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RE: Optimized clocksource with AMD AVIC enabled for Windows guest

From: Vitaly Kuznetsov
Subject: RE: Optimized clocksource with AMD AVIC enabled for Windows guest
Date: Thu, 04 Feb 2021 13:24:28 +0100

Kechen Lu <kechenl@nvidia.com> writes:

> Hi Vitaly and Paolo,
> Thanks so much for quick reply. This makes sense to me. From my 
> understanding, basically this can be two part of it to resolve it. 
> First, we make sure to set and expose 0x40000004.EAX Bit9 to windows guest, 
> like in kvm_vcpu_ioctl_get_hv_cpuid(), having this recommendation bit :
> -----------------------
> ...
> -----------------------

This also needs to be wired through userspace (e.g. QEMU) as this
doesn't go to the guest directly.

> Second, although the above could tell guest to deprecate AutoEOI, older 
> Windows OSes would not acknowledge this (I checked the Hyper-v TLFS, from 
> spec v3.0 (i.e. Windows Server 2012), it starts having bit9 defined in 
> 0x40000004.EAX), we may want to dynamically toggle off APICv/AVIC if we found 
> the SynIC SINT vector has AutoEOI, under synic_update_vector(). E.g. like:
> -----------------------------
> if (synic_has_vector_auto_eoi(synic, vector)) {
>       kvm_request_apicv_update(vcpu->kvm, false, APICV_INHIBIT_REASON_HYPERV);
>       __set_bit(vector, synic->auto_eoi_bitmap);
> } else {
>       kvm_request_apicv_update(vcpu->kvm, true, APICV_INHIBIT_REASON_HYPERV);
>       __clear_bit(vector, synic->auto_eoi_bitmap);
> }
> ---------------------------------

APICV_INHIBIT_REASON_HYPERV is per-VM so we need to count how many
AutoEOI SINTs were set in *all* SynICs (an atomic in 'struct kvm_hv'
would do).

> Curious about what current plan/status of upstream is for this. If
> that's doable and not current pending patch covering this, I can make
> a quick draft patch tested and sent out for reviewing. 

I checked Linux VMs on genuine Hyper-V and surprisingly
'HV_DEPRECATING_AEOI_RECOMMENDED' is not exposed. I'm going to pass it
to WS2016/2019 and see what happens. If it all works as expected and if
you don't beat me to it I'll be sending a patch.


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