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Wed, 09 Oct 2024 03:24:22 -0700 (PDT) Received: from [10.125.226.166] ([185.25.67.249]) by smtp.gmail.com with ESMTPSA id 4fb4d7f45d1cf-5c927e4403asm404944a12.51.2024.10.09.03.24.21 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 09 Oct 2024 03:24:22 -0700 (PDT) Message-ID: Date: Wed, 9 Oct 2024 11:24:19 +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: [tip: x86/urgent] x86/bugs: Use code segment selector for VERW operand To: Borislav Petkov , Peter Zijlstra Cc: linux-kernel@vger.kernel.org, linux-tip-commits@vger.kernel.org, Robert Gill , stable@vger.kernel.org, #@tip-bot2.tec.linutronix.de, 5.10+@tip-bot2.tec.linutronix.de, Dave Hansen , Brian Gerst , Pawan Gupta , x86@kernel.org References: <172842753652.1442.15253433006014560776.tip-bot2@tip-bot2> <20241009061102.GBZwYediMceBEfSEFo@fat_crate.local> <20241009073437.GG17263@noisy.programming.kicks-ass.net> <20241009093257.GDZwZNyfIjw0lTZJqL@fat_crate.local> 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 09/10/2024 10:32 am, Borislav Petkov wrote: > On Wed, Oct 09, 2024 at 09:34:37AM +0200, Peter Zijlstra wrote: >> You need ifdeffery either way around, either directly like this or for >> that macro. This is simple and straight forward. > Nothing in this file full of macros is simple. In any case, I would've done > this as the ifdeffery is shorter and the macro is simpler. We have this coding > pattern in a lot of headers, abstracting 32-bit vs 64-bit machine details, and > it is a very common and familiar one: > > /* > * In 32bit mode, the memory operand must be a %cs reference. The data > * segments may not be usable (vm86 mode), and the stack segment may not be > * flat (ESPFIX32). > */ > #ifdef CONFIG_X86_64 > #define VERW_ARG "verw mds_verw_sel(%rip)" > #else /* CONFIG_X86_32 */ > #define VERW_ARG "verw %cs:mds_verw_sel" > #endif > > /* > * Macro to execute VERW instruction that mitigate transient data sampling > * attacks such as MDS. On affected systems a microcode update overloaded VERW > * instruction to also clear the CPU buffers. VERW clobbers CFLAGS.ZF. > * > * Note: Only the memory operand variant of VERW clears the CPU buffers. > */ > .macro CLEAR_CPU_BUFFERS > ALTERNATIVE "", VERW_ARG, X86_FEATURE_CLEAR_CPU_BUF > .endm > I'll bite.  Why do you think this form is is better? You've now got VERW_ARG leaking across the whole kernel, and a layer of obfuscatio^W indirection in CLEAR_CPU_BUFFERS. Admittedly, when I wrote this fragment as a suggestion[1], the 32bit comment was in the main comment because there really is no need for it to be separate. But abstracting away VERW_ARG like this hampers legibility/clarity, rather than improving it IMO. ~Andrew [1] https://lore.kernel.org/lkml/5703f2d8-7ca0-4f01-a954-c0eb1de930b4@citrix.com/