Re: [PATCH 1/2] x86 swiotlb: Verify we can perform the remapping requested.

[ebiederm@xmission.com (Eric W. Biederman)]

Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> writes:

> On Mon, Oct 17, 2011 at 02:19:18PM -0700, Eric W. Biederman wrote:
>> 
>> Recently I had a driver try with a peculiar 2G dma memory limit.
>> It failed in weird and strange ways because my bounce buffers were
>> being allocated above 2G where the driver could not reach, and
>> no error was reported when the mappings were setup.
>
> OK, so the overflow buffer was used instead.. which presumarily
> also was allocated above the 2G? That seems to point that
> alloc_bootmem_low_pages is not doing its job?

I just looked alloc_bootmem_low_pages allocates memory below
ARCH_ADDRESS_LIMIT which for everything except s390 is 4G.

I know I was mostly using the amd gart driver.  So I may be mistaken
that the swiotlb driver had the same issue.  However my only solution
at the time was to boot with mem=2G.  So I believe the swiotlb did
have this issue.

Mostly the patch was.  Hmm.  That looks stupid not wiring up the
swiotlb address space limit check when someone has already written it.

>> Use the swiotlb_dma_supported to avoid silent problems like this
>> in the future.
>
> Which driver was it that had this limit?

An unmerged driver.  The driver has since been fixed to use it's
hardware in a way that doesn't hit this limit.

At the time that I looked drivers in the kernel with a dma limit
above 16MB and blow 4GB only existed on exotic architectures in
very special cases.

Unfortunately I no longer have access to this hardware to test with.

Eric