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

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Message-ID: <281354d3-1f04-483d-a6d0-baf6fdcec376@intel.com>
Date: Wed, 30 Apr 2025 08:01:44 -0700
From: "Chang S. Bae" <chang.seok.bae@...el.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
CC: "ebiggers@...gle.com" <ebiggers@...gle.com>, "kvm@...r.kernel.org"
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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 8:36 PM, Edgecombe, Rick P wrote:
> 
> KVM_GET_XSAVE is part of KVM's API. It uses fields configured in struct
> fpu_guest. If fpu_user_cfg.default_features changes value (in the current code)
> it would change KVM's uABI. 

Not quite. The ABI reflects the XSAVE format directly. The XSAVE header 
indicates which feature states are present, so while the _contents_ of 
the buffer may vary depending on the feature set, the _format_ itself 
remains unchanged. That doesn't constitute a uABI change.

> It should be simple. Two new configuration fields are added in this patch that
> match the existing concept and values of existing configurations fields. Per
> Sean, there are no plans to have them diverge. So why add them. 

I'm fine with dropping them -- as long as the resulting code remains 
clear and avoids unnecessary complexity around VCPU allocation.

Here are some of the considerations that led me to suggest them in the 
first place:

  * The guest-only feature model should be established in a clean and
    structured way.
  * The initialization logic should stay explicit -- especially to make
    it clear what constitutes guest features, even when they match host
    features. That naturally led to introducing a dedicated data
    structure.
  * Since the VCPU FPU container includes struct fpstate, it felt
    appropriate to mirror relevant fields where useful.
  * Including user_size and user_xfeatures made the VCPU allocation logic
    more straightforward and self-contained.

And to clarify -- this addition doesn’t necessarily imply divergence 
from fpu_guest_cfg. Its usage is local to setting up the guest fpstate, 
and nothing more.