From: David Woodhouse <dwmw2@infradead.org>
To: Paul Durrant <paul@xen.org>, Paolo Bonzini <pbonzini@redhat.com>,
Jonathan Corbet <corbet@lwn.net>,
Sean Christopherson <seanjc@google.com>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
Dave Hansen <dave.hansen@linux.intel.com>,
x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
Shuah Khan <shuah@kernel.org>,
kvm@vger.kernel.org, linux-doc@vger.kernel.org,
linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org
Subject: Re: [PATCH v10 18/19] KVM: pfncache: check the need for invalidation under read lock first
Date: Thu, 14 Dec 2023 14:08:05 +0000 [thread overview]
Message-ID: <8cba6a556233ae4e8cb401cb4ffa56b9d809e337.camel@infradead.org> (raw)
In-Reply-To: <20231204144334.910-19-paul@xen.org>
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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);
> }
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next prev parent reply other threads:[~2023-12-14 14:08 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-12-04 14:43 [PATCH v10 00/19] KVM: xen: update shared_info and vcpu_info handling Paul Durrant
2023-12-04 14:43 ` [PATCH v10 01/19] KVM: pfncache: Add a map helper function Paul Durrant
2023-12-04 14:43 ` [PATCH v10 02/19] KVM: pfncache: remove unnecessary exports Paul Durrant
2023-12-04 14:43 ` [PATCH v10 03/19] KVM: xen: mark guest pages dirty with the pfncache lock held Paul Durrant
2023-12-04 14:43 ` [PATCH v10 04/19] KVM: pfncache: add a mark-dirty helper Paul Durrant
2023-12-04 14:43 ` [PATCH v10 05/19] KVM: pfncache: remove KVM_GUEST_USES_PFN usage Paul Durrant
2023-12-04 14:43 ` [PATCH v10 06/19] KVM: pfncache: stop open-coding offset_in_page() Paul Durrant
2023-12-04 14:43 ` [PATCH v10 07/19] KVM: pfncache: include page offset in uhva and use it consistently Paul Durrant
2023-12-04 14:43 ` [PATCH v10 08/19] KVM: pfncache: allow a cache to be activated with a fixed (userspace) HVA Paul Durrant
2023-12-04 14:43 ` [PATCH v10 09/19] KVM: xen: separate initialization of shared_info cache and content Paul Durrant
2023-12-14 13:41 ` David Woodhouse
2023-12-14 14:00 ` Paul Durrant
2023-12-04 14:43 ` [PATCH v10 10/19] KVM: xen: (re-)initialize shared_info if guest (32/64-bit) mode is set Paul Durrant
2023-12-14 13:44 ` David Woodhouse
2023-12-04 14:43 ` [PATCH v10 11/19] KVM: xen: allow shared_info to be mapped by fixed HVA Paul Durrant
2023-12-04 14:43 ` [PATCH v10 12/19] KVM: xen: allow vcpu_info " Paul Durrant
2023-12-04 14:43 ` [PATCH v10 13/19] KVM: selftests / xen: map shared_info using HVA rather than GFN Paul Durrant
2023-12-04 14:43 ` [PATCH v10 14/19] KVM: selftests / xen: re-map vcpu_info using HVA rather than GPA Paul Durrant
2023-12-04 14:43 ` [PATCH v10 15/19] KVM: xen: advertize the KVM_XEN_HVM_CONFIG_SHARED_INFO_HVA capability Paul Durrant
2023-12-04 14:43 ` [PATCH v10 16/19] KVM: xen: split up kvm_xen_set_evtchn_fast() Paul Durrant
2023-12-14 14:00 ` David Woodhouse
2023-12-14 14:20 ` David Woodhouse
2023-12-04 14:43 ` [PATCH v10 17/19] KVM: xen: don't block on pfncache locks in kvm_xen_set_evtchn_fast() Paul Durrant
2023-12-14 14:03 ` David Woodhouse
2023-12-04 14:43 ` [PATCH v10 18/19] KVM: pfncache: check the need for invalidation under read lock first Paul Durrant
2023-12-14 14:08 ` David Woodhouse [this message]
2023-12-04 14:43 ` [PATCH v10 19/19] KVM: xen: allow vcpu_info content to be 'safely' copied Paul Durrant
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