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

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Date:   Fri, 18 Mar 2022 18:47:32 +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 06:28:37PM +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.

> Which is a tripple prefix xor %eax, %eax, which, IIRC should still clear
> the whole 64bit on 64bit and *should* still not trigger the prefix
> decoding penalty some frontends have (which is >3 IIRC).
> 
> Or we can emit:
> 
> 	0xb8, 0x00, 0x00, 0x00, 0x00
> 
> which decodes to: mov $0x0,%eax, which is less efficient in some
> front-ends since it doesn't always get picked up in register rename
> stage.
> 
>