linux-kernel - Re: [PATCH] KVM: x86: Update Xen-specific CPUID leaves during mangling

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Message-ID: <06e9f951afb46098983dc009c0efbcef3fc1b246.camel@infradead.org>
Date: Wed, 22 Jan 2025 10:53:03 -0800
From: David Woodhouse <dwmw2@...radead.org>
To: Vitaly Kuznetsov <vkuznets@...hat.com>, paul@....org, Fred Griffoul
	 <fgriffo@...zon.co.uk>, kvm@...r.kernel.org
Cc: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini
 <pbonzini@...hat.com>,  Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar
 <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, Dave Hansen
 <dave.hansen@...ux.intel.com>, x86@...nel.org, "H. Peter Anvin"
 <hpa@...or.com>,  linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: Update Xen-specific CPUID leaves during
 mangling

On Wed, 2025-01-22 at 18:44 +0100, Vitaly Kuznetsov wrote:
>  
> > 
> > What is the purpose of the comparison anyway? IIUC we want to ensure 
> > that a VMM does not change its mind after KVM_RUN so should we not be 
> > stashing what was set by the VMM and comparing against that *before* 
> > mangling any values?
> > 
> 
> I guess it can be done this way but we will need to keep these 'original'
> unmangled values for the lifetime of the vCPU with very little gain (IMO):
> KVM_SET_CPUID{,2} either fails (if the data is different) or does (almost)
> nothing when the data is the same.

If they're supposed to be entirely unchanged, would it suffice just to
keep a hash of them?

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