Re: [Xen-devel] kmap_atomic issue with SLES11SP1 32bit XEN driver code

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 16.12.11 at 00:19, dbrace <dab@xxxxxx> wrote:
> Can you give me some URLs to documentation that can help me understand 
> my issue?

I'm not aware of any.

> Is sounds like I cannot go back to the correct virtual address after I 
> map it for DMA.

Is it really a problem to insert a proper bus_to_phys() (or mfn_to_pfn()
if you are willing to tolerate Xen-specific code) translation?

Jan

> On 12/15/2011 10:37 AM, Jan Beulich wrote:
>>>>> On 15.12.11 at 17:15, "Brace, Don"<dab@xxxxxx>  wrote:
>>> I do not understand what this means:
>>> " That is, we'd be susceptible to this problem only if within a
>>> lazy-mode enabled section k{,un}map_atomic were used
>>> synchronously (which I'd consider a coding mistake)."
>>>
>>> Could you elaborate on this?
>> Not sure what additional explanation is needed. kmap_atomic() is
>> intended to be used in atomic context, and if you use it elsewhere
>> that's a mis-use.
>>
>>> Here is what I am trying to do.
>>> I am in a LLD I have bus addresses that are DMA-able that I would normally
>>> DMA to except this driver is emulating hardware in software and so it does
>>> not need to DMA is needs to transfer the data with the CPU.
>> So this makes clear that what I told you about a possibly missing
>> translation is likely the culprit. Quoting from your original mail
>>
>>                   linux_page = __pfn_to_page(physical_address>>  PAGE_SHIFT);
>>
>> I have to assume that physical_address really isn't a physical address,
>> but a bus one. Hence you first need to translate it to a physical one
>> before passing it to __pfn_to_page().
>>
>>> On non-XEN 32bit kernels I use kmap_atomic() and it handles both highmem
>>> addresses and non highmem addresses with no issues.
>> And so does it on Xen.
>>
>>> When I am running 32bit XEN kernels I run into issues like this:
>>>      "BUG: unable to handle kernel paging request at 9822bf40"
>> Because you pass in a random struct page *.
>>
>> Jan
>>
> 
> -- 
> Don Brace
> SPSN Linux Development
> Hewlett-Packard Company




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