Hi,
Xen community is currently reviewing a new implementation for reading
I{S,C}ACTIVER registers (see [1]).
The implementation is based on vgic_mmio_read_active() in KVM, i.e the
active state of the interrupts is based on the vGIC state stored in memory.
While reviewing the patch on xen-devel, I noticed a potential deadlock
at least with Xen implementation. I know that Xen vGIC and KVM vGIC are
quite different, so I looked at the implementation to see how this is dealt.
With my limited knowledge of KVM, I wasn't able to rule it out. I am
curious to know if I missed anything.
vCPU A may read the active state of an interrupt routed to vCPU B. When
vCPU A is reading the state, it will read the state stored in memory.
The only way the memory state can get synced with the HW state is when
vCPU B exit guest context.
AFAICT, vCPU B will not exit when deactivating HW mapped interrupts and
virtual edge interrupts. So vCPU B may run for an abritrary long time
before been exiting and syncing the memory state with the HW state.
Looking at Linux (5.4 and onwards) use of the active state, vCPU A would
loop until the interrupt is not active anymore. So wouldn't the task on
vCPU A be blocked for an arbitrary long time?
Cheers,
[1]
https://lists.xenproject.org/archives/html/xen-devel/2020-03/msg01844.html
--
Julien Grall
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