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authorVitaly Kuznetsov <vkuznets@redhat.com>2020-05-25 16:41:21 +0200
committerPaolo Bonzini <pbonzini@redhat.com>2020-06-01 04:26:08 -0400
commit557a961abbe06ed9dfd3b55ef7bd6e68295cda3d (patch)
tree257d2ac75925f214eb581e5cf25baa2e0114575c /Documentation/virt
parent2635b5c4a0e407b84f68e188c719f28ba0e9ae1b (diff)
KVM: x86: acknowledgment mechanism for async pf page ready notifications
If two page ready notifications happen back to back the second one is not delivered and the only mechanism we currently have is kvm_check_async_pf_completion() check in vcpu_run() loop. The check will only be performed with the next vmexit when it happens and in some cases it may take a while. With interrupt based page ready notification delivery the situation is even worse: unlike exceptions, interrupts are not handled immediately so we must check if the slot is empty. This is slow and unnecessary. Introduce dedicated MSR_KVM_ASYNC_PF_ACK MSR to communicate the fact that the slot is free and host should check its notification queue. Mandate using it for interrupt based 'page ready' APF event delivery. As kvm_check_async_pf_completion() is going away from vcpu_run() we need a way to communicate the fact that vcpu->async_pf.done queue has transitioned from empty to non-empty state. Introduce kvm_arch_async_page_present_queued() and KVM_REQ_APF_READY to do the job. Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com> Message-Id: <20200525144125.143875-7-vkuznets@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Diffstat (limited to 'Documentation/virt')
-rw-r--r--Documentation/virt/kvm/msr.rst15
1 files changed, 14 insertions, 1 deletions
diff --git a/Documentation/virt/kvm/msr.rst b/Documentation/virt/kvm/msr.rst
index be08df12f31a..9b107889b033 100644
--- a/Documentation/virt/kvm/msr.rst
+++ b/Documentation/virt/kvm/msr.rst
@@ -236,7 +236,9 @@ data:
of these bytes is a token which was previously delivered as 'page not
present' event. The event indicates the page in now available. Guest is
supposed to write '0' to 'token' when it is done handling 'page ready'
- event so the next one can be delivered.
+ event and to write 1' to MSR_KVM_ASYNC_PF_ACK after clearing the location;
+ writing to the MSR forces KVM to re-scan its queue and deliver the next
+ pending notification.
Note, MSR_KVM_ASYNC_PF_INT MSR specifying the interrupt vector for 'page
ready' APF delivery needs to be written to before enabling APF mechanism
@@ -359,3 +361,14 @@ data:
Interrupt vector for asynchnonous 'page ready' notifications delivery.
The vector has to be set up before asynchronous page fault mechanism
is enabled in MSR_KVM_ASYNC_PF_EN.
+
+MSR_KVM_ASYNC_PF_ACK:
+ 0x4b564d07
+
+data:
+ Asynchronous page fault (APF) acknowledgment.
+
+ When the guest is done processing 'page ready' APF event and 'token'
+ field in 'struct kvm_vcpu_pv_apf_data' is cleared it is supposed to
+ write '1' to bit 0 of the MSR, this causes the host to re-scan its queue
+ and check if there are more notifications pending.