Re: [PATCH 00/11] KVM: x86/mmu: Restore fast invalidate/zap flow

[Paolo Bonzini]

On 13/09/19 04:46, Sean Christopherson wrote:
> Restore the fast invalidate flow for zapping shadow pages and use it
> whenever vCPUs can be active in the VM.  This fixes (in theory, not yet
> confirmed) a regression reported by James Harvey where KVM can livelock
> in kvm_mmu_zap_all() when it's invoked in response to a memslot update.
> 
> The fast invalidate flow was removed as it was deemed to be unnecessary
> after its primary user, memslot flushing, was reworked to zap only the
> memslot in question instead of all shadow pages.  Unfortunately, zapping
> only the memslot being (re)moved during a memslot update introduced a
> regression for VMs with assigned devices.  Because we could not discern
> why zapping only the relevant memslot broke device assignment, or if the
> regression extended beyond device assignment, we reverted to zapping all
> shadow pages when a memslot is (re)moved.
> 
> The revert to "zap all" failed to account for subsequent changes that
> have been made to kvm_mmu_zap_all() between then and now.  Specifically,
> kvm_mmu_zap_all() now conditionally drops reschedules and drops mmu_lock
> if a reschedule is needed or if the lock is contended.  Dropping the lock
> allows other vCPUs to add shadow pages, and, with enough vCPUs, can cause
> kvm_mmu_zap_all() to get stuck in an infinite loop as it can never zap all
> pages before observing lock contention or the need to reschedule.
> 
> The reasoning behind having kvm_mmu_zap_all() conditionally reschedule was
> that it would only be used when the VM is inaccesible, e.g. when its
> mm_struct is dying or when the VM itself is being destroyed.  In that case,
> playing nice with the rest of the kernel instead of hogging cycles to free
> unused shadow pages made sense.
> 
> Since it's unlikely we'll root cause the device assignment regression any
> time soon, and that simply removing the conditional rescheduling isn't
> guaranteed to return us to a known good state, restore the fast invalidate
> flow for zapping on memslot updates, including mmio generation wraparound.
> Opportunisticaly tack on a bug fix and a couple enhancements.
> 
> Alex and James, it probably goes without saying... please test, especially
> patch 01/11 as a standalone patch as that'll likely need to be applied to
> stable branches, assuming it works.  Thanks!
> 
> Sean Christopherson (11):
>   KVM: x86/mmu: Reintroduce fast invalidate/zap for flushing memslot
>   KVM: x86/mmu: Treat invalid shadow pages as obsolete
>   KVM: x86/mmu: Use fast invalidate mechanism to zap MMIO sptes
>   KVM: x86/mmu: Revert "Revert "KVM: MMU: show mmu_valid_gen in shadow
>     page related tracepoints""
>   KVM: x86/mmu: Revert "Revert "KVM: MMU: add tracepoint for
>     kvm_mmu_invalidate_all_pages""
>   KVM: x86/mmu: Revert "Revert "KVM: MMU: zap pages in batch""
>   KVM: x86/mmu: Revert "Revert "KVM: MMU: collapse TLB flushes when zap
>     all pages""
>   KVM: x86/mmu: Revert "Revert "KVM: MMU: reclaim the zapped-obsolete
>     page first""
>   KVM: x86/mmu: Revert "KVM: x86/mmu: Remove is_obsolete() call"
>   KVM: x86/mmu: Explicitly track only a single invalid mmu generation
>   KVM: x86/mmu: Skip invalid pages during zapping iff root_count is zero
> 
>  arch/x86/include/asm/kvm_host.h |   4 +-
>  arch/x86/kvm/mmu.c              | 154 ++++++++++++++++++++++++++++----
>  arch/x86/kvm/mmutrace.h         |  42 +++++++--
>  arch/x86/kvm/x86.c              |   1 +
>  4 files changed, 173 insertions(+), 28 deletions(-)
> 

Thanks, I'm testing patch 1 and should send a pull request to Linus
tomorrow morning as soon as I get the results.

Paolo