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

[Juergen Gross <jgross@suse.com>]

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On 16.03.23 14:45, Alex Deucher wrote:
> On Thu, Mar 16, 2023 at 3:50 AM Jan Beulich <jbeulich@suse.com> wrote:
>>
>> On 16.03.2023 00:25, Stefano Stabellini wrote:
>>> On Wed, 15 Mar 2023, Jan Beulich wrote:
>>>> 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.
>>>
>>> In short, it is going to work as long as Linux has guest physical
>>> addresses (not machine addresses, those could be anything) lower than
>>> 4GB.
>>>
>>> If the address-restricted device does DMA via an IOMMU, then the device
>>> gets programmed by Linux using its guest physical addresses (not machine
>>> addresses).
>>>
>>> The 32-bit restriction would be applied by Linux to its choice of guest
>>> physical address to use to program the device, the same way it does on
>>> native. The device would be fine as it always uses Linux-provided <4GB
>>> addresses. After the IOMMU translation (pagetable setup by Xen), we
>>> could get any address, including >4GB addresses, and that is expected to
>>> work.
>>
>> I understand that's the "normal" way of working. But whatever the swiotlb
>> is used for in baremetal Linux, that would similarly require its use in
>> PVH (or HVM) aiui. So unconditionally disabling it in PVH would look to
>> me like an incomplete attempt to disable its use altogether on x86. What
>> difference of PVH vs baremetal am I missing here?
> 
> swiotlb is not usable for GPUs even on bare metal.  They often have
> hundreds or megs or even gigs of memory mapped on the device at any
> given time.  Also, AMD GPUs support 44-48 bit DMA masks (depending on
> the chip family).

But the swiotlb isn't per device, but system global.


Juergen


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