Re: [RESEND][PATCH 1/6] x86/bugs: Add asm helpers for executing VERW

Andrew Cooper
2023-10-20T23:55:55Z

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