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Date: Mon, 14 Apr 2025 13:47:50 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: x86@kernel.org
Cc: kys@microsoft.com, haiyangz@microsoft.com, wei.liu@kernel.org,
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Subject: Re: [PATCH 4/6] x86,hyperv: Clean up hv_do_hypercall()
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 <20250414113754.285564821@infradead.org>
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In-Reply-To: <20250414113754.285564821@infradead.org>

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.