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Re: A question about the translation granule size supported by the vSMMU

From: Auger Eric
Subject: Re: A question about the translation granule size supported by the vSMMU
Date: Thu, 8 Apr 2021 09:27:36 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.0

Hi Kunkun,

On 4/7/21 11:26 AM, Kunkun Jiang wrote:
> Hi Eric,
> On 2021/4/7 3:50, Auger Eric wrote:
>> Hi Kunkun,
>> On 3/27/21 3:24 AM, Kunkun Jiang wrote:
>>> Hi all,
>>> Recently, I did some tests on SMMU nested mode. Here is
>>> a question about the translation granule size supported by
>>> vSMMU.
>>> There is such a code in SMMUv3_init_regs():
>>>>     /* 4K and 64K granule support */
>>>>      s->idr[5] = FIELD_DP32(s->idr[5], IDR5, GRAN4K, 1);
>>>>      s->idr[5] = FIELD_DP32(s->idr[5], IDR5, GRAN64K, 1);
>>>>      s->idr[5] = FIELD_DP32(s->idr[5], IDR5, OAS, SMMU_IDR5_OAS); /* 44
>>>> bits */
>>> Why is the 16K granule not supported? I modified the code
>>> to support it and did not encounter any problems in the
>>> test. Although 4K and 64K minimal granules are "strongly
>>> recommended", I think vSMMU should still support 16K.😉
>>> Are there other reasons why 16K is not supported here?
>> no there aren't any. The main reasons were 16KB support is optional and
>> supporting it increases the test matrix. Also it seems quite a few
>> machines I have access to do support 16KB granule. On the others I get
>> "EFI stub: ERROR: This 16 KB granular kernel is not supported by your
>> CPU".
>> Nevertheless I am not opposed to support it as it seems to work without
>> trouble. Just need to have an extra look at implied validity checks but
>> there shouldn't be much.
>> Thanks
>> Eric
> Yes, you are right. In my opinion, it is necessary to check whether pSMMU
> supports 16K to avoid the situation I mentioned below.
> In SMMU nested mode, if vSMMU supports 16K and set pasid table to
> pSMMU, it may get errors when pSMMU does translation table walk if
> pSMMU doesn't support 16K (not tested). Do you think we need to add
> an interface to get some pSMMU info?>
> Maybe my consideration was superfluous.😁
No it is not. At qemu level we have
memory_region_iommu_set_page_size_mask() that is called from the VFIO
device. It allows to pass such info to the IOMMU device (qemu

iommu_set_page_size_mask() cb needs to be implemented at SMMU QEMU
device level. Also [PATCH 0/2] Domain nesting info for arm-smmu may
allow to return other constraints from the pSMMU.


> Thanks,
> Kunkun Jiang
>>> When in SMMU nested mode, it may get errors if pSMMU
>>> doesn't support 16K but vSMMU supports 16K. But we
>>> can get some settings of pSMMU to avoid this situation.
>>> I found some discussions between Eric and Linu about
>>> this [1], but this idea does not seem to be implemented.
>>> [1] https://lists.gnu.org/archive/html/qemu-arm/2017-09/msg00149.html
>>> Best regards,
>>> Kunkun Jiang
>> .

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