linux-kernel - Re: [RFC/RFT PATCH 10/19] x86/kvm: Use RIP-relative addressing

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Message-ID: <20260120194358.1bd909fa@pumpkin>
Date: Tue, 20 Jan 2026 19:43:58 +0000
From: David Laight <david.laight.linux@...il.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Ard Biesheuvel <ardb@...nel.org>, linux-kernel@...r.kernel.org,
 x86@...nel.org, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar
 <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, Dave Hansen
 <dave.hansen@...ux.intel.com>, "H. Peter Anvin" <hpa@...or.com>, Josh
 Poimboeuf <jpoimboe@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
 Kees Cook <kees@...nel.org>, Uros Bizjak <ubizjak@...il.com>, Brian Gerst
 <brgerst@...il.com>, linux-hardening@...r.kernel.org
Subject: Re: [RFC/RFT PATCH 10/19] x86/kvm: Use RIP-relative addressing

On Tue, 20 Jan 2026 09:04:26 -0800
Sean Christopherson <seanjc@...gle.com> wrote:

> On Thu, Jan 08, 2026, Ard Biesheuvel wrote:
> > Replace absolute references in inline asm with RIP-relative ones, to
> > avoid the need for relocation fixups at boot time. This is a
> > prerequisite for PIE linking, which only permits 64-bit wide
> > loader-visible absolute references.
> > 
> > Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
> > ---
> >  arch/x86/kernel/kvm.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> > index df78ddee0abb..1a0335f328e1 100644
> > --- a/arch/x86/kernel/kvm.c
> > +++ b/arch/x86/kernel/kvm.c
> > @@ -807,8 +807,9 @@ extern bool __raw_callee_save___kvm_vcpu_is_preempted(long);
> >   * restoring to/from the stack.
> >   */
> >  #define PV_VCPU_PREEMPTED_ASM						     \
> > - "movq   __per_cpu_offset(,%rdi,8), %rax\n\t"				     \
> > - "cmpb   $0, " __stringify(KVM_STEAL_TIME_preempted) "+steal_time(%rax)\n\t" \
> > + "0:leaq 0b(%rip), %rax\n\t"						     \  
> 
> Please use something other than '0' for the label, it took me forever (and looking
> at disassembly) to realize "0b" was just a backwards label and not some fancy
> syntax I didn't know.

I remember taking a while to grok that as well.

Can't you just use . as in:
	leaq	.(%rip), %rax

shame the assembler doesn't understand:
	movq	%rip, %rax
(maybe it does...)

	David