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

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Date:   Thu, 9 Sep 2021 14:32:55 +0800
From:   Kai-Heng Feng <kai.heng.feng@...onical.com>
To:     Joerg Roedel <joro@...tes.org>
Cc:     Konrad Rzeszutek Wilk <konrad@...nok.org>,
        Robin Murphy <robin.murphy@....com>, will@...nel.org,
        "open list:AMD IOMMU (AMD-VI)" <iommu@...ts.linux-foundation.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] iommu/amd: Enable swiotlb if any device supports iommu v2
 and uses identity mapping

On Wed, Jul 14, 2021 at 6:25 PM Joerg Roedel <joro@...tes.org> 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