Re: [PATCH] KVM: PCIPT: VT-d: fix guest unmap

[Ben-Ami Yassour <benami@il.ibm.com>]

On Tue, 2008-06-17 at 16:29 -0500, Anthony Liguori wrote:
> I think the current VT-d code needs some reworking.
> 
> We should build the table as the shadow page table gets built.  We 
> should suppress iotlb flushes unless the table is actually being updated.
> 

I'm not sure what you mean.
The current implementation of vtd for passthrough is a direct map, which
means that we map the entire guest memory (and pin it).
In this case there are no iotlb flushes after the first initialization.

Obviously, pinning the entire guest is not desirable since we waste a
lot of memory resources, but this is the approach that we currently
have. Do you find it good enough for a merge with the main KVM tree, and
optimize later?

When you mentioned building a table as the shadow page table, did you
mean that we should map the IOMMU on demand?
I'm not sure how we can do that... the guest can send a guest physical
address to the device for DMA, even without generating a page-fault on
the host for that address... which implies that the host must pin the
entire guest memory in advance. agree?

The only way I can think of avoiding that is PVDMA with VT-d, which
means that there is a hyper call for each DMA request, but this is a
different solution, cause it only applies to PV guests.

Do you see a way to avoid mapping (and pinning) the entire guest memory
for fully virtual guests (and without parsing every transaction between
the guest and the device to figure out the DMA addresses)?

Regards,
Ben

> Regards,
> 
> Anthony Liguori
>