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Re: [RFC PATCH 1/5] x86/xen: disable swiotlb for xen pvh
- To: Stefano Stabellini <sstabellini@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Wed, 15 Mar 2023 07:49:55 +0100
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- Cc: Huang Rui <ray.huang@xxxxxxx>, Alex Deucher <alexander.deucher@xxxxxxx>, Christian König <christian.koenig@xxxxxxx>, Stewart Hildebrand <Stewart.Hildebrand@xxxxxxx>, Xenia Ragiadakou <burzalodowa@xxxxxxxxx>, Honglei Huang <honglei1.huang@xxxxxxx>, Julia Zhang <julia.zhang@xxxxxxx>, Chen Jiqian <Jiqian.Chen@xxxxxxx>, Juergen Gross <jgross@xxxxxxxx>, Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, dri-devel@xxxxxxxxxxxxxxxxxxxxx, amd-gfx@xxxxxxxxxxxxxxxxxxxxx
- Delivery-date: Wed, 15 Mar 2023 06:50:26 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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
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