linux-kernel - Re: [PATCH] x86/calgary: fix bitcast type warnings

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Message-ID: <3275e86a-5b12-29ff-9b1f-d8eba099ae98@deltatee.com>
Date:   Fri, 29 Mar 2019 15:32:07 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Jann Horn <jannh@...gle.com>, Mukesh Ojha <mojha@...eaurora.org>,
        Horia Geantă <horia.geanta@....com>
Cc:     Muli Ben-Yehuda <mulix@...ix.org>, Jon Mason <jdmason@...zu.us>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        iommu@...ts.linux-foundation.org,
        kernel list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/calgary: fix bitcast type warnings



On 2019-03-29 3:19 p.m., Jann Horn wrote:
>>> Can the existing api's not be used here like iowrite64be/ioread64be/ or
>>> similar variant in "include/asm-generic/io.h"
>>
>> Oooh! I didn't realize that those exist. I'll change that and send a v2.
Yes, they are very new! It took me a while to get those patches in as it
is a bit more complicated than it seems.

> Actually, that doesn't work at the moment on x86-64:
> 
> include/asm-generic/io.h only defines these things if
> CONFIG_GENERIC_IOMAP isn't defined; and X86 unconditionally defines
> it. With CONFIG_GENERIC_IOMAP set, these functions are provided by
> include/asm-generic/iomap.h.

For CONFIG_GENERIC_IOMAP, lib/iomap.c provides _lo_hi and _hi_lo
implementations seeing the pio regions must be emulated with 32bit
operations and we have to define the order.

> The definitions for these are in lib/iomap.c, except that there are no
> definitions for ioread64be() and iowrite64be(); if you try to use
> them, you get linker errors.

Some platforms implement these but most do not. If I recall correctly
only powerpc does.

If you want to use 64 bit operations in a portable fashion, you should
include "linux/io-64-nonatomic-hi-lo.h" or
"linux/io-64-nonatomic-lo-hi.h", depending on weather you want the lower
bits or the higher bits to be written or read first in cases where an
atomic operation is not available.

Logan