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Wed, 03 Jul 2024 09:00:57 -0700 (PDT) Received: from [10.125.226.166] ([185.25.67.249]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-6b59e6146bcsm54777846d6.120.2024.07.03.09.00.55 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 03 Jul 2024 09:00:56 -0700 (PDT) Message-ID: <15f56e6a-6edd-43d0-8e83-bb6430096514@citrix.com> Date: Wed, 3 Jul 2024 17:00:53 +0100 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 v1 2/4] x86/fred: Write to FRED MSRs with wrmsrns() To: Borislav Petkov , dave.hansen@intel.com Cc: xin@zytor.com, linux-kernel@vger.kernel.org, hpa@zytor.com, tglx@linutronix.de, mingo@redhat.com, dave.hansen@linux.intel.com, x86@kernel.org, peterz@infradead.org, nik.borisov@suse.com, houwenlong.hwl@antgroup.com References: <9063b0fe-e8f3-44ff-b323-b2b6c338690f@intel.com> <172002205406.3280081.14523962650685954182@Ubuntu-2204-jammy-amd64-base> Content-Language: en-GB From: Andrew Cooper Autocrypt: addr=andrew.cooper3@citrix.com; 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charset=UTF-8 Content-Transfer-Encoding: 8bit On 03/07/2024 4:54 pm, Borislav Petkov wrote: > Dave Hansen wrote: >> On 7/3/24 01:54, Xin Li (Intel) wrote: >> > Do FRED MSR writes with wrmsrns() rather than wrmsrl(). >> >> A longer changelog would be appreciated here. The wrmsrns() is >> presumably to avoid the WRMSR serialization overhead and the CR4 write >> provides all of the serialization that we need. > Also, all those wrmsrns() writes better be behind a CPUID check. They're not, in Linux. For the $N'th time, here is the primitive that Linux wants to stea^w borrow for this to be sane. /* Non-serialising WRMSR, when available.  Falls back to a serialising WRMSR. */ static inline void wrmsrns(uint32_t msr, uint32_t lo, uint32_t hi) {     /*      * WRMSR is 2 bytes.  WRMSRNS is 3 bytes.  Pad WRMSR with a redundant CS      * prefix to avoid a trailing NOP.      */     alternative_input(".byte 0x2e; wrmsr",                       ".byte 0x0f,0x01,0xc6", X86_FEATURE_WRMSRNS,                       "c" (msr), "a" (lo), "d" (hi)); } ~Andrew