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 10:32:48 +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 9/15/2023 6:39 AM, Edgecombe, Rick P wrote:
> On Thu, 2023-09-14 at 02:33 -0400, Yang Weijiang wrote:
>> Remove XFEATURE_CET_USER entry from dependency array as the entry
>> doesn't
>> reflect true dependency between CET features and the xstate bit,
>> instead
>> manually check and add the bit back if either SHSTK or IBT is
>> supported.
>>
>> Both user mode shadow stack and indirect branch tracking features
>> depend
>> on XFEATURE_CET_USER bit in XSS to automatically save/restore user
>> mode
>> xstate registers, i.e., IA32_U_CET and IA32_PL3_SSP whenever
>> necessary.
>>
>> Although in real world a platform with IBT but no SHSTK is rare, but
>> in
>> virtualization world it's common, guest SHSTK and IBT can be
>> controlled
>> independently via userspace app.
> Nit, not sure we can assert it's common yet. It's true in general that
> guests can have CPUID combinations that don't appear in real world of
> course. Is that what you meant?

Yes, guest CPUID features can be configured by userspace flexibly.

>
> 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!

>> Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
> Otherwise:
>
> Reviewed-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
> Tested-by: Rick Edgecombe <rick.p.edgecombe@...el.com>