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

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Date:   Wed, 14 Jul 2021 12:25:31 +0200
From:   Joerg Roedel <joro@...tes.org>
To:     Konrad Rzeszutek Wilk <konrad@...nok.org>
Cc:     Robin Murphy <robin.murphy@....com>,
        Kai-Heng Feng <kai.heng.feng@...onical.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 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