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

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Message-ID: <CAMj1kXEW4yZqHAu_cm=ZVMwah-MO+ZZeBtHSO=_UcaSFkY49iQ@mail.gmail.com>
Date: Tue, 20 Jan 2026 21:54:30 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: David Laight <david.laight.linux@...il.com>
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 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?