Re: [PATCH v3 6/6] KVM: x86: allow defining return-0 static calls

Peter Zijlstra
2022-03-18T18:02:34Z

On Fri, Mar 18, 2022 at 06:47:32PM +0100, Peter Zijlstra wrote:
> On Fri, Mar 18, 2022 at 06:28:37PM +0100, Peter Zijlstra wrote:
> > > Related to this, I don't see anything in arch/x86/kernel/static_call.c that
> > > limits this code to x86-64:
> > >
> > > if (func == &__static_call_return0) {
> > > emulate = code;
> > > code = &xor5rax;
> > > }
> > >
> > >
> > > On 32-bit, it will be patched as "dec ax; xor eax, eax" or something like
> > > that. Fortunately it doesn't corrupt any callee-save register but it is not
> > > just a bit funky, it's also not a single instruction.
> >
> > Urggghh.. that's fairly yuck. So there's two options I suppose:
> >
> > 0x66, 0x66, 0x66, 0x31, 0xc0
>
> Argh, that turns into: xorw %ax, %ax.
>
> Let me see if there's another option.

Amazingly:

0x2e, 0x2e, 0x2e, 0x31, 0xc0

seems to actually work.. I've build and ran and decoded the below on
32bit and 64bit (arguably on the same 64bit host).

---
#include <stdio.h>

long zero(void)
{
long z = -1L;

asm (".byte 0x2e, 0x2e, 0x2e, 0x31, 0xc0" : "=a" (z) );

return z;
}

void main(void)
{
printf("%ld\n", zero());
}