linux-kernel - Re: [PATCH 2/3] x86/boot: Allow to hook up alternative port I/O helpers

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Date:   Thu, 20 Jan 2022 19:38:26 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     bp@...en8.de
Cc:     aarcange@...hat.com, ak@...ux.intel.com, dan.j.williams@...el.com,
        dave.hansen@...el.com, david@...hat.com, hpa@...or.com,
        jgross@...e.com, jmattson@...gle.com, joro@...tes.org,
        jpoimboe@...hat.com, kirill.shutemov@...ux.intel.com,
        knsathya@...nel.org, linux-kernel@...r.kernel.org, luto@...nel.org,
        mingo@...hat.com, pbonzini@...hat.com, peterz@...radead.org,
        sathyanarayanan.kuppuswamy@...ux.intel.com, sdeep@...are.com,
        seanjc@...gle.com, tglx@...utronix.de, tony.luck@...el.com,
        vkuznets@...hat.com, wanpengli@...cent.com, x86@...nel.org
Subject: Re: [PATCH 2/3] x86/boot: Allow to hook up alternative port I/O
 helpers

On Thu, Jan 20, 2022 at 05:15:43AM +0300, Kirill A. Shutemov wrote:
> diff --git a/arch/x86/boot/io.h b/arch/x86/boot/io.h
> new file mode 100644
> index 000000000000..640daa3925fb
> --- /dev/null
> +++ b/arch/x86/boot/io.h
> @@ -0,0 +1,30 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef BOOT_IO_H
> +#define BOOT_IO_H
> +
> +#include <asm/shared/io.h>
> +
> +struct port_io_ops {
> +	unsigned char (*inb)(int port);
> +	unsigned short (*inw)(int port);
> +	unsigned int (*inl)(int port);
> +	void (*outb)(unsigned char v, int port);
> +	void (*outw)(unsigned short v, int port);
> +	void (*outl)(unsigned int v, int port);
> +};
> +
> +extern struct port_io_ops pio_ops;
> +
> +static inline void init_io_ops(void)
> +{
> +	pio_ops = (struct port_io_ops){
> +		.inb = inb,
> +		.inw = inw,
> +		.inl = inl,
> +		.outb = outb,
> +		.outw = outw,
> +		.outl = outl,
> +	};
> +}
> +
> +#endif

It works fine on x86-64, but breaks on i386:

ld: Unexpected run-time relocations (.rel) detected!

I'll change it to

	pio_ops.inb = inb;
	pio_ops.inw = inw;
	pio_ops.inl = inl;
	pio_ops.outb = outb;
	pio_ops.outw = outw;
	pio_ops.outl = outl;

It works, but I hate that I don't really have control here. I have no clue
why compiler generate different code after the change. It is very fragile.

Do we really have no way to say compiler to avoid relactions here?

-- 
 Kirill A. Shutemov