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: <8734hd8rrx.fsf@redhat.com>
Date: Mon, 20 Jan 2025 15:49:06 +0100
From: Vitaly Kuznetsov <vkuznets@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>, Sean Christopherson
 <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>, David Woodhouse
 <dwmw2@...radead.org>, Paul Durrant <paul@....org>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
 syzbot+352e553a86e0d75f5120@...kaller.appspotmail.com, Paul Durrant
 <pdurrant@...zon.com>, David Woodhouse <dwmw@...zon.co.uk>
Subject: Re: [PATCH 09/10] KVM: x86: Setup Hyper-V TSC page before Xen PV
 clocks (during clock update)

Sean Christopherson <seanjc@...gle.com> writes:

> When updating paravirtual clocks, setup the Hyper-V TSC page before
> Xen PV clocks.  This will allow dropping xen_pvclock_tsc_unstable in favor
> of simply clearing PVCLOCK_TSC_STABLE_BIT in the reference flags.
>
> Cc: Vitaly Kuznetsov <vkuznets@...hat.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/x86.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 9eabd70891dd..c68e7f7ba69d 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3280,6 +3280,8 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
>  		hv_clock.flags &= ~PVCLOCK_GUEST_STOPPED;
>  	}
>  
> +	kvm_hv_setup_tsc_page(v->kvm, &hv_clock);
> +
>  #ifdef CONFIG_KVM_XEN
>  	if (vcpu->xen.vcpu_info_cache.active)
>  		kvm_setup_guest_pvclock(&hv_clock, v, &vcpu->xen.vcpu_info_cache,
> @@ -3289,7 +3291,6 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
>  		kvm_setup_guest_pvclock(&hv_clock, v, &vcpu->xen.vcpu_time_info_cache, 0,
>  					xen_pvclock_tsc_unstable);
>  #endif
> -	kvm_hv_setup_tsc_page(v->kvm, &hv_clock);
>  	return 0;
>  }

"No functional change detected".

Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>

(What I'm wondering is if (from mostly theoretical PoV) it's OK to pass
*some* of the PV clocks as stable and some as unstable to the same
guest, i.e. if it would make sense to disable Hyper-V TSC page when
KVM_XEN_HVM_CONFIG_PVCLOCK_TSC_UNSTABLE too. I don't know if anyone
combines Xen and Hyper-V emulation capabilities for the same guest on
KVM though.)

-- 
Vitaly