linux-kernel - Re: [RFC 29/33] KVM: VMX: Save instruction length on EPT violation

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Message-ID: <ZUu0FzbW5tr2Werz@google.com>
Date:   Wed, 8 Nov 2023 08:15:19 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Alexander Graf <graf@...zon.com>
Cc:     Nicolas Saenz Julienne <nsaenz@...zon.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-hyperv@...r.kernel.org,
        pbonzini@...hat.com, vkuznets@...hat.com, anelkz@...zon.com,
        dwmw@...zon.co.uk, jgowans@...zon.com, corbert@....net,
        kys@...rosoft.com, haiyangz@...rosoft.com, decui@...rosoft.com,
        x86@...nel.org, linux-doc@...r.kernel.org
Subject: Re: [RFC 29/33] KVM: VMX: Save instruction length on EPT violation

On Wed, Nov 08, 2023, Alexander Graf wrote:
> 
> On 08.11.23 12:18, Nicolas Saenz Julienne wrote:
> > Save the length of the instruction that triggered an EPT violation in
> > struct kvm_vcpu_arch. This will be used to populate Hyper-V VSM memory
> > intercept messages.
> > 
> > Signed-off-by: Nicolas Saenz Julienne <nsaenz@...zon.com>
> 
> 
> In v1, please do this for SVM as well :)

Why?  KVM caches values on VMX because VMREAD is measurable slower than memory
accesses, especially when running nested.  SVM has no such problems.  I wouldn't
be surprised if adding a "cache" is actually less performant due to increased
pressure and misses on the hardware cache.