linux-kernel - Re: [PATCH v10 04/27] x86/fpu/xstate: Introduce XFEATURE_MASK_KERNEL

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Message-ID: <1159c4e1-dae6-462a-8e34-6f74be4c83b3@intel.com>
Date: Tue, 7 May 2024 16:17:09 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Yang Weijiang <weijiang.yang@...el.com>, pbonzini@...hat.com,
 x86@...nel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
 peterz@...radead.org, chao.gao@...el.com, rick.p.edgecombe@...el.com,
 mlevitsk@...hat.com, john.allen@....com
Subject: Re: [PATCH v10 04/27] x86/fpu/xstate: Introduce
 XFEATURE_MASK_KERNEL_DYNAMIC xfeature set

On 5/7/24 15:57, Sean Christopherson wrote:
>> So I still prefer calling it "KERNEL" over "GUEST".  But I also don't
>> feel strongly about it and I've said my peace.  I won't NAK it one way
>> or the other.
> I assume you mean "DYNAMIC" over "GUEST"?  I'm ok with DYNAMIC, reflecting the
> impact on each buffer makes sense.

Yes.  Silly thinko/typo on my part.

> My one request would be to change the WARN in os_xsave() to fire on CET_KERNEL,
> not KERNEL_DYNAMIC, because it's specifically CET_KERNEL that is guest-only.
> Future dynamic xfeatures could be guest-only, but they could also be dynamic for
> some completely different reason.  That was my other hang-up with "DYNAMIC";
> as-is, os_xsave() implies that it really truly is GUEST_ONLY.
> 
> diff --git a/arch/x86/kernel/fpu/xstate.h b/arch/x86/kernel/fpu/xstate.h
> index 83ebf1e1cbb4..2a1ff49ccfd5 100644
> --- a/arch/x86/kernel/fpu/xstate.h
> +++ b/arch/x86/kernel/fpu/xstate.h
> @@ -185,8 +185,7 @@ static inline void os_xsave(struct fpstate *fpstate)
>         WARN_ON_FPU(!alternatives_patched);
>         xfd_validate_state(fpstate, mask, false);
>  
> -       WARN_ON_FPU(!fpstate->is_guest &&
> -                   (mask & XFEATURE_MASK_KERNEL_DYNAMIC));
> +       WARN_ON_FPU(!fpstate->is_guest && (mask & XFEATURE_MASK_CET_KERNEL));
>  
>         XSTATE_XSAVE(&fpstate->regs.xsave, lmask, hmask, err);

Yeah, that would make a lot of sense.  We could add a more generic
#define for it later if another feature gets added like this.