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

[Kunkun Jiang <jiangkunkun@huawei.com>]

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.😁

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
>>
>
> .