linux-kernel - Re: [PATCH v3] arch/x86: port I/O tracing on x86

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Message-ID: <20231004195001.76a57417@gandalf.local.home>
Date:   Wed, 4 Oct 2023 19:50:01 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Dan Raymond <raymod2@...il.com>
Cc:     Greg KH <gregkh@...uxfoundation.org>, linux-kernel@...r.kernel.org,
        x86@...nel.org, linux-serial <linux-serial@...r.kernel.org>,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com, peterz@...radead.org,
        andriy.shevchenko@...ux.intel.com, quic_saipraka@...cinc.com
Subject: Re: [PATCH v3] arch/x86: port I/O tracing on x86

On Wed, 4 Oct 2023 16:54:20 -0600
Dan Raymond <raymod2@...il.com> wrote:

> With one exception io.h is included from boot.h or misc.h which is where
> the include guards are defined:
> 
> # find arch/x86/boot -type f -print0 | xargs -0 grep "#include.*[^a-z]io\.h"
> arch/x86/boot/boot.h:#include "io.h"
> arch/x86/boot/compressed/misc.h:#include "../io.h"
> arch/x86/boot/compressed/tdx.c:#include "../io.h"
> arch/x86/boot/io.h:#include <asm/shared/io.h>
> 
> I agree this is fragile but the problem is not confined to this patch.
> If I add a call to rdmsr() or wrmsr() in arch/x86/boot/compressed/misc.c
> I get the same compiler error.  It has something to do with the inline
> assembly inside arch/x86/include/asm/jump_label.h.

Doesn't arch/x86/boot/* code create an image that is separate from the core
vmlinux? That is, that code doesn't implement jump label logic nor sections.

> 
> I've copied Steven Rostedt who is the maintainer of tracefs to see if he
> has any comment.  I just noticed arch/x86/boot/msr.h and I see that it
> redefines rdmsr() and wrmsr() and omits the tracepoints.  A comment there
> explains:
> 
> /*
>  * The kernel proper already defines rdmsr()/wrmsr(), but they are not for the
>  * boot kernel since they rely on tracepoint/exception handling infrastructure
>  * that's not available here.
>  */
> 
> We could do something similar for inb()/outb() and redefine them in
> arch/x86/boot/io.h instead of including <asm/shared/io.h> there.

That would be a saner approach.

-- Steve