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

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Message-ID: <f5e03f2a-8176-528f-e885-9a97940367c0@suse.com>
Date:   Wed, 15 Mar 2023 07:49:55 +0100
From:   Jan Beulich <jbeulich@...e.com>
To:     Stefano Stabellini <sstabellini@...nel.org>
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>,
        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 15.03.2023 01:52, Stefano Stabellini wrote:
> 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).

But how does Xen using an IOMMU help with, as said, address-restricted
devices? They may still need e.g. a 32-bit address to be programmed in,
and if the kernel has memory beyond the 4G boundary not all I/O buffers
may fulfill this requirement.

Jan