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vmm: fix vcpu atomic load
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Authored by br on Oct 28 2024, 1:15 PM.
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Details

Summary

Load vcpu with acquire semantics as we are making a critical section between creating vcpu and using it.

Test Plan

tested on risc-v only

Diff Detail

Repository
rG FreeBSD src repository
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Event Timeline

br requested review of this revision.Oct 28 2024, 1:15 PM

In particular, this is paired with an atomic_store_rel_ptr a few lines below. I think in practice this is probably a no-op as any dependent memory accesses are reads of data that vcpu points to, and if you had a mispredicted read it would never dereference a "wrong" pointer, it would just see a NULL pointer and stop once speculation ran into a fault. That said, I do think this is more correct.

This revision is now accepted and ready to land.Oct 28 2024, 1:29 PM
In D47306#1078749, @jhb wrote:

In particular, this is paired with an atomic_store_rel_ptr a few lines below. I think in practice this is probably a no-op as any dependent memory accesses are reads of data that vcpu points to, and if you had a mispredicted read it would never dereference a "wrong" pointer, it would just see a NULL pointer and stop once speculation ran into a fault. That said, I do think this is more correct.

This is perhaps a use-case for atomic_load_consume_ptr()? I think I agree that an acq fence is not formally required, but it looks wrong at first glance.

As an aside, we have #define atomic_load_ptr(p) (*(volatile __typeof(*p) *)(p)) - I wonder why atomic_load_acq_ptr isn't similarly defined.

This revision was automatically updated to reflect the committed changes.