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

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Message-ID: <a7eb34n6gkwg6kafh7r76tkwtweuflyfoczgxya2k63al2qdoe@phmszu6ilk4w>
Date: Tue, 4 Feb 2025 19:55:39 +0530
From: Naveen N Rao <naveen@...nel.org>
To: Maxim Levitsky <mlevitsk@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Sean Christopherson <seanjc@...gle.com>, 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 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?

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.


Thanks,
Naveen