linux-kernel - Re: [PATCH V2 03/14] x86/set_memory: Add x86_set_memory

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Message-ID: <YQv0bRBUq1N5+jgG@hirez.programming.kicks-ass.net>
Date:   Thu, 5 Aug 2021 16:23:41 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Tianyu Lan <ltykernel@...il.com>
Cc:     Dave Hansen <dave.hansen@...el.com>, kys@...rosoft.com,
        haiyangz@...rosoft.com, sthemmin@...rosoft.com, wei.liu@...nel.org,
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Subject: Re: [PATCH V2 03/14] x86/set_memory: Add x86_set_memory_enc static
 call support

On Thu, Aug 05, 2021 at 10:05:17PM +0800, Tianyu Lan wrote:
>  static int __set_memory_enc_dec(unsigned long addr, int numpages, bool enc)
>  {
> +	return static_call(x86_set_memory_enc)(addr, numpages, enc);
>  }

Hurpmh... So with a bit of 'luck' you get code-gen like:

__set_memory_enc_dec:
	jmp __SCT_x86_set_memory_enc;

set_memory_encrypted:
	mov $1, %rdx
	jmp __set_memory_enc_dec

set_memory_decrypted:
	mov $0, %rdx
	jmp __set_memory_enc_dec


Which, to me, seems exceedingly daft. Best to make all 3 of those
inlines and use EXPORT_STATIC_CALL_TRAMP_GPL(x86_set_memory_enc) or
something.

This is assuming any of this is actually performance critical, based off
of this using static_call() to begin with.