linux-kernel - Re: [PATCH] ia64: SN specific version of dma_get_required

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Sun, 23 Nov 2008 14:37:22 +0100
From:	Bernhard Walle <bwalle@...e.de>
To:	John Keller <jpk@....com>
Cc:	linux-ia64@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ia64: SN specific version of dma_get_required_mask()

Hi,

[Sorry for the late reply and for not following the whole thread, I'm
just busy.]

* John Keller [2008-11-18 08:08]:
> 
> This patch addresses a problem on SN Altix systems with < 4GB, where
> device drivers using the dma_get_required_mask() API would be told
> to use 32 bit DMA, when 64 bit is more efficient.
> 
> How exactly the use of dma_get_required_mask() relates to the crash
> kernel code you refer to is unclear to me.

I'm not sure myself. The crashkernel reservation code on IA64 (for
other architectures I don't know any machines that have basically their
whole memory except a small amount which is used for booting mapped
above 4 GiB physical address space) needs to check if it's okay to
use memory for the crashkernel that is *all* above 4 GiB.

This is only possible if a hardware IO/MMU is present (and working
correctly in the kdump case which isn't the case on HP IA64) and SWIOTBL
is not used because SWIOTBL needs some memory below that 4 GiB margin.

Now I thought that there's a relationship between "memory above 4 GiB
can be used for DMA" and the return value of dma_get_required_mask().
My assumption was:

  (dma_get_required_mask() & 0xffffffff00000000ull) > 0
  -> memory above 4 GiB can be used for DMA and so the
     crashkernel memory can reside above 4 GiB

  (dma_get_required_mask() & 0xffffffff00000000ull) == 0
  -> memory above 4 GiB can not be used for DMA and so the
     crashkernel memory can not all reside above 4 GiB

Is that wrong?

> If, for all platforms, the crash kernel code could use the mask returned
> from dma_get_required_mask() to do its check, then switching the code
> might be OK. But, if that's not possible for some platforms, then I'd
> wonder if dma_get_required_mask() is being used in the wrong context in
> this case.

The crashkernel reservation code is different for every platform, so it
does not matter. However, in theory I think the check would return
correct results.

Regards,
Bernhard
-- 
Bernhard Walle, SUSE LINUX Products GmbH, Architecture Development
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/