linux-kernel - Re: [PATCH v3 03/10] x86/fpu/xstate: Correct xfeatures cache in guest pseudo fpu container

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Message-ID: <8f94cb20-68a3-48ca-ae4d-c6609d63e30a@intel.com>
Date: Fri, 7 Mar 2025 09:48:25 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Chao Gao <chao.gao@...el.com>, tglx@...utronix.de, x86@...nel.org,
 seanjc@...gle.com, pbonzini@...hat.com, linux-kernel@...r.kernel.org,
 kvm@...r.kernel.org
Cc: peterz@...radead.org, rick.p.edgecombe@...el.com,
 weijiang.yang@...el.com, john.allen@....com, bp@...en8.de
Subject: Re: [PATCH v3 03/10] x86/fpu/xstate: Correct xfeatures cache in guest
 pseudo fpu container

On 3/7/25 08:41, Chao Gao wrote:
> The xfeatures field in struct fpu_guest is designed to track the enabled
> xfeatures for guest FPUs. However, during allocation in
> fpu_alloc_guest_fpstate(), gfpu->xfeatures is initialized to
> fpu_user_cfg.default_features, while the corresponding
> fpstate->xfeatures is set to fpu_kernel_cfg.default_features
> 
> Correct the mismatch to avoid confusion.
> 
> Note this mismatch does not cause any functional issues. The
> gfpu->xfeatures is checked in fpu_enable_guest_xfd_features() to
> verify if XFD features are already enabled:
> 
> 	xfeatures &= ~guest_fpu->xfeatures;
> 	if (!xfeatures)
> 		return 0;
> 
> It gets updated in fpstate_realloc() after enabling some XFD features:
> 
> 	guest_fpu->xfeatures |= xfeatures;
> 
> So, backport is not needed.

I don't have any great suggestions for improving this, but I just don't
seem to find this changelog compelling. I can't put my finger on it, though.

I think I'd find it more convincing if you argued what the *CORRECT*
value is and why rather than just arguing for consistency with a random
value. I also don't get the pivot over the XFD for explaining why it is
harmless. XFD isn't even used in most cases, so I'd find a justification
separate from XFD more compelling.