linux-kernel - Re: [PATCH 0/2] KVM: Protect vCPU's PID with a rwlock

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Message-ID: <173039527102.1510350.14562788857137656763.b4-ty@google.com>
Date: Thu, 31 Oct 2024 12:51:29 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Marc Zyngier <maz@...nel.org>, 
	Oliver Upton <oliver.upton@...ux.dev>, Paolo Bonzini <pbonzini@...hat.com>
Cc: linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev, 
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Steve Rutherford <srutherford@...gle.com>
Subject: Re: [PATCH 0/2] KVM: Protect vCPU's PID with a rwlock

On Fri, 02 Aug 2024 13:01:34 -0700, Sean Christopherson wrote:
> Protect vcpu->pid with a rwlock instead of RCU, so that running a vCPU
> with a different task doesn't require a full RCU synchronization, which
> can introduce a non-trivial amount of jitter, especially on large systems.
> 
> I've had this mini-series sitting around for ~2 years, pretty much as-is.
> I could have sworn past me thought there was a flaw in using a rwlock, and
> so I never posted it, but for the life of me I can't think of any issues.
> 
> [...]

Applied to kvm-x86 generic, with a comment explaining the vcpu->mutex is the
true protector of vcpu->pid.

Oliver, I didn't add the requested lockdep notification, I really want to make
that a separate discussion:

https://lore.kernel.org/all/20241009150455.1057573-7-seanjc@google.com

[1/2] KVM: Return '0' directly when there's no task to yield to
      https://github.com/kvm-x86/linux/commit/6cf9ef23d942
[2/2] KVM: Protect vCPU's "last run PID" with rwlock, not RCU
      https://github.com/kvm-x86/linux/commit/3e7f43188ee2

--
https://github.com/kvm-x86/linux/tree/next