Re: [Xen-devel] [RFC PATCH 28/33] Add Xen grant table support

[Harry Butterworth <harry@hebutterworth.freeserve.co.uk>]

I don't think that the current grant-table API/implementation quite
meets the requirements for revocation.

Specifically I think the requirements are that:

1) When a domain starts to reclaim a set of resources it allocated for
inter-domain communication this process is guaranteed to complete
without cooperation from any other domain.

2) When a domain reclaims resources it allocated for inter-domain
communication it can't cause a page fault or other exception in any
other domain.

The current API/implementation meets the second requirement but not the
first.

It's possible to make the current implementation meet the first
requirement if you relax it a bit and allow the superuser to kill an
offending domain via domain 0 but this expands the scope of the failure
from a single inter-domain communication channel to a whole domain which
is undesirable.

Another potential issue with grant-tables is support for efficient N-way
communication but I haven't investigated that personally.

A final point is that quite a lot of fairly tricky code is required to
combine grant-tables, event-channels and xenbus before you can create an
inter-domain communication channel which can be correctly disconnected
and reconnected across module load and unload.

It's appropriate to have a low-level API at the hypervisor because it
keeps the hypervisor as small as possible and therefore easier to audit.

But, whatever the low-level API, whether grant-tables or something which
has better support for revocation and n-way communication, I think there
needs to be a small library to implement a higher level API that is more
convenient for driver authors to use directly.

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