Re: [PATCH 3/4] UefiCpuPkg/MpInitLib: allow platforms to provide a known CPU count upfront

[Laszlo Ersek <lersek@redhat.com>]

On 11/23/16 06:36, Fan, Jeff wrote:
> Laszlo,
> 
> I ever submitted one bugzilla for such request at
> https://bugzilla.tianocore.org/show_bug.cgi?id=116

Huh, I didn't know about that. :)

> My point is not to add new PCD and just to use existing PCD
> PcdCpuMaxLogicalProcessorNumber that supports dynamic type.
> 
> Platform Peim could update it is platform pei module before memory ready.
> 
> MpInitLib just exits from wait loop once the discovered processors
> number reaches the PcdCpuMaxLogicalProcessorNumber.
> 
> Does it meet your requirement?

This was the first idea I had as well, yes. However, I considered the
following case:

- QEMU is started (and the guest is booted) with N online processors and
M > N *possible* processors.

- During OS runtime, the user hotplugs one or more additional
processors, so that the number of currently online processors is now
larger than N.

- The user suspends and resumes (S3) the virtual machine.

- During S3 resume, QEMU will report the increased number of boot
processors. (At S3 resume, QEMU specifically refreshes the fw_cfg item
that returns this value to the firmware, from the increased number of
VCPUs.)

- During S3 resume, we cannot modify PcdCpuMaxLogicalProcessorNumber any
longer, because the UefiCpuPkg modules that consume the PCD have already
allocated their data structures using the first (lower) value. Those
allocations cannot be resized / extended during S3 resume.

So the idea is, set PcdCpuMaxLogicalProcessorNumber statically to the
number of the expected maximum VCPU count, which will ensure the proper
allocations during first boot. Then allow
PcdCpuKnownLogicalProcessorNumber to change -- hot-unplug is also
possible -- from normal boot to S3 resume.

(The concept of "expected maximum" is not specific to VCPU hotplug in
QEMU, it applies to memory (DIMM) hotplug and memory ballooning as well.)

Of course, I don't know (or expect) that the UefiCpuPkg modules fully
support this use case right now; it's just that I didn't want to
*prevent* this use case. So I figured I'd preserve the original role of
PcdCpuMaxLogicalProcessorNumber, and introduce a more dynamic value for
the "current boot VCPU count".

Do you agree that it makes sense to keep these quantities separate?

If you think we should just set PcdCpuMaxLogicalProcessorNumber
dynamically at this point, and introduce no new PCD for now, I can do
that too. I just wanted to make sure that we discuss VCPU hot-plug /
hot-unplug and S3 resume first.

Thanks!
Laszlo

> 
> Thanks!
> Jeff
> 
> -----Original Message-----
> From: Laszlo Ersek [mailto:lersek@redhat.com] 
> Sent: Wednesday, November 23, 2016 4:26 AM
> To: edk2-devel-01
> Cc: Igor Mammedov; Fan, Jeff
> Subject: [PATCH 3/4] UefiCpuPkg/MpInitLib: allow platforms to provide a known CPU count upfront
> 
> On QEMU/KVM, the VCPU topology for a guest is specified dynamically. It can be a low number or a high number.
> 
> Waiting for PcdCpuApInitTimeOutInMicroSeconds during the initial AP collection is impractical, because in a VM, the time needed for an AP to wake up can vary widely:
> 
> - if the APs report back quickly, then we could be wasting time,
> 
> - if the APs report back late (due to scheduling artifacts or VCPU
>   over-subscription on the virtualization host), then the timeout can
>   elapse before all APs increment CpuMpData->CpuCount in
>   ApWakeupFunction().
> 
> Trying to set PcdCpuApInitTimeOutInMicroSeconds dynamically is also impractical, as scheduling artifacts on the KVM host may delay AP threads arbitrarily.
> 
> Instead, allow OVMF (and other platforms) to tell MpInitLib the number of boot-time CPUs in advance.
> 
> Cc: Igor Mammedov <imammedo@redhat.com>
> Cc: Jeff Fan <jeff.fan@intel.com>
> Contributed-under: TianoCore Contribution Agreement 1.0
> Signed-off-by: Laszlo Ersek <lersek@redhat.com>
> ---
>  UefiCpuPkg/UefiCpuPkg.dec                     | 11 +++++++++++
>  UefiCpuPkg/Library/MpInitLib/DxeMpInitLib.inf |  1 +  UefiCpuPkg/Library/MpInitLib/PeiMpInitLib.inf |  4 ++--
>  UefiCpuPkg/Library/MpInitLib/MpLib.c          | 16 ++++++++++++----
>  4 files changed, 26 insertions(+), 6 deletions(-)
> 
> diff --git a/UefiCpuPkg/UefiCpuPkg.dec b/UefiCpuPkg/UefiCpuPkg.dec index ca560398bbef..35903c4386e4 100644
> --- a/UefiCpuPkg/UefiCpuPkg.dec
> +++ b/UefiCpuPkg/UefiCpuPkg.dec
> @@ -225,5 +225,16 @@ [PcdsDynamic, PcdsDynamicEx]
>    # @ValidList   0x80000001 | 0
>    gUefiCpuPkgTokenSpaceGuid.PcdCpuHotPlugDataAddress|0x0|UINT64|0x60000011
>  
> +  ## On platforms where the number of boot-time CPUs can be dynamically  
> + #  retrieved from a platform-specific information source, the BSP does 
> + not  #  have to wait for PcdCpuApInitTimeOutInMicroSeconds in order to 
> + detect all  #  APs for the first time. On such platforms, this PCD 
> + specifies the number  #  of CPUs available at boot. If the platform 
> + doesn't support this feature,  #  this PCD should be set to 0. The 
> + platform is responsible for ensuring that  #  this PCD is never set to 
> + a value larger than  #  PcdCpuMaxLogicalProcessorNumber.
> +  # @Prompt Known number of CPUs available at boot.
> +  
> + gUefiCpuPkgTokenSpaceGuid.PcdCpuKnownLogicalProcessorNumber|0|UINT32|0
> + x60000012
> +
>  [UserExtensions.TianoCore."ExtraFiles"]
>    UefiCpuPkgExtra.uni
> diff --git a/UefiCpuPkg/Library/MpInitLib/DxeMpInitLib.inf b/UefiCpuPkg/Library/MpInitLib/DxeMpInitLib.inf
> index 11b230174ec8..dc18eaf6152d 100644
> --- a/UefiCpuPkg/Library/MpInitLib/DxeMpInitLib.inf
> +++ b/UefiCpuPkg/Library/MpInitLib/DxeMpInitLib.inf
> @@ -61,6 +61,7 @@ [Guids]
>  
>  [Pcd]
>    gUefiCpuPkgTokenSpaceGuid.PcdCpuMaxLogicalProcessorNumber        ## CONSUMES
> +  gUefiCpuPkgTokenSpaceGuid.PcdCpuKnownLogicalProcessorNumber      ## SOMETIMES_CONSUMES
>    gUefiCpuPkgTokenSpaceGuid.PcdCpuApInitTimeOutInMicroSeconds      ## SOMETIMES_CONSUMES
>    gUefiCpuPkgTokenSpaceGuid.PcdCpuApStackSize                      ## CONSUMES
>    gUefiCpuPkgTokenSpaceGuid.PcdCpuMicrocodePatchAddress            ## CONSUMES
> diff --git a/UefiCpuPkg/Library/MpInitLib/PeiMpInitLib.inf b/UefiCpuPkg/Library/MpInitLib/PeiMpInitLib.inf
> index 0c6873da79db..2bcfa70ae7e5 100644
> --- a/UefiCpuPkg/Library/MpInitLib/PeiMpInitLib.inf
> +++ b/UefiCpuPkg/Library/MpInitLib/PeiMpInitLib.inf
> @@ -61,10 +61,10 @@ [Ppis]
>  
>  [Pcd]
>    gUefiCpuPkgTokenSpaceGuid.PcdCpuMaxLogicalProcessorNumber        ## CONSUMES
> -  gUefiCpuPkgTokenSpaceGuid.PcdCpuApInitTimeOutInMicroSeconds      ## CONSUMES
> +  gUefiCpuPkgTokenSpaceGuid.PcdCpuKnownLogicalProcessorNumber      ## CONSUMES
> +  gUefiCpuPkgTokenSpaceGuid.PcdCpuApInitTimeOutInMicroSeconds      ## SOMETIMES_CONSUMES
>    gUefiCpuPkgTokenSpaceGuid.PcdCpuApStackSize                      ## CONSUMES
>    gUefiCpuPkgTokenSpaceGuid.PcdCpuMicrocodePatchAddress            ## CONSUMES
>    gUefiCpuPkgTokenSpaceGuid.PcdCpuMicrocodePatchRegionSize         ## CONSUMES
>    gUefiCpuPkgTokenSpaceGuid.PcdCpuApLoopMode                       ## CONSUMES
>    gUefiCpuPkgTokenSpaceGuid.PcdCpuApTargetCstate                   ## SOMETIMES_CONSUMES
> -
> diff --git a/UefiCpuPkg/Library/MpInitLib/MpLib.c b/UefiCpuPkg/Library/MpInitLib/MpLib.c
> index 15dbfa1e7d6c..f7dfbd5bad13 100644
> --- a/UefiCpuPkg/Library/MpInitLib/MpLib.c
> +++ b/UefiCpuPkg/Library/MpInitLib/MpLib.c
> @@ -707,6 +707,7 @@ WakeUpAP (
>    CPU_AP_DATA                      *CpuData;
>    BOOLEAN                          ResetVectorRequired;
>    CPU_INFO_IN_HOB                  *CpuInfoInHob;
> +  UINT32                           KnownProcessorCount;
>  
>    CpuMpData->FinishedCount = 0;
>    ResetVectorRequired = FALSE;
> @@ -745,10 +746,17 @@ WakeUpAP (
>        SendInitSipiSipiAllExcludingSelf ((UINT32) ExchangeInfo->BufferStart);
>      }
>      if (CpuMpData->InitFlag == ApInitConfig) {
> -      //
> -      // Wait for all potential APs waken up in one specified period
> -      //
> -      MicroSecondDelay (PcdGet32(PcdCpuApInitTimeOutInMicroSeconds));
> +      KnownProcessorCount = PcdGet32 (PcdCpuKnownLogicalProcessorNumber);
> +      if (KnownProcessorCount > 0) {
> +        while (CpuMpData->FinishedCount < (KnownProcessorCount - 1)) {
> +          CpuPause ();
> +        }
> +      } else {
> +        //
> +        // Wait for all potential APs waken up in one specified period
> +        //
> +        MicroSecondDelay (PcdGet32(PcdCpuApInitTimeOutInMicroSeconds));
> +      }
>      } else {
>        //
>        // Wait all APs waken up if this is not the 1st broadcast of SIPI
> --
> 2.9.2
> 
>