Grant operation batching

[Demi Marie Obenour <demi@invisiblethingslab.com>]

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It is well known that mapping and unmapping grants is expensive, which
is why blkback has persistent grants.  Could this cost be mitigated by
batching, and if it was, would it affect the tradeoff of memcpy() vs
grant table operations?

Alternatively, would there be any interest in an “unsafe” mode for
blkback that skips both the copy and the grant operations?  This is
obviously unsafe (hence the name!), but in many cases that unsafety does
not actually matter.  For instance, a Qubes dom0 can execute any shell
command it wants in any Qubes VM via qvm-run.  Much easier than trying
to exploit some UaF or race condition 🙂.  More generally, when the
backend is the all-powerful dom0, trying to defend against a malicious
backend is (at least in the absence of SEV-SNP or TDX) pointless, so one
might as well not bother and take the free performance.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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