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

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Date: Wed, 8 May 2024 09:19:52 +0800
From: "Yang, Weijiang" <weijiang.yang@...el.com>
To: Dave Hansen <dave.hansen@...el.com>, Sean Christopherson
	<seanjc@...gle.com>
CC: <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 5/8/2024 7:17 AM, Dave Hansen wrote:
> On 5/7/24 15:57, Sean Christopherson wrote:

[...]

>> 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.

Thank you for getting alignment! I will change the code accordingly.