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

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Message-ID: <CAKocOOPqNPYU1dcDdHpKqMZtJnSOhT6tBGY7k4qGEy6uKSAr7Q@mail.gmail.com>
Date:	Fri, 4 Jan 2013 14:02:18 -0700
From:	Shuah Khan <shuahkhan@...il.com>
To:	Yinghai Lu <yinghai@...nel.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...e.hu>,
	"H. Peter Anvin" <hpa@...or.com>,
	"Eric W. Biederman" <ebiederm@...ssion.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,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH v7u1 26/31] x86: Don't enable swiotlb if there is not
 enough ram for it

On Fri, Jan 4, 2013 at 1:34 PM, Yinghai Lu <yinghai@...nel.org> wrote:
> On Fri, Jan 4, 2013 at 9:50 AM, Shuah Khan <shuahkhan@...il.com> wrote:
>>
>> This change doesn't take into account what swiolb was when
>> pci_swiotlb_detect_override() is called. Instead of returning
>> use_swiotlb like the original code did, it returns swiotlb which could
>> be zero, if !enough_mem_for_swiotlb().
>>
>> Might work fine on Intel platforms, but not on systems where the IOMMU
>> driver wants to enable swiotlb for some devices as in the case of AMD.
>>
>> AMD IOMMU driver enables swiotlb for devices that are not specified in
>> IVRs and/or not in the AMD IOMMU scope, after it successfully
>> initializes IOMMU. It will explicitely set switolb=1 to make sure
>> reserved swiotlb memory is not released. This change will break that
>> case.
>
> in that case, we have to panic....
>
> please check attached patch.
>
> Also for those kind of systems, users must specify crashkernel_low=72M or
> what ever for kdump.

Pani'cing the system doesn't sound like a good option to me in this
case. This change to disable swiotlb is made for kdump. However, with
this change several system fail to boot, unless crashkernel_low=72M is
specified.

I would the say the right approach to solve this would be to not
change the current pci_swiotlb_detect_override() behavior and treat
swiotlb =1 upon entry equivalent to swiotlb_force set.

Thanks,
-- Shuah
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