Re: IOAT DMA w/IOMMU

[Robin Murphy <robin.murphy@arm.com>]

On 14/08/18 00:50, Logan Gunthorpe wrote:
> On 13/08/18 05:48 PM, Kit Chow wrote:
>> On 08/13/2018 04:39 PM, Logan Gunthorpe wrote:
>>>
>>> On 13/08/18 05:30 PM, Kit Chow wrote:
>>>> In arch/x86/include/asm/page.h, there is the following comment in
>>>> regards to validating the virtual address.
>>>>
>>>> /*
>>>>    * virt_to_page(kaddr) returns a valid pointer if and only if
>>>>    * virt_addr_valid(kaddr) returns true.
>>>>    */
>>>> #define virt_to_page(kaddr)     pfn_to_page(__pa(kaddr) >> PAGE_SHIFT)
>>>>
>>>> So it looks like the validation by virt_addr_valid was somehow dropped
>>>> from the virt_to_page code path. Does anyone have any ideas what
>>>> happended to it?
>>> I don't think it was ever validated (though I haven't been around long
>>> enough to say for certain). What the comment is saying is that you
>>> shouldn't rely on virt_to_page() unless you know virt_addr_valid() is
>>> true (which in most cases you can without the extra check). virt_to_page
>>> is meant to be really fast so adding an extra validation would probably
>>> be a significant performance regression for the entire kernel.
>>>
>>> The fact that this can happen through dma_map_single() is non-ideal at
>>> best. It assumes the caller is mapping regular memory and doesn't check
>>> this at all. It may make sense to fix that but I think people expect
>>> dma_map_single() to be as fast as possible as well...

dma_map_single() is already documented as only supporting lowmem (for 
which virt_to_page() can be assumed to be valid). You might get away 
with feeding it bogus addresses on x86, but on non-coherent 
architectures which convert the page back to a virtual address to 
perform cache maintenance you can expect that to crash and burn rapidly.

There may be some minimal-overhead sanity checking of fundamentals, but 
in general it's not really the DMA API's job to police its callers 
exhaustively; consider that the mm layer doesn't go out of its way to 
stop you from doing things like "kfree(kfree);" either.

>>>
>> Perhaps include the validation with some debug turned on?
> 
> The problem is how often do you develop code with any of the debug
> config options turned on?
> 
> There's already a couple of BUG_ONs in dma_map_single so maybe another
> one with virt_addr_valid wouldn't be so bad.

Note that virt_addr_valid() may be pretty heavyweight in itself. For 
example the arm64 implementation involves memblock_search(); that really 
isn't viable in a DMA mapping fastpath.

Robin.