Re: [PATCH RESEND v2 3/3] x86/hyperv: Remove ASM_CALL_CONSTRAINT with VMMCALL insn

["H. Peter Anvin" <hpa@zytor.com>]

On 2026-01-05 01:02, Uros Bizjak wrote:
> Unlike CALL instruction, VMMCALL does not push to the stack, so it's
> OK to allow the compiler to insert it before the frame pointer gets
> set up by the containing function. ASM_CALL_CONSTRAINT is for CALLs
> that must be inserted after the frame pointer is set up, so it is
> over-constraining here and can be removed.
> 
> Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
> Tested-by: Michael Kelley <mhklinux@outlook.com>
> Cc: "K. Y. Srinivasan" <kys@microsoft.com>
> Cc: Haiyang Zhang <haiyangz@microsoft.com>
> Cc: Wei Liu <wei.liu@kernel.org>
> Cc: Dexuan Cui <decui@microsoft.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> ---
> v2: Expand commit message and include ASM_CALL_CONSTRAINT explanation
> ---
>  arch/x86/hyperv/ivm.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/hyperv/ivm.c b/arch/x86/hyperv/ivm.c
> index 7365d8f43181..be7fad43a88d 100644
> --- a/arch/x86/hyperv/ivm.c
> +++ b/arch/x86/hyperv/ivm.c
> @@ -392,7 +392,7 @@ u64 hv_snp_hypercall(u64 control, u64 param1, u64 param2)
>  
>  	register u64 __r8 asm("r8") = param2;
>  	asm volatile("vmmcall"
> -		     : "=a" (hv_status), ASM_CALL_CONSTRAINT,
> +		     : "=a" (hv_status),
>  		       "+c" (control), "+d" (param1), "+r" (__r8)
>  		     : : "cc", "memory", "r9", "r10", "r11");
>  

Looks good to me; I'm not familiar directly with HyperV, but vmmcall is a
system call-type instruction which doesn't use the stack.

Acked-by: H. Peter Anvin (Intel) <hpa@zytor.com>