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

From: Joerg Roedel
Date: Wed Jul 14 2021 - 06:26:06 EST


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.

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

Regards,

Joerg