linux-kernel - Re: [PATCH 4/6] x86,hyperv: Clean up hv_do

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Message-ID: <20250414114750.GG5600@noisy.programming.kicks-ass.net>
Date: Mon, 14 Apr 2025 13:47:50 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: x86@...nel.org
Cc: kys@...rosoft.com, haiyangz@...rosoft.com, wei.liu@...nel.org,
	decui@...rosoft.com, tglx@...utronix.de, mingo@...hat.com,
	bp@...en8.de, dave.hansen@...ux.intel.com, hpa@...or.com,
	jpoimboe@...nel.org, pawan.kumar.gupta@...ux.intel.com,
	seanjc@...gle.com, pbonzini@...hat.com, ardb@...nel.org,
	kees@...nel.org, Arnd Bergmann <arnd@...db.de>,
	gregkh@...uxfoundation.org, linux-hyperv@...r.kernel.org,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	linux-efi@...r.kernel.org, samitolvanen@...gle.com,
	ojeda@...nel.org
Subject: Re: [PATCH 4/6] x86,hyperv: Clean up hv_do_hypercall()

On Mon, Apr 14, 2025 at 01:11:44PM +0200, Peter Zijlstra wrote:

> +u64 hv_pg_hypercall(u64 control, u64 param1, u64 param2)
> +{
> +	u64 hv_status;
> +
> +	if (!hv_hypercall_pg)
> +		return U64_MAX;
> +
> +	register u64 __r8 asm("r8") = param2;
> +	asm volatile (CALL_NOSPEC
> +		      : "=a" (hv_status), ASM_CALL_CONSTRAINT,
> +		        "+c" (control), "+d" (param1)
> +		      : "r" (__r8),
> +		        THUNK_TARGET(hv_hypercall_pg)
> +		      : "cc", "memory", "r9", "r10", "r11");
> +
> +	return hv_status;
> +}

I've tried making this a tail-call, but even without the
ASM_CALL_CONSTRAINT on it confuses the compiler (objtool warns on
frame-pointer builds about non-matching stack).

I could of course have written the entire thing in asm, but meh.