linux-kernel - Re: [PATCH 09/10] KVM: x86: Setup Hyper-V TSC page before Xen PV clocks (during clock update)

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Message-ID: <8a9460aa-7ac7-4e6f-91d5-f1170a1963f3@xen.org>
Date: Tue, 21 Jan 2025 17:30:14 +0000
From: Paul Durrant <xadimgnik@...il.com>
To: David Woodhouse <dwmw2@...radead.org>,
 Sean Christopherson <seanjc@...gle.com>,
 Vitaly Kuznetsov <vkuznets@...hat.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
 linux-kernel@...r.kernel.org,
 syzbot+352e553a86e0d75f5120@...kaller.appspotmail.com,
 Paul Durrant <pdurrant@...zon.com>
Subject: Re: [PATCH 09/10] KVM: x86: Setup Hyper-V TSC page before Xen PV
 clocks (during clock update)

On 21/01/2025 17:16, David Woodhouse wrote:
> On Tue, 2025-01-21 at 15:59 +0000, Paul Durrant wrote:
>> On 21/01/2025 15:44, Sean Christopherson wrote:
>> [snip]
>>>
>>> I think it's ok to keep the Hyper-V TSC page in this case.  It's not that the Xen
>>> PV clock is truly unstable, it's that some guests get tripped up by the STABLE
>>> flag.  A guest that can't handle the STABLE flag has bigger problems than the
>>> existence of a completely unrelated clock that is implied to be stable.
>>>
>>
>> Agreed.
>>
>>>> I don't know if anyone combines Xen and Hyper-V emulation capabilities for
>>>> the same guest on KVM though.)
>>>
>>> That someone would have to be quite "brave" :-D
>>
>> Maybe :-)
> 
> Xen itself does offer some Hyper-V enlightenments, and we might
> reasonably expect KVM-based hypervisors to offer the same. We
> explicitly do account for the KVM CPUID leaves moving up to let the
> Hyper-V ones exist.
> 
> I don't recall if Xen's Hyper-V support includes the TSC page though.

It does :-)