Re: [PATCH] iommu/amd: Enable swiotlb if any device supports iommu v2 and uses identity mapping

[Kai-Heng Feng]

On Wed, Jul 14, 2021 at 6:25 PM Joerg Roedel <joro@xxxxxxxxxx> wrote:
>
> On Tue, Jul 13, 2021 at 07:57:40PM -0400, Konrad Rzeszutek Wilk wrote:
> > The SWIOTLB does have support to do late initialization (xen-pcifront
> > does that for example - so if you add devices that can't do 64-bit it
> > will allocate something like 4MB).
>
> That sounds like a way to evaluate. I suggest to allocate the SWIOTLB
> memory at boot and when the IOMMUs are initialized we re-evaluate what
> we ended up with and free the SWIOTLB memory if its not needed.
>
> If that turns out to be wrong during runtime (e.g. because a device is
> switched to a passthrough default domain at runtime), we allocate a
> small aperture for this device like the above mentioned 4MB.

I am currently working on this but I found that 4MB is not enough,
16MB is the minimal size to make the device work.
How do I know the right SWIOTLB size for each device?

>
> (A boot option to always keep the aperture around might also be helpful
>  for some setups)

OK, will also implement this in next iteration.

Kai-Heng

>
> Regards,
>
>         Joerg