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

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Message-ID: <4eb50ee5-fd5e-4dc1-bee1-629da687bdb5@amazon.com>
Date:   Wed, 8 Nov 2023 18:11:34 +0100
From:   Alexander Graf <graf@...zon.com>
To:     Sean Christopherson <seanjc@...gle.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 08.11.23 17:15, Sean Christopherson wrote:
>
> 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.


My understanding was that this patch wasn't about caching it, it was 
about storing it somewhere generically so we can use it for the fault 
injection code path in the following patch. And if we don't set this 
variable for SVM, it just means Credential Guard fault injection would 
be broken there.


Alex




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