Messages in this thread | | | Date | Fri, 18 Mar 2022 19:02:25 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH v3 6/6] KVM: x86: allow defining return-0 static calls |
| |
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()); }
| |