Re: [PATCH v2 2/2] KVM: VMX: guard regparm(0) on vmread_error_trampoline for x86_32 only

["Hanne-Lotta Mäenpää" <hannelotta@gmail.com>]

On 6/17/26 4:13 AM, Clark Williams wrote:
> [ Upstream commit 0b5e7a16a0a79a3742f0df9e45bca46f01b40e6a ]
> 
> regparm(0) overrides the kernel's -mregparm=3 convention on x86-32 so
> that vmread_error_trampoline receives its arguments on the stack, matching
> the inline asm callers that push args before the call.  On x86-64 the
> attribute is a no-op and newer GCC now emits -Wattributes for it, which
> becomes a build error under -Werror.  Guard it with CONFIG_X86_32.
> 
> [ clrkwllms: the upstream commit redesigns the trampoline declaration as
>    an opaque symbol; this simpler approach guards the regparm(0) attribute
>    with CONFIG_X86_32 since the attribute is only meaningful on x86-32. ]
> 
> Assisted-by: Claude:claude-sonnet-4.6
> Signed-off-by: Clark Williams <clrkwllms@kernel.org>
> ---
>   arch/x86/kvm/vmx/vmx_ops.h | 7 +++++--
>   1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx_ops.h b/arch/x86/kvm/vmx/vmx_ops.h
> index 5edab28dfb2e..50328be40b2b 100644
> --- a/arch/x86/kvm/vmx/vmx_ops.h
> +++ b/arch/x86/kvm/vmx/vmx_ops.h
> @@ -11,8 +11,11 @@
>   #include "../x86.h"
>   
>   void vmread_error(unsigned long field, bool fault);
> -__attribute__((regparm(0))) void vmread_error_trampoline(unsigned long field,
> -							 bool fault);
> +/* regparm(0) overrides -mregparm=3 so args are stack-passed, matching asm callers */
> +#ifdef CONFIG_X86_32
> +__attribute__((regparm(0)))
> +#endif
> +void vmread_error_trampoline(unsigned long field, bool fault);
>   void vmwrite_error(unsigned long field, unsigned long value);
>   void vmclear_error(struct vmcs *vmcs, u64 phys_addr);
>   void vmptrld_error(struct vmcs *vmcs, u64 phys_addr);

Hi,

I've sent a backport patch for this commit, see here:

https://lore.kernel.org/lkml/20260617105100.22094-1-hannelotta@gmail.com/

The commit applies cleanly without any modifications, and compiles 
without errors with gcc-16, and boots.

For commits that apply cleanly, you can simply use:

     git cherry-pick -s -x <commit-id>

And after that

     git commit --amend

to add the required [ Upstream commit <commit-id> ] line.

I wasn't able to see other compilation errors for v6.1.175 using gcc-16. 
I tried with allyesconfig and my local config.

Hope this helps.

Best regards,

Hanne-Lotta Mäenpää