Re: [PATCH v3 5/6] iommu/vt-d: Flush PASID-based iotlb for iova over first level

Lu Baolu
2019-12-17T01:20:09Z

Hi Yi,

On 12/15/19 5:22 PM, Liu, Yi L wrote:
> Ok, let me explain more... default pasid is meaningful only when
> the domain has been attached to a device as an aux-domain. right?

No exactly. Each domain has a specific default pasid, no matter normal
domain (RID based) or aux-domain (PASID based). The difference is for a
normal domain RID2PASID value is used, for an aux-domain the pasid is
allocated from a global pool.

The same concept used in VT-d 3.x scalable mode. For RID based DMA
translation RID2PASID value is used when walking the tables; For PASID
based DMA translation a real pasid in the transaction is used.

> If a domain only has one device, and it is attached to this device as
> normal domain (normal domain means non aux-domain here). Then
> you should flush cache with domain-id and RID2PASID value.
> If a domain has one device, and it is attached to this device as
> aux-domain. Then you may want to flush cache with domain-id
> and default pasid. right?

A domain's counterpart is IOMMU group. So we say attach/detach domain
to/from devices in a group. We don't allow devices with different
default pasid sitting in a same group, right?

> Then let's come to the case I mentioned in previous email. a mdev
> and another device assigned to a single VM. In host, you will have
> a domain which has two devices, one device(deva) is attached as

No. We will have two IOMMU groups and two domains. Correct me if my
understanding is not right.

Best regards,
baolu

> normal domain, another one (devB) is attached as aux-domain. Then
> which pasid should be used when the mapping in IOVA page table is
> modified? RID2PASID or default pasid? I think both should be used
> since the domain means differently to the two devices. If you just
> use default pasid, then deva may still be able to use stale caches.
>
> Regards,
> Yi Liu