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

[H. Peter Anvin]

On November 21, 2025 6:14:11 AM PST, Uros Bizjak <ubizjak@xxxxxxxxx> 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@xxxxxxxxx>
>Tested-by: Michael Kelley <mhklinux@xxxxxxxxxxx>
>Cc: "K. Y. Srinivasan" <kys@xxxxxxxxxxxxx>
>Cc: Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>
>Cc: Wei Liu <wei.liu@xxxxxxxxxx>
>Cc: Dexuan Cui <decui@xxxxxxxxxxxxx>
>Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>Cc: Ingo Molnar <mingo@xxxxxxxxxx>
>Cc: Borislav Petkov <bp@xxxxxxxxx>
>Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
>Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
>---
>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");
> 

I think it would be good to have a comment at the point where ASM_CALL_CONSTRAINT is defined explaining its proper use.

Specifically, instructions like syscall, vmcall, vmfunc, vmmcall, int xx and VM-specific escape instructions are not "calls" because they either don't modify the stack or create an exception frame (kernel) or signal frame (user space) which is completely special.