| Message ID | 20231204144334.910-19-paul@xen.org |
|---|---|
| State | Superseded |
| Headers | show |
| Series | KVM: xen: update shared_info and vcpu_info handling | expand |
On Mon, 2023-12-04 at 14:43 +0000, Paul Durrant wrote: > From: Paul Durrant <pdurrant@amazon.com> > > 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@amazon.com> Reviewed-by: David Woodhouse <dwmw@amazon.co.uk> 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@google.com> > Cc: Paolo Bonzini <pbonzini@redhat.com> > Cc: David Woodhouse <dwmw2@infradead.org> > > 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); > }
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); }