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@xxxxxxxxxx/
