[Feature Request] Host-side guest vmcore dump on kernel panic (pvpanic)

Feature Request

Automatically capture a guest vmcore on kernel panic via a host-side pvpanic device, without reserving any guest memory.

Today the only supported way to obtain a full memory image of a panicked guest kernel is in-guest kdump/kexec, which requires reserving memory via the crashkernel= kernel command line parameter — in practice a fixed ~100–200 MB per VM. For microVMs, which are often provisioned with only a few hundred MB of RAM, that reservation consumes a large fraction of guest memory.

But the VMM already has the entire guest physical memory mapped. If the guest signals the host on panic, the host can produce a complete crash dump itself, with zero guest-side memory reservation.

Describe the desired solution

Add an opt-in, per-VM paravirtualized pvpanic device. The guest-side driver is already in mainline Linux (CONFIG_PVPANIC), so the guest needs no changes, it simply signals the device on panic.

When Firecracker receives that event, it automatically writes a vmcore to a configured host path that is directly analyzable with crash/gdb, with no in-guest crashkernel= reservation.

Describe possible alternatives

• In-guest kdump via crashkernel=. Works, but requires a fixed ~100–200 MB reservation per guest.
• Serial-console panic logs only. No memory reservation, but only yields the backtrace/log text.

Additional context

I have a working x86_64 PoC (the pvpanic device plus the host-side ELF vmcore dumper) that produces crash/gdb-analyzable dumps, with negligible impact on Firecracker's process RSS.

However, I'm not sure whether a full guest vmcore dump really fits Firecracker's scope, given the minimalist core tenet, or whether a lighter mechanism would be preferred. I also noticed that pvpanic config isn't enabled in the CI kernels.

If there's a better way to dump a panicked kernel without adding a new device, it would also be much appreciated. And if the community is interested in this feature, I'm very happy to open a PR for this.

Checks

• Have you searched the Firecracker Issues database for similar requests?
• Have you read all the existing relevant Firecracker documentation?
• Have you read and understood Firecracker's core tenets?