Re: [edk2-devel] [PATCH v2] OvmfPkg/PlatformInitLib: catch QEMU's CPU hotplug reg block regression

["Michael Brown" <mcb30@ipxe.org>]

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