_struct *next,
> > >       cpumask_clear_cpu(cpu, mm_cpumask(prev));
> > >       cpumask_set_cpu(cpu, mm_cpumask(next));
> > >
> > > +     if (use_asid_allocator)
> > > +             set_mm_asid(next, cpu);
> > > +     else
> > > +             set_mm_noasid(next);
> > > +
> > > +     flush_icache_deferred(next);
> > > +}
> > > +
> > > +static int asids_init(void)
> > > +{
> > > +     unsigned long old;
> > > +
> > > +     /* Figure-out number of ASID bits in HW */
> > > +     old = csr_read(sptbr);
> > > +     asid_bits = old | (SATP_ASID_MASK << SATP_ASID_SHIFT);
> > > +     csr_write(sptbr, asid_bits);
> > > +     asid_bits = (csr_read(sptbr) >> SATP_ASID_SHIFT)  & SATP_ASID_MASK;
> > > +     asid_bits = fls_long(asid_bits);
> > > +     csr_write(sptbr, old);
> > > +
> > >       /*
> > > -      * Use the old spbtr name instead of using the current satp
> > > -      * name to support binutils 2.29 which doesn't know about the
> > > -      * privileged ISA 1.10 yet.
> > > +      * In the process of determining number of ASID bits (above)
> > > +      * we polluted the TLB of current HART so let's do TLB flushed
> > > +      * to remove unwanted TLB enteries.
> > >        */
> > > -     csr_write(sptbr, virt_to_pfn(next->pgd) | SATP_MODE);
> > > +     local_flush_tlb_all();
> > > +
> > > +     /* Pre-compute ASID details */
> > > +     num_asids = 1 << asid_bits;
> > > +     asid_mask = num_asids - 1;
> > >
> > >       /*
> > > -      * sfence.vma after SATP write. We call it on MM context instead of
> > > -      * calling local_flush_tlb_all to prevent global mappings from being
> > > -      * affected.
> > > +      * Use ASID allocator only if number of HW ASIDs are
> > > +      * at-least twice more than CPUs
> > >        */
> > > -     local_flush_tlb_mm(next);
> > > +     use_asid_allocator =
> > > +             (num_asids <= (2 * num_possible_cpus())) ? false : true;
> > >
> > > -     flush_icache_deferred(next);
> > > -}
> > > +     /* Setup ASID allocator if available */
> > > +     if (use_asid_allocator) {
> > > +             atomic_long_set(&current_version, num_asids);
> > > +
> > > +             context_asid_map = kcalloc(BITS_TO_LONGS(num_asids),
> > > +                                sizeof(*context_asid_map), GFP_KERNEL);
> > > +             if (!context_asid_map)
> > > +                     panic("Failed to allocate bitmap for %lu ASIDs\n",
> > > +                           num_asids);
> > > +
> > > +             __set_bit(0, context_asid_map);
> > >
> > > +             pr_info("ASID allocator using %lu entries\n", num_asids);
> > > +     } else {
> > > +             pr_info("ASID allocator disabled\n");
> > > +     }
> > > +
> > > +     return 0;
> > > +}
> > > +early_initcall(asids_init);