On 03/12/21 07:26, Ankur Arora wrote:
> Add logic in EjectCpu() to do the actual the CPU ejection.
>
> On the BSP, ejection happens by first selecting the CPU via
> its QemuSelector and then sending the QEMU "eject" command.
> QEMU in-turn signals the remote VCPU thread which context-switches
> the CPU out of the SMI handler.
>
> Meanwhile the CPU being ejected, waits around in its holding
> area until it is context-switched out. Note that it is possible
> that a slow CPU gets ejected before it reaches the wait loop.
> However, this would never happen before it has executed the
> "AllCpusInSync" loop in SmiRendezvous().
> It can mean that an ejected CPU does not execute code after
> that point but given that the CPU state will be destroyed by
> QEMU, the missed cleanup is no great loss.
>
> Cc: Laszlo Ersek <ler...@redhat.com>
> Cc: Jordan Justen <jordan.l.jus...@intel.com>
> Cc: Ard Biesheuvel <ard.biesheu...@arm.com>
> Cc: Igor Mammedov <imamm...@redhat.com>
> Cc: Boris Ostrovsky <boris.ostrov...@oracle.com>
> Cc: Aaron Young <aaron.yo...@oracle.com>
> Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=3132
> Signed-off-by: Ankur Arora <ankur.a.ar...@oracle.com>
> ---
>
> Notes:
> Addresses the following comments from v8:
>
> (1a,1b) CheckIfBsp(): get rid of ProcessorNum, document retval.
> (2) Line up IsBsp and ApicBaseMsr
> (3) s/ongoing SMI iteration/ongoing SMI/
> (4) Get rid of the allusions to alignment in the comment in EjectCpu().
> () Also reduce some of the repetitive detail in this comment.
> (5) EjectCpu(): reorder logic to cleanly separate the AP and the BSP
> portions.
> (6) Get rid of unnecessary MemoryFence() between QemuCpuhpWrite
> and clearing of the eject status.
> (7) Change type of QemuSelector to %Lu in DEBUG statement
> (8) Get rid of the repetitive comment in SmmCpuFeaturesRendezvousExit().
> The necessary parts of this got moved to patch-7.
>
> OvmfPkg/Include/IndustryStandard/QemuCpuHotplug.h | 1 +
> OvmfPkg/CpuHotplugSmm/CpuHotplug.c | 113
> ++++++++++++++++++++--
> 2 files changed, 108 insertions(+), 6 deletions(-)
Reviewed-by: Laszlo Ersek <ler...@redhat.com>
Thanks
Laszlo
>
> diff --git a/OvmfPkg/Include/IndustryStandard/QemuCpuHotplug.h
> b/OvmfPkg/Include/IndustryStandard/QemuCpuHotplug.h
> index 2ec7a107a64d..d0e83102c13f 100644
> --- a/OvmfPkg/Include/IndustryStandard/QemuCpuHotplug.h
> +++ b/OvmfPkg/Include/IndustryStandard/QemuCpuHotplug.h
> @@ -34,6 +34,7 @@
> #define QEMU_CPUHP_STAT_ENABLED BIT0
> #define QEMU_CPUHP_STAT_INSERT BIT1
> #define QEMU_CPUHP_STAT_REMOVE BIT2
> +#define QEMU_CPUHP_STAT_EJECT BIT3
> #define QEMU_CPUHP_STAT_FW_REMOVE BIT4
>
> #define QEMU_CPUHP_RW_CMD_DATA 0x8
> diff --git a/OvmfPkg/CpuHotplugSmm/CpuHotplug.c
> b/OvmfPkg/CpuHotplugSmm/CpuHotplug.c
> index 2eeb4567a262..ae3abd525900 100644
> --- a/OvmfPkg/CpuHotplugSmm/CpuHotplug.c
> +++ b/OvmfPkg/CpuHotplugSmm/CpuHotplug.c
> @@ -18,6 +18,7 @@
> #include <Pcd/CpuHotEjectData.h> // CPU_HOT_EJECT_DATA
> #include <Protocol/MmCpuIo.h> // EFI_MM_CPU_IO_PROTOCOL
> #include <Protocol/SmmCpuService.h> // EFI_SMM_CPU_SERVICE_PROTOCOL
> +#include <Register/Intel/ArchitecturalMsr.h> // MSR_IA32_APIC_BASE_REGISTER
> #include <Uefi/UefiBaseType.h> // EFI_STATUS
>
> #include "ApicId.h" // APIC_ID
> @@ -193,12 +194,40 @@ RevokeNewSlot:
> }
>
> /**
> + EjectCpu needs to know the BSP at SMI exit at a point when
> + some of the EFI_SMM_CPU_SERVICE_PROTOCOL state has been torn
> + down.
> + Reuse the logic from OvmfPkg::PlatformSmmBspElection() to
> + do that.
> +
> + @retval TRUE If the CPU executing this function is the BSP.
> +
> + @retval FALSE If the CPU executing this function is an AP.
> +**/
> +STATIC
> +BOOLEAN
> +CheckIfBsp (
> + VOID
> + )
> +{
> + MSR_IA32_APIC_BASE_REGISTER ApicBaseMsr;
> + BOOLEAN IsBsp;
> +
> + ApicBaseMsr.Uint64 = AsmReadMsr64 (MSR_IA32_APIC_BASE);
> + IsBsp = (BOOLEAN)(ApicBaseMsr.Bits.BSP == 1);
> + return IsBsp;
> +}
> +
> +/**
> CPU Hot-eject handler, called from SmmCpuFeaturesRendezvousExit()
> on each CPU at exit from SMM.
>
> - If, the executing CPU is not being ejected, nothing to be done.
> + If, the executing CPU is neither the BSP, nor being ejected, nothing
> + to be done.
> If, the executing CPU is being ejected, wait in a halted loop
> until ejected.
> + If, the executing CPU is the BSP, set QEMU CPU status to eject
> + for CPUs being ejected.
>
> @param[in] ProcessorNum ProcessorNum denotes the CPU exiting SMM,
> and will be used as an index into
> @@ -214,6 +243,83 @@ EjectCpu (
> {
> UINT64 QemuSelector;
>
> + if (CheckIfBsp ()) {
> + UINT32 Idx;
> +
> + for (Idx = 0; Idx < mCpuHotEjectData->ArrayLength; Idx++) {
> + UINT64 QemuSelector;
> +
> + QemuSelector = mCpuHotEjectData->QemuSelectorMap[Idx];
> +
> + if (QemuSelector != CPU_EJECT_QEMU_SELECTOR_INVALID) {
> + //
> + // This to-be-ejected-CPU has already received the BSP's SMI exit
> + // signal and will execute SmmCpuFeaturesRendezvousExit()
> + // followed by this callback or is already penned in the
> + // CpuSleep() loop below.
> + //
> + // Tell QEMU to context-switch it out.
> + //
> + QemuCpuhpWriteCpuSelector (mMmCpuIo, (UINT32) QemuSelector);
> + QemuCpuhpWriteCpuStatus (mMmCpuIo, QEMU_CPUHP_STAT_EJECT);
> +
> + //
> + // Now that we've ejected the CPU corresponding to
> QemuSelectorMap[Idx],
> + // clear its eject status to ensure that an invalid future SMI does
> + // not end up trying a spurious eject or a newly hotplugged CPU does
> + // not get penned in the CpuSleep() loop.
> + //
> + // Note that the QemuCpuhpWriteCpuStatus() command above is a write
> to
> + // a different address space and uses the EFI_MM_CPU_IO_PROTOCOL.
> + //
> + // This means that we are guaranteed that the following assignment
> + // will not be reordered before the eject. And, so we can safely
> + // do this write here.
> + //
> + mCpuHotEjectData->QemuSelectorMap[Idx] =
> + CPU_EJECT_QEMU_SELECTOR_INVALID;
> +
> + DEBUG ((DEBUG_INFO, "%a: Unplugged ProcessorNum %u, "
> + "QemuSelector %Lu\n", __FUNCTION__, Idx, QemuSelector));
> + }
> + }
> +
> + //
> + // We are done until the next hot-unplug; clear the handler.
> + //
> + // mCpuHotEjectData->Handler is a NOP for any CPU not under ejection.
> + // So, once we are done with all the ejections, we can safely reset it
> + // here since any CPU dereferencing it would only see either the old
> + // or the new value (since it is aligned at a natural boundary.)
> + //
> + mCpuHotEjectData->Handler = NULL;
> + return;
> + }
> +
> + //
> + // Reached only on APs
> + //
> +
> + //
> + // mCpuHotEjectData->QemuSelectorMap[ProcessorNum] is updated
> + // on the BSP in the ongoing SMI at two places:
> + //
> + // - UnplugCpus() where the BSP determines if a CPU is under ejection
> + // or not. As a comment in UnplugCpus() at set-up, and in
> + // SmmCpuFeaturesRendezvousExit() where it is dereferenced describe,
> + // any such updates are guaranteed to be ordered-before the
> + // dereference below.
> + //
> + // - EjectCpu() on the BSP (above) updates QemuSelectorMap[ProcessorNum]
> + // for a CPU once it's ejected.
> + //
> + // The CPU under ejection: might be executing anywhere between the
> + // AllCpusInSync loop in SmiRendezvous(), to about to dereference
> + // QemuSelectorMap[ProcessorNum].
> + // As described in the comment above where we do the reset, this
> + // is not a problem since the ejected CPU never sees the after value.
> + // CPUs not-under ejection: never see any changes so they are fine.
> + //
> QemuSelector = mCpuHotEjectData->QemuSelectorMap[ProcessorNum];
> if (QemuSelector == CPU_EJECT_QEMU_SELECTOR_INVALID) {
> return;
> @@ -495,11 +601,6 @@ CpuHotplugMmi (
> if (EFI_ERROR (Status)) {
> goto Fatal;
> }
> - if (ToUnplugCount > 0) {
> - DEBUG ((DEBUG_ERROR, "%a: hot-unplug is not supported yet\n",
> - __FUNCTION__));
> - goto Fatal;
> - }
>
> if (PluggedCount > 0) {
> Status = ProcessHotAddedCpus (mPluggedApicIds, PluggedCount);
>
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