Re: [RFC] xen/arm: introduce XENFEAT_ARM_dom0_iommu

[Jan Beulich <jbeulich@xxxxxxxx>]

On 17.02.2021 16:34, Bertrand Marquis wrote:
>> On 17 Feb 2021, at 02:00, Stefano Stabellini <sstabellini@xxxxxxxxxx> wrote:
>> Today Linux uses the swiotlb-xen driver (drivers/xen/swiotlb-xen.c) to
>> translate addresses for DMA operations in Dom0. Specifically,
>> swiotlb-xen is used to translate the address of a foreign page (a page
>> belonging to a domU) mapped into Dom0 before using it for DMA.
>>
>> This is important because although Dom0 is 1:1 mapped, DomUs are not. On
>> systems without an IOMMU swiotlb-xen enables PV drivers to work as long
>> as the backends are in Dom0. Thanks to swiotlb-xen, the DMA operation
>> ends up using the MFN, rather than the GFN.
>>
>>
>> On systems with an IOMMU, this is not necessary: when a foreign page is
>> mapped in Dom0, it is added to the Dom0 p2m. A new GFN->MFN translation
>> is enstablished for both MMU and SMMU. Dom0 could safely use the GFN
>> address (instead of the MFN) for DMA operations and they would work. It
>> would be more efficient than using swiotlb-xen.
>>
>> If you recall my presentation from Xen Summit 2020, Xilinx is working on
>> cache coloring. With cache coloring, no domain is 1:1 mapped, not even
>> Dom0. In a scenario where Dom0 is not 1:1 mapped, swiotlb-xen does not
>> work as intended.
>>
>>
>> The suggested solution for both these issues is to add a new feature
>> flag "XENFEAT_ARM_dom0_iommu" that tells Dom0 that it is safe not to use
>> swiotlb-xen because IOMMU translations are available for Dom0. If
>> XENFEAT_ARM_dom0_iommu is set, Linux should skip the swiotlb-xen
>> initialization. I have tested this scheme with and without cache
>> coloring (hence with and without 1:1 mapping of Dom0) on ZCU102 and it
>> works as expected: DMA operations succeed.
> 
> Wouldn’t it be easier to name the flag XENFEAT_ARM_swiotlb_needed ?

Except that "swiotlb" is Linux terminology, which I don't think a
Xen feature should be named after. Names should be generic, except
maybe when they're really targeting exactly one kind of guest
(which imo would better never be the case).

Jan