Re: [RFC v3 14/25] intel_iommu: add virtual command capability support

[Peter Xu]

On Wed, Jan 29, 2020 at 04:16:45AM -0800, Liu, Yi L wrote:
> +/*
> + * The basic idea is to let hypervisor to set a range for available
> + * PASIDs for VMs. One of the reasons is PASID #0 is reserved by
> + * RID_PASID usage. We have no idea how many reserved PASIDs in future,
> + * so here just an evaluated value. Honestly, set it as "1" is enough
> + * at current stage.
> + */
> +#define VTD_MIN_HPASID              1
> +#define VTD_MAX_HPASID              0xFFFFF

One more question: I see that PASID is defined as 20bits long.  It's
fine.  However I start to get confused on how the Scalable Mode PASID
Directory could service that much of PASID entries.

I'm looking at spec 3.4.3, Figure 3-8.

Firstly, we only have two levels for a PASID table.  The context entry
of a device stores a pointer to the "Scalable Mode PASID Directory"
page. I see that there're 2^14 entries in "Scalable Mode PASID
Directory" page, each is a "Scalable Mode PASID Table".
However... how do we fit in the 4K page if each entry is a pointer of
x86_64 (8 bytes) while there're 2^14 entries?  A simple math gives me
4K/8 = 512, which means the "Scalable Mode PASID Directory" page can
only have 512 entries, then how the 2^14 come from?  Hmm??

Apart of this: also I just noticed (when reading the latter part of
the series) that the time that a pasid table walk can consume will
depend on this value too.  I'd suggest to make this as small as we
can, as long as it satisfies the usage.  We can even bump it in the
future.

-- 
Peter Xu