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KVM: x86: acknowledgment mechanism for async pf page ready notifications

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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>
This commit is contained in:
Vitaly Kuznetsov
2020-05-25 16:41:21 +02:00
committed by Paolo Bonzini
parent 2635b5c4a0
commit 557a961abb
6 changed files with 52 additions and 5 deletions
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15
Documentation/virt/kvm/msr.rst
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@@ -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.