From: Peter Zijlstra <peterz@infradead.org>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Maxim Levitsky <mlevitsk@redhat.com>,
linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
seanjc@google.com
Subject: Re: [PATCH v3 6/6] KVM: x86: allow defining return-0 static calls
Date: Fri, 18 Mar 2022 19:02:25 +0100 [thread overview]
Message-ID: <20220318180225.GF14330@worktop.programming.kicks-ass.net> (raw)
In-Reply-To: <20220318174732.GE14330@worktop.programming.kicks-ass.net>
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());
}
next prev parent reply other threads:[~2022-03-18 18:02 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-02-17 18:08 [PATCH v3 0/6] kvm: x86: better handling of optional kvm_x86_ops Paolo Bonzini
2022-02-17 18:08 ` [PATCH v3 1/6] KVM: x86: return 1 unconditionally for availability of KVM_CAP_VAPIC Paolo Bonzini
2022-02-18 15:44 ` Sean Christopherson
2022-02-23 13:21 ` Maxim Levitsky
2022-02-17 18:08 ` [PATCH v3 2/6] KVM: x86: use static_call_cond for optional callbacks Paolo Bonzini
2022-02-18 15:46 ` Sean Christopherson
2022-02-17 18:08 ` [PATCH v3 3/6] KVM: x86: remove KVM_X86_OP_NULL and mark optional kvm_x86_ops Paolo Bonzini
2022-02-18 15:52 ` Sean Christopherson
2022-02-17 18:08 ` [PATCH v3 4/6] KVM: x86: warn on incorrectly NULL members of kvm_x86_ops Paolo Bonzini
2022-02-18 16:18 ` Sean Christopherson
2022-02-17 18:08 ` [PATCH v3 5/6] KVM: x86: make several AVIC callbacks optional Paolo Bonzini
2022-02-18 16:23 ` Sean Christopherson
2022-02-18 17:17 ` Paolo Bonzini
2022-02-18 17:37 ` Sean Christopherson
2022-02-18 17:41 ` Paolo Bonzini
2022-02-17 18:08 ` [PATCH v3 6/6] KVM: x86: allow defining return-0 static calls Paolo Bonzini
2022-02-18 16:29 ` Sean Christopherson
2022-02-18 17:20 ` Paolo Bonzini
2022-02-18 17:33 ` Sean Christopherson
2022-03-17 17:43 ` Maxim Levitsky
2022-03-17 22:30 ` David Laight
2022-03-18 16:29 ` Paolo Bonzini
2022-03-18 17:28 ` Peter Zijlstra
2022-03-18 17:47 ` Peter Zijlstra
2022-03-18 18:02 ` Peter Zijlstra [this message]
2022-03-18 23:03 ` David Laight
2022-03-20 13:58 ` Maxim Levitsky
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