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Re: Disable IOMMU in Dom0

Hi Roman

On 01/09/2021 10:59, Roman Skakun wrote:
If you have a setup  where Dom0 is not 1:1 mapped (which is not currently
possible with upstream  Xen but it is possible with cache coloring) and
uses the IOMMU to make  device DMA work like regular DomUs, then passing
XENFEAT_not_direct_mapped  to Linux would make it work. Without
XENFEAT_not_direct_mapped,  Linux would try to use swiotlb-xen which has
code that relies on  Linux being 1:1 mapped to work properly.

I'm using 1:1 Dom0.
According to your patch series, xen-swiotlb fops will be applied for all devices
because XENFEAT_direct_mapped is active, as shown here:
https://elixir.bootlin.com/linux/v5.14/source/arch/arm64/mm/dma-mapping.c#L56 <https://elixir.bootlin.com/linux/v5.14/source/arch/arm64/mm/dma-mapping.c#L56>

I agreed, that xen-swiotlb should work correctly, but in my case, I retrieved MFN here: https://elixir.bootlin.com/linux/v5.14/source/drivers/xen/swiotlb-xen.c#L366 <https://elixir.bootlin.com/linux/v5.14/source/drivers/xen/swiotlb-xen.c#L366> which is greater than 32bit and xen-swiotlb tries to use bounce buffer as expected.
It can lead to decrease a performance because I have a long buffer ~4MB.

I thought, that we can disable swiotlb fops for devices which are controlled by IOMMU.

Yes you can disable swiotlb for devices which are controlled by the IOMMU. But this will not make your problem disappear, it simply hides until it bites you in a more subttle way.

From what you wrote, you have a 32-bit DMA capable. So you always need to have an address below 4GB. For foreign mapping, there is no guarantee the Guest Physical Address will actually be below 4GB.

Today, the ballooning code only ask Linux to steal *a* RAM page for mapping the foreign page. This may or may not be below 4GB depending on how you assigned the RAM to dom0 (IIRC you had some RAM above 4GB).

But that's the current behavior. One of your work colleague is looking at avoid to steal RAM page to avoid exhausting the memory. So foreign mapping may end up to be a lot higher in memory.

IOW, you will need to be able to bounce the DMA buffer for your device. If you want to avoid bouncing, the proper way would be to rework the ballonning code so pages are allocated below 4GB.


Julien Grall



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