Re: [PATCH 0/4] UefiCpuPkg, OvmfPkg: separate PCDs for boot CPU count vs. max CPU count

["Ard Biesheuvel" <ard.biesheuvel@linaro.org>]

On Tue, 8 Oct 2019 at 13:27, Laszlo Ersek <lersek@redhat.com> wrote:
>
> Repo:   https://github.com/lersek/edk2.git
> Branch: max_cpus_bz_1515
> Ref:    https://bugzilla.tianocore.org/show_bug.cgi?id=1515
>
> UefiCpuPkg/MpInitLib currently lacks support for the following use case:
>
> - time-limited AP enumeration is not reliable on the platform
>   (individual AP check-in may take arbitrarily long), and
>
> - APs may finish the wakeup routine, and report in to the BSP, in any
>   sequence whatsoever, and
>
> - the number of boot CPUs (which is known in advance) is strictly less
>   than the number of maximum CPUs (which is also known in advance).
>
> In the above case, the platform cannot tell UefiCpuPkg/MpInitLib to wait
> exactly until all boot APs check in. That is, the platform can't request
> that the AP enumeration never time out, but also not wait for too long.
>
> For supporting this use case, the patch series introduces
> PcdCpuBootLogicalProcessorNumber to UefiCpuPkg, and makes MpInitLib wait
> for exactly that many CPUs (= BSP + APs) to show up during CPU
> enumeration.
>
> Working towards VCPU hotplug with OVMF, OvmfPkg/PlatformPei fetches both
> the boot and the max CPU counts from QEMU, co-operating with the
> following QEMU patch set:
>
>   [qemu-devel] [PATCH 0/4] hw/i386: pass "MachineState.smp.max_cpus" to OVMF
>   http://mid.mail-archive.com/20191008105259.5378-1-lersek@redhat.com
>
> and passes them to UefiCpuPkg via PcdCpuBootLogicalProcessorNumber and
> PcdCpuMaxLogicalProcessorNumber.
>
> As a result, PcdCpuApInitTimeOutInMicroSeconds becomes irrelevant for,
> and unused by, OVMF -- time-limited AP enumeration is never going to be
> used.
>
> When OVMF is built with -D SMM_REQUIRE, this patch series is just a
> small building block, towards the full VCPU hotplug feature. However,
> when OVMF is built without -D SMM_REQUIRE, this series (together with
> the counterpart patch set for QEMU) completes the VCPU hotplug feature:
> it allows S3 resume to work with VCPUs hot-plugged previously (at OS
> runtime, of course).
>
> Cc: Anthony Perard <anthony.perard@citrix.com>
> Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> Cc: Eric Dong <eric.dong@intel.com>
> Cc: Igor Mammedov <imammedo@redhat.com>
> Cc: Jordan Justen <jordan.l.justen@intel.com>
> Cc: Julien Grall <julien.grall@arm.com>
> Cc: Ray Ni <ray.ni@intel.com>
>
> Thanks
> Laszlo
>
> Laszlo Ersek (4):
>   UefiCpuPkg/MpInitLib: honor the platform's boot CPU count in AP
>     detection

Assuming this ^^^ patch gets accepted by the maintainers,

>   OvmfPkg/OvmfXen.dsc: remove PcdCpu* dynamic defaults
>   OvmfPkg/IndustryStandard: define FW_CFG_X86_TOPOLOGY structure type
>   OvmfPkg/PlatformPei: rewrite MaxCpuCountInitialization() for CPU
>     hotplug
>

Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>

for the OvmfPkg changes.