linux-kernel - Re: [PATCH v5 3/7] x86/fpu/xstate: Differentiate default features for host and guest FPUs

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Message-ID: <9925d172-94e1-4e7a-947e-46261ac83864@intel.com>
Date: Mon, 28 Apr 2025 18:11:24 -0700
From: "Chang S. Bae" <chang.seok.bae@...el.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
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Subject: Re: [PATCH v5 3/7] x86/fpu/xstate: Differentiate default features for
 host and guest FPUs

On 4/28/2025 8:42 AM, Edgecombe, Rick P wrote:
> 
> Right, so there should be no need to keep a separate features and buffer size
> for KVM's xsave UABI, as this patch does. Let's just leave it using the core
> kernels UABI version.

Hmm, why so?

As I see it, the vcpu->arch.guest_fpu structure is already exposed to 
KVM. This series doesn’t modify those structures (fpu_guest and 
fpstate), other than removing a dead field (patch 2).

Both ->usersize and ->user_xfeatures fields are already exposed -- 
currently KVM just doesn’t reference them at all.

All the changes introduced here are transparent to KVM. Organizing the 
initial values and wiring up guest_perm and fpstate are entirely 
internal to the x86 core, no?