linux-kernel - Re: [PATCH 2/2] KVM: x86: Clear pv_unhalted on all transitions to KVM_MP_STATE

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Message-ID: <Z6zREu-pShh-ivK-@google.com>
Date: Wed, 12 Feb 2025 08:49:22 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Jim Mattson <jmattson@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, 
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H . Peter Anvin" <hpa@...or.com>, David Woodhouse <dwmw2@...radead.org>, Paul Durrant <paul@....org>, 
	Gleb Natapov <gleb@...hat.com>, Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>, 
	Suzuki Poulose <suzuki@...ibm.com>, Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>, 
	Jan Kiszka <jan.kiszka@...mens.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] KVM: x86: Clear pv_unhalted on all transitions to KVM_MP_STATE_RUNNABLE

On Wed, Feb 05, 2025, Jim Mattson wrote:
> On Mon, Jan 13, 2025 at 12:02 PM Jim Mattson <jmattson@...gle.com> wrote:
> >
> > In kvm_set_mp_state(), ensure that vcpu->arch.pv.pv_unhalted is always
> > cleared on a transition to KVM_MP_STATE_RUNNABLE, so that the next HLT
> > instruction will be respected.
> >
> > The "fixes" list may be incomplete.
> 
> The only commit I'm not sure of is commit 1a65105a5aba ("KVM: x86/xen:
> handle PV spinlocks slowpath"). That commit introduces an mp_state
> transition to KVM_MP_STATE_RUNNABLE  without clearing pv_unhalted, so
> perhaps it should be in the "fixes" list. OTOH, this seems to be an
> independent implementation of PV spinlocks, so maybe it's not a
> problem.

I'll add it to the list, and drop the "list may be incomplete" line.
KVM_HC_KICK_CPU is unreachable if Xen hypercalls are enabled, but nothing would
prevent a clever guest from sending an virtual IPI with APIC_DM_REMRD.  Whether
or not that makes the Xen code a KVM bug is definitely debatable, but I can't
imagine will care about Fixes being slightly overzealous.