linux-kernel - Re: [RFC PATCH 1/5] x86/xen: disable swiotlb for xen pvh

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Message-ID: <alpine.DEB.2.22.394.2303141747350.863724@ubuntu-linux-20-04-desktop>
Date:   Tue, 14 Mar 2023 17:52:30 -0700 (PDT)
From:   Stefano Stabellini <sstabellini@...nel.org>
To:     Jan Beulich <jbeulich@...e.com>
cc:     Huang Rui <ray.huang@....com>,
        Alex Deucher <alexander.deucher@....com>,
        Christian König <christian.koenig@....com>,
        Stewart Hildebrand <Stewart.Hildebrand@....com>,
        Xenia Ragiadakou <burzalodowa@...il.com>,
        Honglei Huang <honglei1.huang@....com>,
        Julia Zhang <julia.zhang@....com>,
        Chen Jiqian <Jiqian.Chen@....com>,
        Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Roger Pau Monné <roger.pau@...rix.com>,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, amd-gfx@...ts.freedesktop.org
Subject: Re: [RFC PATCH 1/5] x86/xen: disable swiotlb for xen pvh

On Mon, 13 Mar 2023, Jan Beulich wrote:
> On 12.03.2023 13:01, Huang Rui wrote:
> > Xen PVH is the paravirtualized mode and takes advantage of hardware
> > virtualization support when possible. It will using the hardware IOMMU
> > support instead of xen-swiotlb, so disable swiotlb if current domain is
> > Xen PVH.
> 
> But the kernel has no way (yet) to drive the IOMMU, so how can it get
> away without resorting to swiotlb in certain cases (like I/O to an
> address-restricted device)?

I think Ray meant that, thanks to the IOMMU setup by Xen, there is no
need for swiotlb-xen in Dom0. Address translations are done by the IOMMU
so we can use guest physical addresses instead of machine addresses for
DMA. This is a similar case to Dom0 on ARM when the IOMMU is available
(see include/xen/arm/swiotlb-xen.h:xen_swiotlb_detect, the corresponding
case is XENFEAT_not_direct_mapped).

Jurgen, what do you think? Would you rather make xen_swiotlb_detect
common between ARM and x86?