linux-kernel - Re: [PATCH v7u1 26/31] x86: Don't enable swiotlb if there is not enough ram for it

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Date:	Mon, 07 Jan 2013 19:13:46 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Yinghai Lu <yinghai@...nel.org>
Cc:	Shuah Khan <shuahkhan@...il.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>, Jan Kiszka <jan.kiszka@....de>,
	Jason Wessel <jason.wessel@...driver.com>,
	linux-kernel@...r.kernel.org, Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH v7u1 26/31] x86: Don't enable swiotlb if there is not enough ram for it

Yinghai Lu <yinghai@...nel.org> writes:

> On Mon, Jan 7, 2013 at 6:22 PM, Eric W. Biederman <ebiederm@...ssion.com> wrote:
>> Yinghai I sat down and read your patch and the approach you are taking
>> is totally wrong.
>
> Thanks for check the patch, did you check v3?

I looked at the version of the patch you had as an attachment.  I don't
know the version number but it was the latest version of the patch I saw
in this thread.

After looking at things having a function enoung_mem_for_swiotlb()
in pci_swiotlb_detect_override() and pic_swiotlb_detect_4gb is brittle
hack.

>> The problem is that swiotlb_init() in lib/swiotlb.c does not know how to
>> fail without panic'ing the system.
>
> I did not put panic in swiotlb, now I put panic in amd_iommu ops init
> when it need extra
> swiotlb for unhandled devices by AMD IOMMU.

But the only reason you need to touch this code at all is that
swiotlb_init() calls panic() if you don't have 64MB of memory below 4G.

>> Which leaves two valid approaches.
>> - Create a variant of swiotlb_init that can fail for use on x86 and
>>   handle the failure.
>> - Delay initializing the swiotlb until someone actually needs a mapping
>>   from it.
>>
>> Delaying the initialization of the swiotlb is out because the code
>> needs an early memory allocation to get a large chunk of contiguous
>> memory for the bounce buffers.
>
> ok.
>
>>
>> Which means the panics that occurr in swiotlb_init() need to be delayed
>> until someone something actually needs bounce buffers from the swiotlb.
>>
>> Although arguably what should actually happen instead of panic() is that
>> swiotlb_map_single should simply fail early when it was not possible to
>> preallocate bounce buffers.
>
> do you mean: actually needed dma buffer is much less than swiotlb
> buffer aka 64M?


I meant we should detect failure to allocate bounce buffers in in
swiotlb_init() instead of panicing.

I meant swiotlb_map_single() should either panic or simply fail.

If I have read lib/swiotlb.c correctly the only place we allocate a
bounce buffer is in swiotlb_map_single.  If there are more places we can
allocate bounce buffers those need to be handled as well.

Eric
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