sidecar: Zero the assist page during ap entry - #4067
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Pull request overview
This PR hardens OpenHCL sidecar x86_64 AP startup by ensuring the per-VP Hyper-V assist page is cleared before it is ever enabled for hypervisor consumption, preventing stale data from influencing hypervisor behavior across boots.
Changes:
- Zero the VP assist page during
ap_entry()bootstrapping, alongside existing zeroing of the register and command pages. - Document why this is necessary (hypervisor may consume fields the kernel never writes) and why the write is safe at that point.
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| // Zero the assist page as well. It may hold data from a previous boot, and | ||
| // the hypervisor consumes fields this kernel never writes, such as the VTL | ||
| // return actions it parses on every VTL return. | ||
| // | ||
| // SAFETY: the assist page has not been enabled with the hypervisor yet, so | ||
| // it is not concurrently accessed. | ||
| unsafe { | ||
| (*addr_space::assist_page().cast::<[u8; PAGE_SIZE]>()).fill(0); | ||
| } | ||
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I don't think there's much point in doing this, given that we subsequently tell the hypervisor to enable the assist page (which maps a new page on top of this memory).
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Ensure that the assist page is zeroed out before being enabled with the hypervisor, preventing potential issues from stale data left over from previous boots.