Re: [PATCH v4 2/8] x86/bugs: Decouple ALTERNATIVE usage from VERW macro definition

[Sean Christopherson <seanjc@google.com>]

On Fri, Oct 31, 2025, Brendan Jackman wrote:
> On Fri Oct 31, 2025 at 12:30 AM UTC, Sean Christopherson wrote:
> > Decouple the use of ALTERNATIVE from the encoding of VERW to clear CPU
> > buffers so that KVM can use ALTERNATIVE_2 to handle "always clear buffers"
> > and "clear if guest can access host MMIO" in a single statement.
> >
> > No functional change intended.
> >
> > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > ---
> >  arch/x86/include/asm/nospec-branch.h | 21 ++++++++++-----------
> >  1 file changed, 10 insertions(+), 11 deletions(-)
> >
> > diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
> > index 08ed5a2e46a5..923ae21cbef1 100644
> > --- a/arch/x86/include/asm/nospec-branch.h
> > +++ b/arch/x86/include/asm/nospec-branch.h
> > @@ -308,24 +308,23 @@
> >   * CFLAGS.ZF.
> >   * Note: Only the memory operand variant of VERW clears the CPU buffers.
> >   */
> > -.macro __CLEAR_CPU_BUFFERS feature
> >  #ifdef CONFIG_X86_64
> > -	ALTERNATIVE "", "verw x86_verw_sel(%rip)", \feature
> > +#define CLEAR_CPU_BUFFERS_SEQ	verw x86_verw_sel(%rip)
> >  #else
> > -	/*
> > -	 * In 32bit mode, the memory operand must be a %cs reference. The data
> > -	 * segments may not be usable (vm86 mode), and the stack segment may not
> > -	 * be flat (ESPFIX32).
> > -	 */
> > -	ALTERNATIVE "", "verw %cs:x86_verw_sel", \feature
> > +/*
> > + * In 32bit mode, the memory operand must be a %cs reference. The data segments
> > + * may not be usable (vm86 mode), and the stack segment may not be flat (ESPFIX32).
> > + */
> > +#define CLEAR_CPU_BUFFERS_SEQ	verw %cs:x86_verw_sel
> >  #endif
> > -.endm
> > +
> > +#define __CLEAR_CPU_BUFFERS	__stringify(CLEAR_CPU_BUFFERS_SEQ)
> 
> Maybe CLEAR_CPU_BUFFERS_SEQ should just be defined as a string in the
> first place?

Heh, I tried that, and AFAICT it simply can't work with the way ALTERNATIVE and
friends are implemented, as each paramater needs to be a single unbroken string.

E.g. this 

diff --git a/arch/x86/kvm/vmx/vmenter.S b/arch/x86/kvm/vmx/vmenter.S
index 61a809790a58..ffa6bc2345e3 100644
--- a/arch/x86/kvm/vmx/vmenter.S
+++ b/arch/x86/kvm/vmx/vmenter.S
@@ -63,6 +63,8 @@
        RET
 .endm
 
+#define CLEAR_CPU_BUFFERS_SEQ_STRING  "verw x86_verw_sel(%rip)"
+
 .section .noinstr.text, "ax"
 
 /**
@@ -169,9 +171,9 @@ SYM_FUNC_START(__vmx_vcpu_run)
 
        /* Clobbers EFLAGS.ZF */
        ALTERNATIVE_2 "",                                                       \
-                     __stringify(jz .Lskip_clear_cpu_buffers;                  \
-                                 CLEAR_CPU_BUFFERS_SEQ;                        \
-                                 .Lskip_clear_cpu_buffers:),                   \
+                     "jz .Lskip_clear_cpu_buffers; "                           \
+                     CLEAR_CPU_BUFFERS_SEQ_STRING;                             \
+                     ".Lskip_clear_cpu_buffers:",                              \
                      X86_FEATURE_CLEAR_CPU_BUF_MMIO,                           \
                      __CLEAR_CPU_BUFFERS, X86_FEATURE_CLEAR_CPU_BUF_VM
 
yields wonderfully helpful error messages like so:

  arch/x86/kvm/vmx/vmenter.S: Assembler messages:
  arch/x86/kvm/vmx/vmenter.S:173: Error: too many positional arguments

If there's a magic incanation to get things to work, it's unknown to me.