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

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Message-ID: <acf2q5da7s2rmh5tugozwhwv5rqafacgah7kjicrfc2qhbuzc6@k6ic4xngr552>
Date: Wed, 5 Feb 2025 16:43:50 +0530
From: Naveen N Rao <naveen@...nel.org>
To: Sean Christopherson <seanjc@...gle.com>
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 at 09:51:22AM -0800, Sean Christopherson wrote:
> 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.

My comment was more from the point of view of correctness, rather than 
performance (with the goal of removing use of apicv_update_lock) -- 
similar to the issue with IRQWIN needing to maintain per-vcpu state. My 
naive understanding of Maxim's mail was that we would introduce per-vcpu 
inhibit field to maintain per-vcpu inhibit state, but not actually 
inhibit AVIC on a per-vcpu basis :)


Thanks,
Naveen