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Date: Tue, 7 May 2024 16:17:09 -0700
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Subject: Re: [PATCH v10 04/27] x86/fpu/xstate: Introduce
 XFEATURE_MASK_KERNEL_DYNAMIC xfeature set
To: Sean Christopherson <seanjc@google.com>
Cc: Yang Weijiang <weijiang.yang@intel.com>, pbonzini@redhat.com,
 x86@kernel.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
 peterz@infradead.org, chao.gao@intel.com, rick.p.edgecombe@intel.com,
 mlevitsk@redhat.com, john.allen@amd.com
References: <20240219074733.122080-1-weijiang.yang@intel.com>
 <20240219074733.122080-5-weijiang.yang@intel.com>
 <ZjKNxt1Sq71DI0K8@google.com>
 <893ac578-baaf-4f4f-96ee-e012dfc073a8@intel.com>
 <Zjqx8-ZPyB--6Eys@google.com>
From: Dave Hansen <dave.hansen@intel.com>
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In-Reply-To: <Zjqx8-ZPyB--6Eys@google.com>
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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.