linux-kernel - Re: [PATCH v5 3/7] x86/fpu/xstate: Differentiate default features for host and guest FPUs

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Message-ID: <87f97718-b105-4385-bfd5-86d7b2e0174b@zytor.com>
Date: Sun, 27 Apr 2025 23:12:35 -0700
From: Xin Li <xin@...or.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        "Gao, Chao" <chao.gao@...el.com>
Cc: "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "ebiggers@...gle.com" <ebiggers@...gle.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "Spassov, Stanislav" <stanspas@...zon.de>,
        "levymitchell0@...il.com" <levymitchell0@...il.com>,
        "samuel.holland@...ive.com" <samuel.holland@...ive.com>,
        "Li, Xin3" <xin3.li@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Yang, Weijiang" <weijiang.yang@...el.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
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        "mlevitsk@...hat.com" <mlevitsk@...hat.com>,
        "seanjc@...gle.com" <seanjc@...gle.com>,
        "Bae, Chang Seok" <chang.seok.bae@...el.com>,
        "vigbalas@....com" <vigbalas@....com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "hpa@...or.com"
 <hpa@...or.com>, "bp@...en8.de" <bp@...en8.de>,
        "aruna.ramakrishna@...cle.com" <aruna.ramakrishna@...cle.com>,
        "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH v5 3/7] x86/fpu/xstate: Differentiate default features for
 host and guest FPUs

On 4/27/2025 10:51 PM, Xin Li wrote:
> On 4/25/2025 9:09 AM, Edgecombe, Rick P wrote:
>>>    And that will create a bit of a
>>>   :snafu if Linux does gain support for SSS.
>>>
>>> *:https://lore.kernel.org/kvm/ZM1jV3UPL0AMpVDI@google.com/
>> I chatted with Xin about this a few weeks ago. It sounds like FRED 
>> bare metal
>> SSS will not need CET_S state, but it wasn't 100% clear.
> 
> FRED reuses one CET_S MSR IA32_PL0_SSP, and give it an alias
> IA32_FRED_SSP0.

Native use of IA32_FRED_SSP0 is very much like IA32_FRED_RSP0:

1) Both are per-task constants.

2) Both are only used for delivering events when running userspace.


IA32_FRED_RSP0 is set on return to userspace:

https://web.git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=fe85ee391966c4cf3bfe1c405314e894c951f521


I suppose we'll likely apply the same approach to IA32_FRED_SSP0 if we
plan to enable SSS for the kernel.  This won't add any extra maintenance
cost, as both x86 and KVM maintainers are well aware.

Thanks!
     Xin