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SubjectRe: [RFC PATCH 0/4] Use 1st-level for DMA remapping in guest
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Hi Peter,

On 9/25/19 4:52 PM, Peter Xu wrote:
> On Wed, Sep 25, 2019 at 08:02:23AM +0000, Tian, Kevin wrote:
>>> From: Peter Xu [mailto:peterx@redhat.com]
>>> Sent: Wednesday, September 25, 2019 3:45 PM
>>>
>>> On Wed, Sep 25, 2019 at 07:21:51AM +0000, Tian, Kevin wrote:
>>>>> From: Peter Xu [mailto:peterx@redhat.com]
>>>>> Sent: Wednesday, September 25, 2019 2:57 PM
>>>>>
>>>>> On Wed, Sep 25, 2019 at 10:48:32AM +0800, Lu Baolu wrote:
>>>>>> Hi Kevin,
>>>>>>
>>>>>> On 9/24/19 3:00 PM, Tian, Kevin wrote:
>>>>>>>>>> '-----------'
>>>>>>>>>> '-----------'
>>>>>>>>>>
>>>>>>>>>> This patch series only aims to achieve the first goal, a.k.a using
>>>>>>> first goal? then what are other goals? I didn't spot such information.
>>>>>>>
>>>>>>
>>>>>> The overall goal is to use IOMMU nested mode to avoid shadow page
>>>>> table
>>>>>> and VMEXIT when map an gIOVA. This includes below 4 steps (maybe
>>> not
>>>>>> accurate, but you could get the point.)
>>>>>>
>>>>>> 1) GIOVA mappings over 1st-level page table;
>>>>>> 2) binding vIOMMU 1st level page table to the pIOMMU;
>>>>>> 3) using pIOMMU second level for GPA->HPA translation;
>>>>>> 4) enable nested (a.k.a. dual stage) translation in host.
>>>>>>
>>>>>> This patch set aims to achieve 1).
>>>>>
>>>>> Would it make sense to use 1st level even for bare-metal to replace
>>>>> the 2nd level?
>>>>>
>>>>> What I'm thinking is the DPDK apps - they have MMU page table already
>>>>> there for the huge pages, then if they can use 1st level as the
>>>>> default device page table then it even does not need to map, because
>>>>> it can simply bind the process root page table pointer to the 1st
>>>>> level page root pointer of the device contexts that it uses.
>>>>>
>>>>
>>>> Then you need bear with possible page faults from using CPU page
>>>> table, while most devices don't support it today.
>>>
>>> Right, I was just thinking aloud. After all neither do we have IOMMU
>>> hardware to support 1st level (or am I wrong?)... It's just that when
>>
>> You are right. Current VT-d supports only 2nd level.
>>
>>> the 1st level is ready it should sound doable because IIUC PRI should
>>> be always with the 1st level support no matter on IOMMU side or the
>>> device side?
>>
>> No. PRI is not tied to 1st or 2nd level. Actually from device p.o.v, it's
>> just a protocol to trigger page fault, but the device doesn't care whether
>> the page fault is on 1st or 2nd level in the IOMMU side. The only
>> relevant part is that a PRI request can have PASID tagged or cleared.
>> When it's tagged with PASID, the IOMMU will locate the translation
>> table under the given PASID (either 1st or 2nd level is fine, according
>> to PASID entry setting). When no PASID is included, the IOMMU locates
>> the translation from default entry (e.g. PASID#0 or any PASID contained
>> in RID2PASID in VT-d).
>>
>> Your knowledge happened to be correct in deprecated ECS mode. At
>> that time, there is only one 2nd level per context entry which doesn't
>> support page fault, and there is only one 1st level per PASID entry which
>> supports page fault. Then PRI could be indirectly connected to 1st level,
>> but this just changed with new scalable mode.
>>
>> Another note is that the PRI capability only indicates that a device is
>> capable of handling page faults, but not that a device can tolerate
>> page fault for any of its DMA access. If the latter is fasle, using CPU
>> page table for DPDK usage is still risky (and specific to device behavior)
>>
>>>
>>> I'm actually not sure about whether my understanding here is
>>> correct... I thought the pasid binding previously was only for some
>>> vendor kernel drivers but not a general thing to userspace. I feel
>>> like that should be doable in the future once we've got some new
>>> syscall interface ready to deliver 1st level page table (e.g., via
>>> vfio?) then applications like DPDK seems to be able to use that too
>>> even directly via bare metal.
>>>
>>
>> using 1st level for userspace is different from supporting DMA page
>> fault in userspace. The former is purely about which structure to
>> keep the mapping. I think we may do the same thing for both bare
>> metal and guest (using 2nd level only for GPA when nested is enabled
>> on the IOMMU). But reusing CPU page table for userspace is more
>> tricky. :-)
>
> Yes I should have mixed up the 1st level page table and PRI a bit, and
> after all my initial question should be irrelevant to this series as
> well so it's already a bit out of topic (sorry for that).

Never mind. Good discussion. :-)

Actually I have plan to use 1st level on bare metal as well. Just
looking forward to more motivation and use cases.

>
> And, thanks for explaining these. :)
>

Thanks for Kevin's explanation. :-)

Best regards,
Baolu

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