Re: kmap_atomic issue with SLES11SP1 32bit XEN driver code

[dbrace <dab@hp.com>]

Can you give me some URLs to documentation that can help me understand 
my issue?

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



On 12/15/2011 10:37 AM, Jan Beulich wrote:
>>>> On 15.12.11 at 17:15, "Brace, Don"<dab@hp.com>  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