linux-kernel - Re: [PATCH] dell-wmi: Stop storing pointers to DMI tables

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Message-ID: <20160119101201.49c4d4c5@endymion.delvare>
Date:	Tue, 19 Jan 2016 10:12:01 +0100
From:	Jean Delvare <jdelvare@...e.de>
To:	Andy Lutomirski <luto@...nel.org>
Cc:	Pali Rohár <pali.rohar@...il.com>,
	platform-driver-x86@...r.kernel.org,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] dell-wmi: Stop storing pointers to DMI tables

Hi Andy,

On Fri, 15 Jan 2016 12:00:02 -0800, Andy Lutomirski wrote:
> UC means uncached.  ioremap, on x86, asks for an uncached mapping, so
> every memory access (load or store) hits main memory individually.
> Assuming that the spec says that whatever physical memory the DMI
> tables live in is permitted to be used with cached accesses, asking
> for the CPU cache to be permitted on those accesses will make them a
> whole lot faster.
> 
> If that isn't safe, you could also just copy each table out of the
> ioremap space into normal RAM as needed using MOVNTDQA.  I forget what
> the helper for that is called, but it basically does a fast streaming
> IO read and then writes to normal RAM, memcpy style.  Most modern CPUs
> support it.

I have no idea what is allowed and what isn't, sorry. You would have to
check the SMBIOS specification but also the UEFI specification.

I have to admit I never understood why dmi_alloc is arch-specific nor
why dmi_remap is needed in the first place (and even less why
dmi_early_remap is different.) So I'm not going to mess up with that
code.

I have no idea how costly dmi_remap() is, but certainly it is being
called more and more as we can see dmi_walk() gaining in popularity. So
if anyone is worried about the performance, I'll be happy to review and
test patches.

-- 
Jean Delvare
SUSE L3 Support