Re: Disable swiotlb for Dom0

[Julien Grall <julien@xen.org>]

On 13/08/2021 10:38, Roman Skakun wrote:
> Hi Julien,

Hi Roman,

>> So 0xb6000000 is most likely the GFN used to mapped the grant from the domU.
>>
>> swiotlb-xen on Arm will convert it to the MFN because it is not aware
>> whether the device is behind an IOMMU.
> 
> If I'm understand right, it seems like that swiotlb-xen is not ready to 
> work properly in case
> when we retrieved MFN instead of proper GFN mapped to Dom0 memory.
> Maybe you know some ideas to overcome this condition?

swiotlb-xen work as intended. You have a DMA buffer at an address that 
your device cannot deal with. So it will try to bounce it.

> 
>>  As the address is too high to be handled by the device, swiotlb will try
>>  to bounce it. I think it is correct to bounce the page but I am not sure
>>  why it can't. What the size of the DMA transaction?
> 
> The DMA map size is 3686400 bytes.

So that's a 3MB buffer. I am slightly confused because in an earlier 
message you wrote that the memory is coming from the guest. How did you 
map it?

> 
> I've added several logs to swiotlb map_single() and see:
> [  151.298455] <SWIOTLB> swiotlb_tbl_map_single() origin_addr: 
> 64af97000, needed: 708,
> avail: 7fc0, stride: 2, index: 4160

I am not sure how to read the logs... Are the number in hexadecimal or 
decimal? It might be useful if you post the diff of your changes.

[...]

> Swiotlb did not fit requested slots because the maximum slot size equals 
> IO_TLB_SEGSIZE=128 by default.

Ok. So it sounds like your problem is the have a DMA buffer that is too 
large for the default swiotlb. Did you try to bump the value from the 
command line?

> But I think, we cannot use64af97000 address in the swiotlb_bounce() 
> directly.

I am not sure to understand what you mean. Can you clarify?

Cheers,

[...]

-- 
Julien Grall