linux-kernel - Re: [Pkg-xen-devel] ioatdma: Boot process hangs then reboots when using Xen + Linux 3.2

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Message-ID: <20120302205600.GB4728@wavehammer.waldi.eu.org>
Date:	Fri, 2 Mar 2012 21:56:00 +0100
From:	Bastian Blank <waldi@...ian.org>
To:	Dan Williams <dan.j.williams@...el.com>
Cc:	Jonathan Nieder <jrnieder@...il.com>,
	xen-devel@...ts.xensource.com,
	pkg-xen-devel@...ts.alioth.debian.org,
	Maciej Sosnowski <maciej.sosnowski@...el.com>,
	linux-kernel@...r.kernel.org, William Dauchy <wdauchy@...il.com>,
	Konrad Rzeszutek Wilk <konrad@...nok.org>,
	Dave Jiang <dave.jiang@...el.com>
Subject: Re: [Pkg-xen-devel] ioatdma: Boot process hangs then reboots when
 using Xen + Linux 3.2

On Fri, Mar 02, 2012 at 12:16:47PM -0800, Dan Williams wrote:
> On Fri, Mar 2, 2012 at 12:08 PM, Bastian Blank <waldi@...ian.org> wrote:
> > On Fri, Mar 02, 2012 at 11:31:56AM -0800, Dan Williams wrote:
> >> On Fri, Mar 2, 2012 at 9:57 AM, Bastian Blank <waldi@...ian.org> wrote:
> >> > phys_complete (a 32 bit value) gets compared to struct
> >> > dma_async_tx_descriptor.phys, which is defined as dma_addr_t, a _64_ bit
> >> > value.
> >> The assumption is that the driver's control structures are not in high
> >> memory so all address values will only have 32-bits of valid data,
> > Can you back that up by some kernel documentation? There is a reason why
> > pci_alloc_pool uses dma_addr_t to store the address and _not_ unsigned
> > long. This are physical addresses, nothing the kernel can access
> > directly without a mapping.
> High memory can only be accessed with kmap(), so the assumption is
> that dma_alloc never gives a buffer address above 32-bits on a 32-bit
> build.  Yes, if HIGHMEM64G is set dma_addr_t becomes 64-bit, but that
> is only to access high memory mapped application buffers via dma_map.

All memory needs to be mapped.  Linux just have a default mapping of 1GiB
of the memory handy. However this is irrelevant for the physical DMA
addresses we talk about.

A assume this devices have a DMA mask of 2^64, so they can address
memory above the 4GiB.  And the kernel will happily assign this memory
if necessary or usefull.

> I'm not aware of any documentation in this area.

There is; the header files qualifies as documentation.

> I don't mind bumping up the size if xen32 is changing the above
> assumptions, but I'd want confirmation that this is the failure
> scenario.

At least it looks pretty wrong to remove four bits from a given address
just for fun.

Bastian

-- 
Respect is a rational process
		-- McCoy, "The Galileo Seven", stardate 2822.3
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