Re: [PATCH v9 4/5] x86/cpuid: fix unbootable VMs by really inlining memcmp() in hypervisor_cpuid_base()

[Borislav Petkov <bp@xxxxxxxxx>]

On Sat, Aug 22, 2026 at 03:33:20PM -0300, Mauricio Faria de Oliveira wrote:
> diff --git a/arch/x86/include/asm/cpuid/api.h 
> b/arch/x86/include/asm/cpuid/api.h
> index 
> 82eddfa2347b32b76c2ea9b85f005ca5416ac71f..2d9f3d4d63de6e721f275d9e80d372edbdfedf30
>  100644
> --- a/arch/x86/include/asm/cpuid/api.h
> +++ b/arch/x86/include/asm/cpuid/api.h
> @@ -204,7 +204,7 @@ static inline u32 cpuid_base_hypervisor(const char *sig, 
> u32 leaves)
>                * from PVH early boot code before instrumentation is set up
>                * and memcmp() itself may be instrumented.
>                */
> -             if (!__builtin_memcmp(sig, signature, 12) &&
> +             if (!__inline_memcmp(sig, signature, 12) &&

Dunno, did you not think of simply doing it by foot and save yourself all
those gymnastics in patches 1-3?

IOW, something like this totally untested thing below:

diff --git a/arch/x86/include/asm/cpuid/api.h b/arch/x86/include/asm/cpuid/api.h
index 2d9f3d4d63de..a2db83d17940 100644
--- a/arch/x86/include/asm/cpuid/api.h
+++ b/arch/x86/include/asm/cpuid/api.h
@@ -192,9 +192,10 @@ static __always_inline bool cpuid_function_is_indexed(u32 
function)
 #define for_each_possible_cpuid_base_hypervisor(function) \
        for (function = 0x40000000; function < 0x40010000; function += 0x100)
 
-static inline u32 cpuid_base_hypervisor(const char *sig, u32 leaves)
+static inline u32 cpuid_base_hypervisor(const char *__sig, u32 leaves)
 {
        u32 base, eax, signature[3];
+       u32 *sig = (u32 *)__sig;
 
        for_each_possible_cpuid_base_hypervisor(base) {
                cpuid(base, &eax, &signature[0], &signature[1], &signature[2]);
@@ -204,9 +205,12 @@ static inline u32 cpuid_base_hypervisor(const char *sig, 
u32 leaves)
                 * from PVH early boot code before instrumentation is set up
                 * and memcmp() itself may be instrumented.
                 */
-               if (!__inline_memcmp(sig, signature, 12) &&
-                   (leaves == 0 || ((eax - base) >= leaves)))
-                       return base;
+               if (sig[0] == signature[0] &&
+                   sig[1] == signature[1] &&
+                   sig[2] == signature[2]) {
+                       if (leaves == 0 || ((eax - base) >= leaves))
+                               return base;
+               }
        }
 
        return 0;


-- 
Regards/Gruss,
    Boris.

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