linux-kernel - Re: [PATCH v2 3/3] x86/hyperv: Remove ASM_CALL

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Message-ID: <90543b26-1e09-4a84-802e-aad737265b7a@zytor.com>
Date: Tue, 10 Feb 2026 14:52:36 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: Uros Bizjak <ubizjak@...il.com>
Cc: linux-hyperv@...r.kernel.org, x86@...nel.org, linux-kernel@...r.kernel.org,
        Michael Kelley <mhklinux@...look.com>,
        "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>
Subject: Re: [PATCH v2 3/3] x86/hyperv: Remove ASM_CALL_CONSTRAINT with
 VMMCALL insn

On 2025-11-22 01:33, Uros Bizjak wrote:
>>
>> 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.
> 
> The existing comment already mentions CALL instruction only:
> 
> /*
>  * This output constraint should be used for any inline asm which has a "call"
>  * instruction.  Otherwise the asm may be inserted before the frame pointer
>  * gets set up by the containing function.  If you forget to do this, objtool
>  * may print a "call without frame pointer save/setup" warning.
>  */
> 

Yes. Some people seem to have misunderstood it to mean any instruction with
"CALL" in the name.

	-hpa