Implement linux-actiond

[dzbarsky]

Author: zbarsky-bot

Adds ARM64 and x86_64 linux-actiond serve-vm binaries. linux-actiond embeds QEMU from the rules_qemu target toolchain and executes QEMU, the Linux kernel, the initramfs, the runtime SquashFS, and x86_64 qboot.rom from sealed memfds. Only the persistent guest-owned cas.ext4 file remains under --root.

The QEMU configuration requires KVM and host CPU passthrough. x86_64 uses microvm,acpi=off,pcie=off,x-option-roms=off,rtc=off with the KVM irqchip. The persistent CAS file uses cache=none, aio=io_uring, one IOThread, and four virtio-blk queues. The runtime SquashFS uses a buffered read-only memfd. Both architectures use virtio-mmio block devices and vhost-vsock-device.

LLVM smoke timing

GitHub Actions run 27484009487 measured revision d37b232f53981aab042bc594a6479d434bf1e1f3 on x86_64. QEMU/actiond and the Linux host used the same Linux musl target and host platform, //e2e:llvm_exec_warmup, Bazel's default job count, fresh output bases, and matching process counts.

Execution	Bazel elapsed	Wall elapsed	Total processes	Executed processes
QEMU/actiond	269.181s	269.273s	2,079	1,998
Linux host	273.404s	273.504s	2,079	1,998

QEMU/actiond was 0.985x the Linux host Bazel elapsed time. VM startup to gRPC readiness was 0.615s. The previous QEMU/TCG actiond measurement was 3,117.899s, so KVM reduced actiond elapsed time by 11.58x.

The hosted ARM64 runner does not expose KVM, so CI builds the ARM64 release binary and runs the LLVM comparison on x86_64.

[dzbarsky]

x86_64 QEMU microvm/TCG LLVM smoke from GitHub Actions run 27474402783:

Execution	Bazel elapsed	Wall elapsed	Total processes	Executed processes
actiond	3,084.336s	3,084.609s	2,079	1,998
Linux host	272.311s	272.419s	2,079	1,998

The actiond / Linux host Bazel elapsed ratio is 11.327x. QEMU startup to gRPC readiness is 2.044s. Both measurements use the same Linux x86_64 musl target and host platform, the same warmup target, Bazel's default job count, and matching process counts.

The earlier q35/TCG result was 2,986.573s actiond / 244.129s host (12.234x), so microvm did not improve absolute actiond time in this CI comparison. The Firecracker/KVM result on PR #17 is 256.334s actiond / 246.639s host (1.039x). Firecracker's actiond time is about 12x faster than this QEMU run, but QEMU uses TCG and the measurements came from separate CI runs, so this is not a hypervisor-only comparison.

-zbarskybot