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

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Message-ID: <fbaf2f8e-f907-4b92-83b9-192f20e6ba9c@intel.com>
Date: Mon, 28 Apr 2025 20:22:38 -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 7:50 PM, Edgecombe, Rick P wrote:
> On Mon, 2025-04-28 at 18:11 -0700, Chang S. Bae wrote:
>> 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?
> 
> This patch adds struct vcpu_fpu_config, with new fields user_size,
> user_features. Then those fields are used to configure the guest FPU, where
> today it just uses fpu_user_cfg.default_features, etc.
> 
> KVM doesn't refer to any of those fields specifically, but since they are used
> to configure struct fpu_guest they become part of KVM's uABI.

Today, fpu_alloc_guest_fpstate() -> __fpstate_reset() sets
vcpu->arch.guest_fpu.fpstate->user_xfeatures using 
fpu_user_cfg.default_features.

Are you really saying that switching this to 
guest_default_cfg.user_features would constitute a uABI change? Do you 
consider fpu_user_cfg.default_features to be part of the uABI or 
anything else?