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

["Laszlo Ersek" <lersek@redhat.com>]

On 10/08/19 13:35, Ard Biesheuvel wrote:
> 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.
> 

Thanks!
Laszlo