Re: [PATCH v10 18/19] KVM: pfncache: check the need for invalidation under read lock first

[David Woodhouse]

On Mon, 2023-12-04 at 14:43 +0000, Paul Durrant wrote:
> From: Paul Durrant <pdurrant@xxxxxxxxxx>
> 
> Taking a write lock on a pfncache will be disruptive if the cache is
> heavily used (which only requires a read lock). Hence, in the MMU notifier
> callback, take read locks on caches to check for a match; only taking a
> write lock to actually perform an invalidation (after a another check).
> 
> Signed-off-by: Paul Durrant <pdurrant@xxxxxxxxxx>

Reviewed-by: David Woodhouse <dwmw@xxxxxxxxxxxx>

In particular, the previous 'don't block on pfncache locks in
kvm_xen_set_evtchn_fast()' patch in this series is easy to justify on
the basis that it only falls back to the slow path if it can't take a
read lock immediately. And surely it should *always* be able to take a
read lock immediately unless there's an actual *writer* — which should
be a rare event, and means the cache was probably going to be
invalidates anyway.

But then we realised the MMU notifier was going to disrupt that.



> ---
> Cc: Sean Christopherson <seanjc@xxxxxxxxxx>
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: David Woodhouse <dwmw2@xxxxxxxxxxxxx>
> 
> v10:
>  - New in this version.
> ---
>  virt/kvm/pfncache.c | 22 +++++++++++++++++++---
>  1 file changed, 19 insertions(+), 3 deletions(-)
> 
> diff --git a/virt/kvm/pfncache.c b/virt/kvm/pfncache.c
> index c2a2d1e145b6..4da16d494f4b 100644
> --- a/virt/kvm/pfncache.c
> +++ b/virt/kvm/pfncache.c
> @@ -29,14 +29,30 @@ void gfn_to_pfn_cache_invalidate_start(struct kvm *kvm, unsigned long start,
>  
>         spin_lock(&kvm->gpc_lock);
>         list_for_each_entry(gpc, &kvm->gpc_list, list) {
> -               write_lock_irq(&gpc->lock);
> +               read_lock_irq(&gpc->lock);
>  
>                 /* Only a single page so no need to care about length */
>                 if (gpc->valid && !is_error_noslot_pfn(gpc->pfn) &&
>                     gpc->uhva >= start && gpc->uhva < end) {
> -                       gpc->valid = false;
> +                       read_unlock_irq(&gpc->lock);
> +
> +                       /*
> +                        * There is a small window here where the cache could
> +                        * be modified, and invalidation would no longer be
> +                        * necessary. Hence check again whether invalidation
> +                        * is still necessary once the write lock has been
> +                        * acquired.
> +                        */
> +
> +                       write_lock_irq(&gpc->lock);
> +                       if (gpc->valid && !is_error_noslot_pfn(gpc->pfn) &&
> +                           gpc->uhva >= start && gpc->uhva < end)
> +                               gpc->valid = false;
> +                       write_unlock_irq(&gpc->lock);
> +                       continue;
>                 }
> -               write_unlock_irq(&gpc->lock);
> +
> +               read_unlock_irq(&gpc->lock);
>         }
>         spin_unlock(&kvm->gpc_lock);
>  }

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