linux-kernel - Re: [PATCH 2/3] KVM: x86: Remove use of apicv_update

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Message-ID: <Z6JTmvrkrLpaJ1nw@google.com>
Date: Tue, 4 Feb 2025 09:51:22 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Naveen N Rao <naveen@...nel.org>
Cc: Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Paolo Bonzini <pbonzini@...hat.com>, Suravee Suthikulpanit <suravee.suthikulpanit@....com>, 
	Vasant Hegde <vasant.hegde@....com>, Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH 2/3] KVM: x86: Remove use of apicv_update_lock when
 toggling guest debug state

On Tue, Feb 04, 2025, Naveen N Rao wrote:
> On Mon, Feb 03, 2025 at 09:00:05PM -0500, Maxim Levitsky wrote:
> > On Mon, 2025-02-03 at 22:33 +0530, Naveen N Rao (AMD) wrote:
> > > apicv_update_lock is not required when querying the state of guest
> > > debug in all the vcpus. Remove usage of the same, and switch to
> > > kvm_set_or_clear_apicv_inhibit() helper to simplify the code.
> > 
> > It might be worth to mention that the reason why the lock is not needed,
> > is because kvm_vcpu_ioctl from which this function is called takes 'vcpu->mutex'
> > and thus concurrent execution of this function is not really possible.
> 
> Looking at this again, that looks to be a vcpu-specific lock, so I guess 
> it is possible for multiple vcpus to run this concurrently?

Correct.

> In reality, this looks to be coming in from a vcpu ioctl from userspace, 
> so this is probably not being invoked concurrently today.
> 
> Regardless, I wonder if moving this to a per-vcpu inhibit might be a 
> better way to address this.

No, this is a slow path.