| Subject | Re: [Xen-devel] [RFC PATCH 28/33] Add Xen grant table support | From | Harry Butterworth <> | Date | Wed, 19 Jul 2006 11:04:10 +0100 |
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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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