linux-kernel - Re: [PATCH 0/5] KVM: x86: Address xstate_required

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Message-ID: <257491b6-f70a-4347-b97d-cc7fa22aac85@redhat.com>
Date: Fri, 13 Dec 2024 19:58:54 +0100
From: Paolo Bonzini <pbonzini@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
 Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH 0/5] KVM: x86: Address xstate_required_size() perf
 regression

On 12/11/24 02:32, Sean Christopherson wrote:
> Fix a hilarious/revolting performance regression (relative to older CPU
> generations) in xstate_required_size() that pops up due to CPUID _in the
> host_ taking 3x-4x longer on Emerald Rapids than Skylake.
> 
> The issue rears its head on nested virtualization transitions, as KVM
> (unnecessarily) performs runtime CPUID updates, including XSAVE sizes,
> multiple times per transition.  And calculating XSAVE sizes, especially
> for vCPUs with a decent number of supported XSAVE features and compacted
> format support, can add up to thousands of cycles.
> 
> To fix the immediate issue, cache the CPUID output at kvm.ko load.  The
> information is static for a given CPU, i.e. doesn't need to be re-read
> from hardware every time.  That's patch 1, and eliminates pretty much all
> of the meaningful overhead.

Queued this one, thanks!

Paolo