Re: [PATCH v3 1/7] memory: associate DMA accesses with the initiator Device

[Stefan Hajnoczi <stefanha@gmail.com>]

The _guarded() calls are required in BHs, timers, fd read/write
callbacks, etc because we're no longer in the memory region dispatch
code with the reentrancy guard set. It's not clear to me whether the
_guarded() calls are actually required in most of these patches
though? Do you plan to convert every DMA API call to a _guarded() call
in the future?

I'm asking because coming up with an API that doesn't require these
code changes will reduce code churn and make existing code safe.

Does it make sense to separate the DMA API and the reentrancy guard
API? That way the reentrancy guard can be put in place once in any BH,
timer, etc callback and then the existing DMA APIs are used within
those callbacks without new _guarded() APIs.

This approach also reduces the number of times that the guard is
toggled. The current approach is fine-grained (per DMA API call) so
the guard needs to be toggled all the time, e.g. in DMA sglist loops.

If we want the compiler to prevent DMA API calls without a reentrancy
guard, then AddressSpace pointers can be hidden behind an API that
sets the guard. This ensures that you cannot access an address space
unless you have a reentrancy guard.

Stefan