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

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Message-ID: <20260120220016.241766b2@pumpkin>
Date: Tue, 20 Jan 2026 22:00:16 +0000
From: David Laight <david.laight.linux@...il.com>
To: Ard Biesheuvel <ardb@...nel.org>
Cc: Sean Christopherson <seanjc@...gle.com>, 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 21:54:30 +0100
Ard Biesheuvel <ardb@...nel.org> wrote:

> On Tue, 20 Jan 2026 at 20:44, David Laight <david.laight.linux@...il.com> wrote:
> >
> > 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
> >  
> 
> How would the other two instructions referring to '0b' in their
> immediates refer to '.' in that case?

I'd forgotten about those, not in the quoted bit of patch :-(

Traditionally (going back to MACRO-11) numeric labels would start at 10
and go up in 10s, TECO had a nice macro to renumber them.
(Showing my age again)

	David