linux-kernel - Re: [PATCH v6 01/25] x86/fpu/xstate: Manually check and add XFEATURE_CET

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Message-ID: <7d783993-1ef5-e1ed-591c-c87a9d879782@intel.com>
Date:   Mon, 18 Sep 2023 15:16:48 +0800
From:   "Yang, Weijiang" <weijiang.yang@...el.com>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     "peterz@...radead.org" <peterz@...radead.org>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "Gao, Chao" <chao.gao@...el.com>,
        "john.allen@....com" <john.allen@....com>
Subject: Re: [PATCH v6 01/25] x86/fpu/xstate: Manually check and add
 XFEATURE_CET_USER xstate bit

On 9/16/2023 12:35 AM, Edgecombe, Rick P wrote:
> On Fri, 2023-09-15 at 10:32 +0800, Yang, Weijiang wrote:
>>> Also, this doesn't discuss the real main reason for this patch, and
>>> that is that KVM will soon use the xfeature for user ibt, and so
>>> there
>>> will now be a reason to have XFEATURE_CET_USER depend on IBT.
>> This is one justification for Linux OS, another reason is there's
>> non-Linux
>> OS which is using the user IBT feature.  I should make the reasons
>> clearer
>> in changelog, thanks for pointing it out!
> The point I was trying to make was today (before this series) nothing
> on the system can use user IBT. Not the host, and not in any guest
> because KVM doesn't support it. So the added xfeature dependency on IBT
> was not previously needed. It is being added only for KVM CET support
> (which, yes, may run on guests with non-standard CPUID).

Agree, I'll highlight this in changelog, thanks!