linux-kernel - Re: [PATCH] iommu/amd: Fix extended features logging

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Message-ID: <87o8ekioo4.fsf@jogness.linutronix.de>
Date:   Sun, 11 Apr 2021 21:52:59 +0200
From:   John Ogness <john.ogness@...utronix.de>
To:     Alexander Monakov <amonakov@...ras.ru>
Cc:     Paul Menzel <pmenzel@...gen.mpg.de>,
        Joerg Roedel <jroedel@...e.de>,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        iommu@...ts.linux-foundation.org,
        LKML <linux-kernel@...r.kernel.org>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joe Perches <joe@...ches.com>
Subject: Re: [PATCH] iommu/amd: Fix extended features logging

On 2021-04-11, Alexander Monakov <amonakov@...ras.ru> wrote:
>>> The second line is emitted via 'pr_cont', which causes it to have a
>>> different ('warn') loglevel compared to the previous line ('info').
>>> 
>>> Commit 9a295ff0ffc9 attempted to rectify this by removing the newline
>>> from the pci_info format string, but this doesn't work, as pci_info
>>> calls implicitly append a newline anyway.
>> 
>> Hmm, did I really screw that up during my testing? I am sorry about that.
>> 
>> I tried to wrap my head around, where the newline is implicitly appended, and
>> only found the definitions below.
>> 
>>     include/linux/pci.h:#define pci_info(pdev, fmt, arg...)
>> dev_info(&(pdev)->dev, fmt, ##arg)
>> 
>>     include/linux/dev_printk.h:#define dev_info(dev, fmt, ...)
>> \
>>     include/linux/dev_printk.h:     _dev_info(dev, dev_fmt(fmt),
>> ##__VA_ARGS__)
>> 
>>     include/linux/dev_printk.h:__printf(2, 3) __cold
>>     include/linux/dev_printk.h:void _dev_info(const struct device *dev, const
>> char *fmt, ...);
>> 
>>     include/linux/compiler_attributes.h:#define __printf(a, b)
>> __attribute__((__format__(printf, a, b)))
>
> Yeah, it's not obvious: it happens in kernel/printk/printk.c:vprintk_store
> where it does
>
> 	if (dev_info)
> 		lflags |= LOG_NEWLINE;
>
> It doesn't seem to be documented; I think it prevents using pr_cont with
> "rich" printk facilities that go via _dev_info.
>
> I suspect it quietly changed in commit c313af145b9bc ("printk() - isolate
> KERN_CONT users from ordinary complete lines").

Yes, this behavior has been around for a while. I see no reason why it
should be that way. These days printk does not care if there is dev_info
included or not.

>> In the discussion *smpboot: CPU numbers printed as warning* [1] John wrote:
>> 
>>> It is supported to provide loglevels for CONT messages. The loglevel is
>>> then only used if the append fails:
>>> 
>>>     pr_cont(KERN_INFO "message part");
>>> 
>>> I don't know if we want to go down that path. But it is supported.
>
> Yeah, I saw that, but decided to go with the 'pr_info("")' solution, because
> it is less magic, and already used in two other drivers.

Note that what I was suggesting was to fix a different issue: If the
pr_cont() caller is interrupted by another printk user, then the
following pr_cont() calls will use the default loglevel. By explicitly
specifying the loglevel in pr_cont(), you can be sure that those pieces
get the desired loglevel, even if those pieces get separated off because
of an interrupting printk user.

So even if we fix dev_info to allow pr_cont continuation, it still may
be desirable to specify the loglevel in the pr_cont pieces.

> pr_info("") will also prepend 'AMD-Vi:' to the feature list, which is
> nice.

I'd rather fix dev_info callers to allow pr_cont and then fix any code
that is using this workaround.

And if the print maintainers agree it is OK to encourage
pr_cont(LOGLEVEL "...") usage, then people should really start using
that if the loglevel on those pieces is important.

John Ogness