Re: [PATCH v5] KVM: x86/mmu: Retry fault before acquiring mmu_lock if mapping is changing

[Yan Zhao]

Reviewed-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>

On Wed, Feb 21, 2024 at 05:26:40PM -0800, Sean Christopherson wrote:
> Retry page faults without acquiring mmu_lock, and without even faulting
> the page into the primary MMU, if the resolved gfn is covered by an active
> invalidation.  Contending for mmu_lock is especially problematic on
> preemptible kernels as the mmu_notifier invalidation task will yield
> mmu_lock (see rwlock_needbreak()), delay the in-progress invalidation, and
> ultimately increase the latency of resolving the page fault.  And in the
> worst case scenario, yielding will be accompanied by a remote TLB flush,
> e.g. if the invalidation covers a large range of memory and vCPUs are
> accessing addresses that were already zapped.
> 
> Faulting the page into the primary MMU is similarly problematic, as doing
> so may acquire locks that need to be taken for the invalidation to
> complete (the primary MMU has finer grained locks than KVM's MMU), and/or
> may cause unnecessary churn (getting/putting pages, marking them accessed,
> etc).
> 
> Alternatively, the yielding issue could be mitigated by teaching KVM's MMU
> iterators to perform more work before yielding, but that wouldn't solve
> the lock contention and would negatively affect scenarios where a vCPU is
> trying to fault in an address that is NOT covered by the in-progress
> invalidation.
> 
> Add a dedicated lockess version of the range-based retry check to avoid
> false positives on the sanity check on start+end WARN, and so that it's
> super obvious that checking for a racing invalidation without holding
> mmu_lock is unsafe (though obviously useful).
> 
> Wrap mmu_invalidate_in_progress in READ_ONCE() to ensure that pre-checking
> invalidation in a loop won't put KVM into an infinite loop, e.g. due to
> caching the in-progress flag and never seeing it go to '0'.
> 
> Force a load of mmu_invalidate_seq as well, even though it isn't strictly
> necessary to avoid an infinite loop, as doing so improves the probability
> that KVM will detect an invalidation that already completed before
> acquiring mmu_lock and bailing anyways.
> 
> Do the pre-check even for non-preemptible kernels, as waiting to detect
> the invalidation until mmu_lock is held guarantees the vCPU will observe
> the worst case latency in terms of handling the fault, and can generate
> even more mmu_lock contention.  E.g. the vCPU will acquire mmu_lock,
> detect retry, drop mmu_lock, re-enter the guest, retake the fault, and
> eventually re-acquire mmu_lock.  This behavior is also why there are no
> new starvation issues due to losing the fairness guarantees provided by
> rwlocks: if the vCPU needs to retry, it _must_ drop mmu_lock, i.e. waiting
> on mmu_lock doesn't guarantee forward progress in the face of _another_
> mmu_notifier invalidation event.
> 
> Note, adding READ_ONCE() isn't entirely free, e.g. on x86, the READ_ONCE()
> may generate a load into a register instead of doing a direct comparison
> (MOV+TEST+Jcc instead of CMP+Jcc), but practically speaking the added cost
> is a few bytes of code and maaaaybe a cycle or three.
> 
> Reported-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
> Closes: https://lore.kernel.org/all/ZNnPF4W26ZbAyGto@xxxxxxxxxxxxxxxxxxxxxxxxx
> Reported-by: Friedrich Weber <f.weber@xxxxxxxxxxx>
> Cc: Kai Huang <kai.huang@xxxxxxxxx>
> Cc: Yan Zhao <yan.y.zhao@xxxxxxxxx>
> Cc: Yuan Yao <yuan.yao@xxxxxxxxxxxxxxx>
> Cc: Xu Yilun <yilun.xu@xxxxxxxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
> 
> v5:
>  - Fix the inverted slot check. [Xu] 
>  - Drop all the other patches (will post separately).
> 
>  arch/x86/kvm/mmu/mmu.c   | 42 ++++++++++++++++++++++++++++++++++++++++
>  include/linux/kvm_host.h | 26 +++++++++++++++++++++++++
>  2 files changed, 68 insertions(+)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 3c193b096b45..274acc53f0e9 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -4405,6 +4405,31 @@ static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
>  	fault->mmu_seq = vcpu->kvm->mmu_invalidate_seq;
>  	smp_rmb();
>  
> +	/*
> +	 * Check for a relevant mmu_notifier invalidation event before getting
> +	 * the pfn from the primary MMU, and before acquiring mmu_lock.
> +	 *
> +	 * For mmu_lock, if there is an in-progress invalidation and the kernel
> +	 * allows preemption, the invalidation task may drop mmu_lock and yield
> +	 * in response to mmu_lock being contended, which is *very* counter-
> +	 * productive as this vCPU can't actually make forward progress until
> +	 * the invalidation completes.
> +	 *
> +	 * Retrying now can also avoid unnessary lock contention in the primary
> +	 * MMU, as the primary MMU doesn't necessarily hold a single lock for
> +	 * the duration of the invalidation, i.e. faulting in a conflicting pfn
> +	 * can cause the invalidation to take longer by holding locks that are
> +	 * needed to complete the invalidation.
> +	 *
> +	 * Do the pre-check even for non-preemtible kernels, i.e. even if KVM
> +	 * will never yield mmu_lock in response to contention, as this vCPU is
> +	 * *guaranteed* to need to retry, i.e. waiting until mmu_lock is held
> +	 * to detect retry guarantees the worst case latency for the vCPU.
> +	 */
> +	if (fault->slot &&
> +	    mmu_invalidate_retry_gfn_unsafe(vcpu->kvm, fault->mmu_seq, fault->gfn))
> +		return RET_PF_RETRY;
> +
>  	ret = __kvm_faultin_pfn(vcpu, fault);
>  	if (ret != RET_PF_CONTINUE)
>  		return ret;
> @@ -4415,6 +4440,18 @@ static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
>  	if (unlikely(!fault->slot))
>  		return kvm_handle_noslot_fault(vcpu, fault, access);
>  
> +	/*
> +	 * Check again for a relevant mmu_notifier invalidation event purely to
> +	 * avoid contending mmu_lock.  Most invalidations will be detected by
> +	 * the previous check, but checking is extremely cheap relative to the
> +	 * overall cost of failing to detect the invalidation until after
> +	 * mmu_lock is acquired.
> +	 */
> +	if (mmu_invalidate_retry_gfn_unsafe(vcpu->kvm, fault->mmu_seq, fault->gfn)) {
> +		kvm_release_pfn_clean(fault->pfn);
> +		return RET_PF_RETRY;
> +	}
> +
>  	return RET_PF_CONTINUE;
>  }
>  
> @@ -4442,6 +4479,11 @@ static bool is_page_fault_stale(struct kvm_vcpu *vcpu,
>  	if (!sp && kvm_test_request(KVM_REQ_MMU_FREE_OBSOLETE_ROOTS, vcpu))
>  		return true;
>  
> +	/*
> +	 * Check for a relevant mmu_notifier invalidation event one last time
> +	 * now that mmu_lock is held, as the "unsafe" checks performed without
> +	 * holding mmu_lock can get false negatives.
> +	 */
>  	return fault->slot &&
>  	       mmu_invalidate_retry_gfn(vcpu->kvm, fault->mmu_seq, fault->gfn);
>  }
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 18e28610749e..97afe4519772 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -2062,6 +2062,32 @@ static inline int mmu_invalidate_retry_gfn(struct kvm *kvm,
>  		return 1;
>  	return 0;
>  }
> +
> +/*
> + * This lockless version of the range-based retry check *must* be paired with a
> + * call to the locked version after acquiring mmu_lock, i.e. this is safe to
> + * use only as a pre-check to avoid contending mmu_lock.  This version *will*
> + * get false negatives and false positives.
> + */
> +static inline bool mmu_invalidate_retry_gfn_unsafe(struct kvm *kvm,
> +						   unsigned long mmu_seq,
> +						   gfn_t gfn)
> +{
> +	/*
> +	 * Use READ_ONCE() to ensure the in-progress flag and sequence counter
> +	 * are always read from memory, e.g. so that checking for retry in a
> +	 * loop won't result in an infinite retry loop.  Don't force loads for
> +	 * start+end, as the key to avoiding infinite retry loops is observing
> +	 * the 1=>0 transition of in-progress, i.e. getting false negatives
> +	 * due to stale start+end values is acceptable.
> +	 */
> +	if (unlikely(READ_ONCE(kvm->mmu_invalidate_in_progress)) &&
> +	    gfn >= kvm->mmu_invalidate_range_start &&
> +	    gfn < kvm->mmu_invalidate_range_end)
> +		return true;
> +
> +	return READ_ONCE(kvm->mmu_invalidate_seq) != mmu_seq;
> +}
>  #endif
>  
>  #ifdef CONFIG_HAVE_KVM_IRQ_ROUTING
> 
> base-commit: 21dbc438dde69ff630b3264c54b94923ee9fcdcf
> -- 
> 2.44.0.rc0.258.g7320e95886-goog
>