The current RFDS mitigation logic relies on verw_clears_cpu_reg_file(),
which directly checks the RFDS_CLEAR bit in the IA32_ARCH_CAPABILITIES MSR.

To support quirks for CPUs that mitigate RFDS but do not enumerate it,
transition to using the synthetic feature flag X86_FEATURE_RFDS_CLEAR.

Replace the MSR-bit-based check with boot_cpu_has(X86_FEATURE_RFDS_CLEAR)
and synthesize this feature bit in rfds_select_mitigation() if the
architectural RFDS_CLEAR bit is present.

Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lore.kernel.org/all/20260129154342.3867-1-moontorise@cfg.kr/
Signed-off-by: Joongsun Moon-Lee <moontorise@cfg.kr>
---
 arch/x86/include/asm/cpufeatures.h |  1 +
 arch/x86/kernel/cpu/bugs.c         | 12 +++++-------
 2 files changed, 6 insertions(+), 7 deletions(-)

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index 63b0f9aa9b3e..3480d9ddc046 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -513,6 +513,7 @@
 						      * and purposes if CLEAR_CPU_BUF_VM is set).
 						      */
 #define X86_FEATURE_X2AVIC_EXT		(21*32+20) /* AMD SVM x2AVIC support for 4k vCPUs */
+#define X86_FEATURE_RFDS_CLEAR		(21*32+21) /* Clear register file via VERW */
 
 /*
  * BUG word(s)
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 83f51cab0b1e..479702bc9c00 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -648,11 +648,6 @@ static const char * const rfds_strings[] = {
 	[RFDS_MITIGATION_UCODE_NEEDED]		= "Vulnerable: No microcode",
 };
 
-static inline bool __init verw_clears_cpu_reg_file(void)
-{
-	return (x86_arch_cap_msr & ARCH_CAP_RFDS_CLEAR);
-}
-
 static void __init rfds_select_mitigation(void)
 {
 	if (!boot_cpu_has_bug(X86_BUG_RFDS)) {
@@ -670,7 +665,10 @@ static void __init rfds_select_mitigation(void)
 	if (rfds_mitigation == RFDS_MITIGATION_OFF)
 		return;
 
-	if (verw_clears_cpu_reg_file())
+	if (x86_arch_cap_msr & ARCH_CAP_RFDS_CLEAR)
+		setup_force_cpu_cap(X86_FEATURE_RFDS_CLEAR);
+
+	if (boot_cpu_has(X86_FEATURE_RFDS_CLEAR))
 		verw_clear_cpu_buf_mitigation_selected = true;
 }
 
@@ -683,7 +681,7 @@ static void __init rfds_update_mitigation(void)
 		rfds_mitigation = RFDS_MITIGATION_VERW;
 
 	if (rfds_mitigation == RFDS_MITIGATION_VERW) {
-		if (!verw_clears_cpu_reg_file())
+		if (!boot_cpu_has(X86_FEATURE_RFDS_CLEAR))
 			rfds_mitigation = RFDS_MITIGATION_UCODE_NEEDED;
 	}
 
-- 
2.52.0

On Fri, Jan 30, 2026 at 09:33:39PM +0900, Joongsun Moon-Lee wrote:
> The current RFDS mitigation logic relies on verw_clears_cpu_reg_file(),
> which directly checks the RFDS_CLEAR bit in the IA32_ARCH_CAPABILITIES MSR.
> 
> To support quirks for CPUs that mitigate RFDS but do not enumerate it,
> transition to using the synthetic feature flag X86_FEATURE_RFDS_CLEAR.
> 
> Replace the MSR-bit-based check with boot_cpu_has(X86_FEATURE_RFDS_CLEAR)
> and synthesize this feature bit in rfds_select_mitigation() if the
> architectural RFDS_CLEAR bit is present.

Please do not explain the diff - talk about why this patch exists and other
details which are not obvious from the code.

> @@ -670,7 +665,10 @@ static void __init rfds_select_mitigation(void)
>  	if (rfds_mitigation == RFDS_MITIGATION_OFF)
>  		return;
>  
> -	if (verw_clears_cpu_reg_file())
> +	if (x86_arch_cap_msr & ARCH_CAP_RFDS_CLEAR)
> +		setup_force_cpu_cap(X86_FEATURE_RFDS_CLEAR);
> +
> +	if (boot_cpu_has(X86_FEATURE_RFDS_CLEAR))
>  		verw_clear_cpu_buf_mitigation_selected = true;
>  }

So:
	if (x86_arch_cap_msr & ARCH_CAP_RFDS_CLEAR) {
		setup_force_cpu_cap(X86_FEATURE_RFDS_CLEAR);
		verw_clear_cpu_buf_mitigation_selected = true;
	}

?

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette