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

[Andrew Cooper <andrew.cooper3@citrix.com>]

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