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: <a71d0215ebc34e132081e856f116ca5fcbda41b4.camel@infradead.org>
Date: Tue, 21 Jan 2025 09:16:29 -0800
From: David Woodhouse <dwmw2@...radead.org>
To: paul@....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 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.

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