linux-kernel - Re: [PATCH 3/3] x86/hyperv: Remove ASM_CALL

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Message-ID: <20251120163121.GB3330456@liuwe-devbox-debian-v2.local>
Date: Thu, 20 Nov 2025 16:31:21 +0000
From: Wei Liu <wei.liu@...nel.org>
To: Uros Bizjak <ubizjak@...il.com>
Cc: linux-hyperv@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org,
	"K. Y. Srinivasan" <kys@...rosoft.com>,
	Haiyang Zhang <haiyangz@...rosoft.com>,
	Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 3/3] x86/hyperv: Remove ASM_CALL_CONSTRAINT with VMMCALL
 insn

On Thu, Nov 20, 2025 at 02:40:24PM +0100, Uros Bizjak wrote:
> Unlike CALL instruction, VMMCALL does not push to the stack, and may be
> inserted before the frame pointer gets set up by the containing function.
> 
> Signed-off-by: Uros Bizjak <ubizjak@...il.com>
> Cc: "K. Y. Srinivasan" <kys@...rosoft.com>
> Cc: Haiyang Zhang <haiyangz@...rosoft.com>
> Cc: Wei Liu <wei.liu@...nel.org>
> Cc: Dexuan Cui <decui@...rosoft.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: "H. Peter Anvin" <hpa@...or.com>

Dexuan and Tianyu are more qualified than I do to review CVM changes.
I will defer this to them.

Wei

> ---
>  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");
>  
> -- 
> 2.51.1
>