Re: [PATCH v10 9/9] KVM: Enable and expose KVM_MEM_PRIVATE

[Chao Peng]

On Wed, Mar 22, 2023 at 05:41:31PM -0700, Isaku Yamahata wrote:
> On Wed, Mar 08, 2023 at 03:40:26PM +0800,
> Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx> wrote:
> 
> > On Wed, Mar 08, 2023 at 12:13:24AM +0000, Ackerley Tng wrote:
> > > Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx> writes:
> > > 
> > > > On Sat, Jan 14, 2023 at 12:01:01AM +0000, Sean Christopherson wrote:
> > > > > On Fri, Dec 02, 2022, Chao Peng wrote:
> > > > ...
> > > > > Strongly prefer to use similar logic to existing code that detects wraps:
> > > 
> > > > > 		mem->restricted_offset + mem->memory_size < mem->restricted_offset
> > > 
> > > > > This is also where I'd like to add the "gfn is aligned to offset"
> > > > > check, though
> > > > > my brain is too fried to figure that out right now.
> > > 
> > > > Used count_trailing_zeros() for this TODO, unsure we have other better
> > > > approach.
> > > 
> > > > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > > > index afc8c26fa652..fd34c5f7cd2f 100644
> > > > --- a/virt/kvm/kvm_main.c
> > > > +++ b/virt/kvm/kvm_main.c
> > > > @@ -56,6 +56,7 @@
> > > >   #include <asm/processor.h>
> > > >   #include <asm/ioctl.h>
> > > >   #include <linux/uaccess.h>
> > > > +#include <linux/count_zeros.h>
> > > 
> > > >   #include "coalesced_mmio.h"
> > > >   #include "async_pf.h"
> > > > @@ -2087,6 +2088,19 @@ static bool kvm_check_memslot_overlap(struct
> > > > kvm_memslots *slots, int id,
> > > >   	return false;
> > > >   }
> > > 
> > > > +/*
> > > > + * Return true when ALIGNMENT(offset) >= ALIGNMENT(gpa).
> > > > + */
> > > > +static bool kvm_check_rmem_offset_alignment(u64 offset, u64 gpa)
> > > > +{
> > > > +	if (!offset)
> > > > +		return true;
> > > > +	if (!gpa)
> > > > +		return false;
> > > > +
> > > > +	return !!(count_trailing_zeros(offset) >= count_trailing_zeros(gpa));
> 
> This check doesn't work expected. For example, offset = 2GB, gpa=4GB
> this check fails.

This case is expected to fail as Sean initially suggested[*]:
  I would rather reject memslot if the gfn has lesser alignment than
  the offset. I'm totally ok with this approach _if_ there's a use case.
  Until such a use case presents itself, I would rather be conservative
  from a uAPI perspective.

I understand that we put tighter restriction on this but if you see such
restriction is really a big issue for real usage, instead of a
theoretical problem, then we can loosen the check here. But at that time
below code is kind of x86 specific and may need improve.

BTW, in latest code, I replaced count_trailing_zeros() with fls64():
  return !!(fls64(offset) >= fls64(gpa));

[*] https://lore.kernel.org/all/Y8HldeHBrw+OOZVm@xxxxxxxxxx/

Chao
> I come up with the following.
> 
> >From ec87e25082f0497431b732702fae82c6a05071bf Mon Sep 17 00:00:00 2001
> Message-Id: <ec87e25082f0497431b732702fae82c6a05071bf.1679531995.git.isaku.yamahata@xxxxxxxxx>
> From: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> Date: Wed, 22 Mar 2023 15:32:56 -0700
> Subject: [PATCH] KVM: Relax alignment check for restricted mem
> 
> kvm_check_rmem_offset_alignment() only checks based on offset alignment
> and GPA alignment.  However, the actual alignment for offset depends
> on architecture.  For x86 case, it can be 1G, 2M or 4K.  So even if
> GPA is aligned for 1G+, only 1G-alignment is required for offset.
> 
> Without this patch, gpa=4G, offset=2G results in failure of memory slot
> creation.
> 
> Fixes: edc8814b2c77 ("KVM: Require gfn be aligned with restricted offset")
> Signed-off-by: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> ---
>  arch/x86/include/asm/kvm_host.h | 15 +++++++++++++++
>  virt/kvm/kvm_main.c             |  9 ++++++++-
>  2 files changed, 23 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 88e11dd3afde..03af44650f24 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -16,6 +16,7 @@
>  #include <linux/irq_work.h>
>  #include <linux/irq.h>
>  #include <linux/workqueue.h>
> +#include <linux/count_zeros.h>
>  
>  #include <linux/kvm.h>
>  #include <linux/kvm_para.h>
> @@ -143,6 +144,20 @@
>  #define KVM_HPAGE_MASK(x)	(~(KVM_HPAGE_SIZE(x) - 1))
>  #define KVM_PAGES_PER_HPAGE(x)	(KVM_HPAGE_SIZE(x) / PAGE_SIZE)
>  
> +#define kvm_arch_required_alignment	kvm_arch_required_alignment
> +static inline int kvm_arch_required_alignment(u64 gpa)
> +{
> +	int zeros = count_trailing_zeros(gpa);
> +
> +	WARN_ON_ONCE(!PAGE_ALIGNED(gpa));
> +	if (zeros >= KVM_HPAGE_SHIFT(PG_LEVEL_1G))
> +		return KVM_HPAGE_SHIFT(PG_LEVEL_1G);
> +	else if (zeros >= KVM_HPAGE_SHIFT(PG_LEVEL_2M))
> +		return KVM_HPAGE_SHIFT(PG_LEVEL_2M);
> +
> +	return PAGE_SHIFT;
> +}
> +
>  #define KVM_MEMSLOT_PAGES_TO_MMU_PAGES_RATIO 50
>  #define KVM_MIN_ALLOC_MMU_PAGES 64UL
>  #define KVM_MMU_HASH_SHIFT 12
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index c9c4eef457b0..f4ff96171d24 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2113,6 +2113,13 @@ static bool kvm_check_memslot_overlap(struct kvm_memslots *slots, int id,
>  	return false;
>  }
>  
> +#ifndef kvm_arch_required_alignment
> +__weak int kvm_arch_required_alignment(u64 gpa)
> +{
> +	return PAGE_SHIFT
> +}
> +#endif
> +
>  /*
>   * Return true when ALIGNMENT(offset) >= ALIGNMENT(gpa).
>   */
> @@ -2123,7 +2130,7 @@ static bool kvm_check_rmem_offset_alignment(u64 offset, u64 gpa)
>  	if (!gpa)
>  		return false;
>  
> -	return !!(count_trailing_zeros(offset) >= count_trailing_zeros(gpa));
> +	return !!(count_trailing_zeros(offset) >= kvm_arch_required_alignment(gpa));
>  }
>  
>  /*
> -- 
> 2.25.1
> 
> 
> 
> -- 
> Isaku Yamahata <isaku.yamahata@xxxxxxxxx>