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

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Message-ID: <Z5FqdjTwPmnV1t-1@google.com>
Date: Wed, 22 Jan 2025 14:00:22 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: David Woodhouse <dwmw2@...radead.org>
Cc: Vitaly Kuznetsov <vkuznets@...hat.com>, paul@....org, Fred Griffoul <fgriffo@...zon.co.uk>, 
	kvm@...r.kernel.org, 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, Jan 22, 2025, David Woodhouse wrote:
> On Wed, 2025-01-22 at 18:44 +0100, Vitaly Kuznetsov wrote:
> > > What is the purpose of the comparison anyway?

To avoid scenarios where KVM has configured state for a set of features X, and
doesn't correctly handle vCPU features suddenly become Y.  Or more commonly,
where correctly handling such transitions (if there's even a "correct" option)
is a complete waste of time and complexity because no sane setup will ever add
and/or remove features from a running VM.

> > > 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.

More importantly, userspace is allowed to set the CPUID returned by KVM_GET_CPUID2.
E.g. selftests do KVM_GET_CPUID2 specifically to read the bits that are managed
by KVM.

Disallowing that would likely break userspace, and would create a weird ABI where
the output of KVM_GET_CPUID2 is rejected by KVM_SET_CPUID2.

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