Re: [PATCH v2 00/10] KVM: Consolidate and optimize MMU notifiers

[Marc Zyngier]

On Fri, 02 Apr 2021 13:17:45 +0100,
Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
> 
> On 02/04/21 02:56, Sean Christopherson wrote:
> > The end goal of this series is to optimize the MMU notifiers to take
> > mmu_lock if and only if the notification is relevant to KVM, i.e. the hva
> > range overlaps a memslot.   Large VMs (hundreds of vCPUs) are very
> > sensitive to mmu_lock being taken for write at inopportune times, and
> > such VMs also tend to be "static", e.g. backed by HugeTLB with minimal
> > page shenanigans.  The vast majority of notifications for these VMs will
> > be spurious (for KVM), and eliding mmu_lock for spurious notifications
> > avoids an otherwise unacceptable disruption to the guest.
> > 
> > To get there without potentially degrading performance, e.g. due to
> > multiple memslot lookups, especially on non-x86 where the use cases are
> > largely unknown (from my perspective), first consolidate the MMU notifier
> > logic by moving the hva->gfn lookups into common KVM.
> > 
> > Based on kvm/queue, commit 5f986f748438 ("KVM: x86: dump_vmcs should
> > include the autoload/autostore MSR lists").
> > 
> > Well tested on Intel and AMD.  Compile tested for arm64, MIPS, PPC,
> > PPC e500, and s390.  Absolutely needs to be tested for real on non-x86,
> > I give it even odds that I introduced an off-by-one bug somewhere.
> > 
> > v2:
> >   - Drop the patches that have already been pushed to kvm/queue.
> >   - Drop two selftest changes that had snuck in via "git commit -a".
> >   - Add a patch to assert that mmu_notifier_count is elevated when
> >     .change_pte() runs. [Paolo]
> >   - Split out moving KVM_MMU_(UN)LOCK() to __kvm_handle_hva_range() to a
> >     separate patch.  Opted not to squash it with the introduction of the
> >     common hva walkers (patch 02), as that prevented sharing code between
> >     the old and new APIs. [Paolo]
> >   - Tweak the comment in kvm_vm_destroy() above the smashing of the new
> >     slots lock. [Paolo]
> >   - Make mmu_notifier_slots_lock unconditional to avoid #ifdefs. [Paolo]
> > 
> > v1:
> >   - https://lkml.kernel.org/r/20210326021957.1424875-1-seanjc@xxxxxxxxxx
> > 
> > Sean Christopherson (10):
> >    KVM: Assert that notifier count is elevated in .change_pte()
> >    KVM: Move x86's MMU notifier memslot walkers to generic code
> >    KVM: arm64: Convert to the gfn-based MMU notifier callbacks
> >    KVM: MIPS/MMU: Convert to the gfn-based MMU notifier callbacks
> >    KVM: PPC: Convert to the gfn-based MMU notifier callbacks
> >    KVM: Kill off the old hva-based MMU notifier callbacks
> >    KVM: Move MMU notifier's mmu_lock acquisition into common helper
> >    KVM: Take mmu_lock when handling MMU notifier iff the hva hits a
> >      memslot
> >    KVM: Don't take mmu_lock for range invalidation unless necessary
> >    KVM: x86/mmu: Allow yielding during MMU notifier unmap/zap, if
> >      possible
> > 
> >   arch/arm64/kvm/mmu.c                   | 117 +++------
> >   arch/mips/kvm/mmu.c                    |  97 ++------
> >   arch/powerpc/include/asm/kvm_book3s.h  |  12 +-
> >   arch/powerpc/include/asm/kvm_ppc.h     |   9 +-
> >   arch/powerpc/kvm/book3s.c              |  18 +-
> >   arch/powerpc/kvm/book3s.h              |  10 +-
> >   arch/powerpc/kvm/book3s_64_mmu_hv.c    |  98 ++------
> >   arch/powerpc/kvm/book3s_64_mmu_radix.c |  25 +-
> >   arch/powerpc/kvm/book3s_hv.c           |  12 +-
> >   arch/powerpc/kvm/book3s_pr.c           |  56 ++---
> >   arch/powerpc/kvm/e500_mmu_host.c       |  27 +-
> >   arch/x86/kvm/mmu/mmu.c                 | 127 ++++------
> >   arch/x86/kvm/mmu/tdp_mmu.c             | 245 +++++++------------
> >   arch/x86/kvm/mmu/tdp_mmu.h             |  14 +-
> >   include/linux/kvm_host.h               |  22 +-
> >   virt/kvm/kvm_main.c                    | 325 +++++++++++++++++++------
> >   16 files changed, 552 insertions(+), 662 deletions(-)
> > 
> 
> For MIPS, I am going to post a series that simplifies TLB flushing
> further.  I applied it, and rebased this one on top, to
> kvm/mmu-notifier-queue.
> 
> Architecture maintainers, please look at the branch and
> review/test/ack your parts.

I've given this a reasonably good beating on arm64 for both VHE and
nVHE HW, and nothing caught fire, although I was left with a conflict
in the x86 code after merging with linux/master.

Feel free to add a

Tested-by: Marc Zyngier <maz@xxxxxxxxxx>

for the arm64 side.

	M.

-- 
Without deviation from the norm, progress is not possible.