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

[Kunkun Jiang]

Hi Eric,

On 2021/4/8 15:27, Auger Eric wrote:
> 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
> b91774984249).
>
> 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
>
> Eric
Ok, it makes sense to me.
I am glad to test them if these patches are ready.

Thanks,
Kunkun Jiang
> > 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
> > > .
> .