Re: [PATCH v2 00/19] iommufd: Add VIOMMU infrastructure (Part-1)

[Yi Liu]

On 2024/10/1 05:59, Nicolin Chen wrote:
> On Sun, Sep 29, 2024 at 03:16:55PM +0800, Yi Liu wrote:
> > > > > I feel these two might act somehow similarly to the two DIDs
> > > > > during nested translations?
> > > >
> > > > not quite the same. Is it possible that the ASID is the same for stage-1?
> > > > Intel VT-d side can have the pasid to be the same. Like the gIOVA, all
> > > > devices use the same ridpasid. Like the scenario I replied to Baolu[1],
> > > > do er choose to use different DIDs to differentiate the caches for the
> > > > two devices.
> > >
> > > On ARM, each S1 domain (either a normal stage-1 PASID=0 domain or
> > > an SVA PASID>0 domain) has a unique ASID.
> >
> > I see. Looks like ASID is not the PASID.
>
> It's not. PASID is called Substream ID in SMMU term. It's used to
> index the PASID table. For cache invalidations, a PASID (ssid) is
> for ATC (dev cache) or PASID table entry invalidation only.

sure. Is there any relationship between PASID and ASID? Per the below
link, ASID is used to tag the TLB entries of an application. So it's
used in the SVA case. right?

https://developer.arm.com/documentation/102142/0100/Stage-2-translation


> > > So it unlikely has the
> > > situation of two identical ASIDs if they are on the same vIOMMU,
> > > because the ASID pool is per IOMMU instance (whether p or v).
> > >
> > > With two vIOMMU instances, there might be the same ASIDs but they
> > > will be tagged with different VMIDs.
> > >
> > > > [1]
> > > > https://lore.kernel.org/linux-iommu/4bc9bd20-5aae-440d-84fd-f530d0747c23@xxxxxxxxx/
> > >
> > > Is "gIOVA" a type of invalidation that only uses "address" out of
> > > "PASID, DID and address"? I.e. PASID and DID are not provided via
> > > the invalidation request, so it's going to broadcast all viommus?
> >
> > gIOVA is just a term v.s. vSVA. Just want to differentiate it from vSVA. :)
> > PASID and DID are still provided in the invalidation.
>
> I am still not getting this gIOVA. What it does exactly v.s. vSVA?
> And should RIDPASID be IOMMU_NO_PASID?
gIOVA is the IOVA in guest. vSVA just the SVA in guest. Maybe the confusion
comes why not use vIOVA instead of gIOVA. is it? I think you are clear
about IOVA v.s. SVA. :)

yes, RIDPASID is the IOMMU_NO_PASID although VT-d arch allows it to be non
IOMMU_NO_PASID.

--
Regards,
Yi Liu