Re: [tip: x86/urgent] x86/bugs: Use code segment selector for VERW operand

[Borislav Petkov]

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

-- 
Regards/Gruss,
    Boris.

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