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

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Date:   Fri, 18 Mar 2022 19:02:25 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Maxim Levitsky <mlevitsk@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, seanjc@...gle.com
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());
}