Re: [patch 00/38] x86/retbleed: Call depth tracking mitigation

[Linus Torvalds]

On Thu, Jul 21, 2022 at 10:56 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> this seems to work, let me go hack the kernel..

Am I missing something?

Isn't this generating

        movl $~IMM,%r10d
        negl %r10d
        cmpl %r10d,-4(%calldest)

for the sequence?

That seems bogus for two reasons:

 (a) 'neg' is not the opposite of '~'. Did you mean 'notl' or did you mean '-'?

     Or am I missing something entirely?

 (b) since you have that r10 use anyway, why can't you just generate the simpler

        movl $-IMM,%r10d
        addl -4(%calldest),%r10d

     instead? You only need ZF anyway.

     Maybe you need to add some "r10 is clobbered" thing, I don't know.

But again: I don't know llvm, so the above is basically me just doing
the "pattern matching monkey" thing.

             Linus