Messages in this thread | | | Date | Sat, 21 Oct 2023 00:55:45 +0100 | Subject | Re: [RESEND][PATCH 1/6] x86/bugs: Add asm helpers for executing VERW | From | Andrew Cooper <> |
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On 20/10/2023 9:44 pm, Pawan Gupta wrote: > +#define EXEC_VERW \ > + __EXEC_VERW(551f); \ > + /* nopl __KERNEL_DS(%rax) */ \ > + .byte 0x0f, 0x1f, 0x80, 0x00, 0x00; \ > +551: .word __KERNEL_DS; \
Is this actually wise from a perf point of view?
You're causing a data access to the instruction stream, and not only that, the immediate next instruction. Some parts don't take kindly to snoops hitting L1I.
A better option would be to simply have
.section .text.entry .align CACHELINE mds_verw_sel: .word __KERNEL_DS int3 .align CACHELINE
And then just have EXEC_VERW be
verw mds_verw_sel(%rip)
in the fastpaths. That keeps the memory operand in .text.entry it works on Meltdown-vulnerable CPUs, but creates effectively a data cacheline that isn't mixed into anywhere in the frontend, which also gets far better locality of reference rather than having it duplicated in 9 different places.
Also it avoids playing games with hiding data inside an instruction. It's a neat trick, but the neater trick is avoid it whenever possible.
~Andrew
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