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

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Date:   Fri, 15 Sep 2023 16:35:04 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Yang, Weijiang" <weijiang.yang@...el.com>,
        "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 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).