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h=Date:Subject:From:To:Cc:References:In-Reply-To:From; b=GWHX7yt4WEzEtf7RsGgQoXXVHdHLYcsnAJTVzpCYMTnmHRw7TjROkpOgKc8rE+dGP Otp8sx97Mp/JMnnzCi8PvemkvC7G2xMOD+BYST/DPP544q/IKCzbZJYK4zeQwdI+iB nSjkD/WE5aR0o7VND8v/xun6uqOF6/Q0jexc7NLo1+4HiuvkaMrrNUtvzmfQ1kq6Yn kiJ17T5XXdYmOjV6d1+qo5/w3pWaRmA1fGhiLiNwY9vRnqnErl+5McULvkocnQHWHr ZTudD2tB03BtfEG9FjGfJvKVuVrmrQC0zv1qoWDiB0eXVFpk6PvkWsP98EQLzVt59E 8QtDbAIGG9oFg== Message-ID: <87f97718-b105-4385-bfd5-86d7b2e0174b@zytor.com> Date: Sun, 27 Apr 2025 23:12:35 -0700 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v5 3/7] x86/fpu/xstate: Differentiate default features for host and guest FPUs From: Xin Li To: "Edgecombe, Rick P" , "Gao, Chao" Cc: "kvm@vger.kernel.org" , "ebiggers@google.com" , "Hansen, Dave" , "dave.hansen@linux.intel.com" , "Spassov, Stanislav" , "levymitchell0@gmail.com" , "samuel.holland@sifive.com" , "Li, Xin3" , "linux-kernel@vger.kernel.org" , "Yang, Weijiang" , "mingo@redhat.com" , "pbonzini@redhat.com" , "tglx@linutronix.de" , "john.allen@amd.com" , "mlevitsk@redhat.com" , "seanjc@google.com" , "Bae, Chang Seok" , "vigbalas@amd.com" , "peterz@infradead.org" , "hpa@zytor.com" , "bp@alien8.de" , "aruna.ramakrishna@oracle.com" , "x86@kernel.org" References: <20250410072605.2358393-1-chao.gao@intel.com> <20250410072605.2358393-4-chao.gao@intel.com> <4a4b1f18d585c7799e5262453e4cfa2cf47c3175.camel@intel.com> <66b3ac69-447c-433f-a907-da20241d55ed@zytor.com> Content-Language: en-US Autocrypt: addr=xin@zytor.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit 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