linux-kernel - Re: [PATCH v5 3/9] x86/bugs: Decouple ALTERNATIVE usage from VERW macro definition

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Message-ID: <aRtAOM040vM9RGfK@google.com>
Date: Mon, 17 Nov 2025 07:33:12 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Borislav Petkov <bp@...en8.de>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, 
	Peter Zijlstra <peterz@...radead.org>, Josh Poimboeuf <jpoimboe@...nel.org>, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, 
	Brendan Jackman <jackmanb@...gle.com>
Subject: Re: [PATCH v5 3/9] x86/bugs: Decouple ALTERNATIVE usage from VERW
 macro definition

On Mon, Nov 17, 2025, Borislav Petkov wrote:
> On Thu, Nov 13, 2025 at 03:37:40PM -0800, Sean Christopherson wrote:
> > +#define __CLEAR_CPU_BUFFERS	__stringify(VERW)
> 
> Let's get rid of one indirection level pls:
> 
> diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
> index 8b4885a1b2ef..59945cb5e5f9 100644
> --- a/arch/x86/include/asm/nospec-branch.h
> +++ b/arch/x86/include/asm/nospec-branch.h
> @@ -309,23 +309,21 @@
>   * Note: Only the memory operand variant of VERW clears the CPU buffers.
>   */
>  #ifdef CONFIG_X86_64
> -#define VERW	verw x86_verw_sel(%rip)
> +#define VERW	__stringify(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).
>   */
> -#define VERW	verw %cs:x86_verw_sel
> +#define VERW	__stringify(verw %cs:x86_verw_sel)
>  #endif

Brendan also brought this up in v4[*].  Unless there's a way to coerce ALTERNATIVE_2
into working with multiple strings, the layer of indirection is needed so that KVM
can emit __stringify() for the entire sequence.

  : 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.

[*] https://lore.kernel.org/all/aQT1JgdgiNae3Ybl@google.com