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.
Reviewed-by: Brendan Jackman <jackmanb@google.com>
Reviewed-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/include/asm/nospec-branch.h | 22 +++++++++++-----------
1 file changed, 11 insertions(+), 11 deletions(-)
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 08ed5a2e46a5..8b4885a1b2ef 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -308,24 +308,24 @@
* 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 VERW 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 VERW verw %cs:x86_verw_sel
#endif
-.endm
+#define __CLEAR_CPU_BUFFERS __stringify(VERW)
+
+/* If necessary, emit VERW on exit-to-userspace to clear CPU buffers. */
#define CLEAR_CPU_BUFFERS \
- __CLEAR_CPU_BUFFERS X86_FEATURE_CLEAR_CPU_BUF
+ ALTERNATIVE "", __CLEAR_CPU_BUFFERS, X86_FEATURE_CLEAR_CPU_BUF
#define VM_CLEAR_CPU_BUFFERS \
- __CLEAR_CPU_BUFFERS X86_FEATURE_CLEAR_CPU_BUF_VM
+ ALTERNATIVE "", __CLEAR_CPU_BUFFERS, X86_FEATURE_CLEAR_CPU_BUF_VM
#ifdef CONFIG_X86_64
.macro CLEAR_BRANCH_HISTORY
--
2.52.0.rc1.455.g30608eb744-googOn 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
-#define __CLEAR_CPU_BUFFERS __stringify(VERW)
-
/* If necessary, emit VERW on exit-to-userspace to clear CPU buffers. */
#define CLEAR_CPU_BUFFERS \
- ALTERNATIVE "", __CLEAR_CPU_BUFFERS, X86_FEATURE_CLEAR_CPU_BUF
+ ALTERNATIVE "", VERW, X86_FEATURE_CLEAR_CPU_BUF
#define VM_CLEAR_CPU_BUFFERS \
- ALTERNATIVE "", __CLEAR_CPU_BUFFERS, X86_FEATURE_CLEAR_CPU_BUF_VM
+ ALTERNATIVE "", VERW, X86_FEATURE_CLEAR_CPU_BUF_VM
#ifdef CONFIG_X86_64
.macro CLEAR_BRANCH_HISTORY
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette
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
On Mon, Nov 17, 2025 at 07:33:12AM -0800, Sean Christopherson wrote:
> 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.
Ah, yah, there was that.
Pls put a small comment above it so that we don't forget.
Thx.
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette
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