On 12/01/2023 08:28, Laszlo Ersek wrote:
In QEMU v5.1.0, the CPU hotplug register block misbehaves: the negotiation
protocol is (effectively) broken such that it suggests that switching from
the legacy interface to the modern interface works, but in reality the
switch never happens. The symptom has been witnessed when using TCG
acceleration; KVM seems to mask the issue. The issue persists with the
following (latest) stable QEMU releases: v5.2.0, v6.2.0, v7.2.0. Currently
there is no stable release that addresses the problem.

The QEMU bug confuses the Present and Possible counting in function
PlatformMaxCpuCountInitialization(), in
"OvmfPkg/Library/PlatformInitLib/Platform.c". OVMF ends up with Present=0
Possible=1. This in turn further confuses MpInitLib in UefiCpuPkg (hence
firmware-time multiprocessing will be broken). Worse, CPU hot(un)plug with
SMI will be summarily broken in OvmfPkg/CpuHotplugSmm, which (considering
the privilege level of SMM) is not that great.

Detect the issue in PlatformMaxCpuCountInitialization(), and print an
error message and *hang* if the issue is present.

Would this mean that OVMF would refuse to start with all current distro versions of qemu (when not using KVM), or am I misunderstanding?

Thanks,

Michael



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