Re: [RFC] [PATCH 0/2] Proposal to add EFI_MP_SERVICES_PROTOCOL support for AARCH64

["Leif Lindholm" <leif@nuviainc.com>]

On Thu, Oct 07, 2021 at 12:03:30 +0200, Ard Biesheuvel wrote:
> On Thu, 7 Oct 2021 at 11:41, Leif Lindholm <leif@nuviainc.com> wrote:
> >
> > Ard/Sami - any comments?
> >
> 
> The code changes by itself look fine to me. The only problem I see is
> that we cannot run arbitrary code on other cores with the MMU off,
> given that in that case,

This is a good point, and something we at the very least should point
out more explicitly - and perhaps ought to provide a helper library to
do - ...

> they don't comply with the UEFI AArch64
> platform requirements, most notably the one that says that unaligned
> accesses must be permitted.

... but I'll note that EFI_MP_SERVICES_PROTOCOL is one of those
incorrectly named protocols defined in PI, not UEFI.

> So either we severely constrain the kind of code that we permit to run
> on other cores, or we enable the MMU and caches on each core as it
> comes out of reset, as well as do any other CPU specific
> initialization that we do for the primary core as well.

The description for StartupAllAPs() has a note:
It is the responsibility of the consumer of the
EFI_MP_SERVICES_PROTOCOL.StartupAllAPs() to make sure that the nature
of the code that is executed on the BSP and the dispatched APs is well
controlled. The MP Services Protocol does not guarantee that the
Procedure function is MP-safe. Hence, the tasks that can be run in
parallel are limited to certain independent tasks and well-controlled
exclusive code. EFI services and protocols may not be called by APs
unless otherwise specified.

So I think this is actually fine, implementation-wise. *Except* for
the SwitchBSP function (where we're currently bailing out anyway).

Regards,

Leif

> > On Tue, Sep 28, 2021 at 12:14:35 +0100, Leif Lindholm wrote:
> > > On Fri, Sep 24, 2021 at 20:17:50 -0600, Rebecca Cran wrote:
> > > > I'd like to propose adding EFI_MP_SERVICES_PROTOCOL support for
> > > > AARCH64 systems. I've attached two patches to implement support for it
> > > > in the DXE phase, based on code in EmulatorPkg and UefiCpuPkg. It's added
> > > > under ArmPkg for now, but longer term it should probably be moved into
> > > > UefiCpuPkg.
> > > >
> > > > Patch 1/2 is the start of addressing the issue that the Aff0 field of
> > > > the MPIDR is no longer guaranteed to be the core, and should be referred
> > > > to in a more generic way: for example it could be the thread, with Aff1
> > > > being the core and Aff2 the cluster. Clearly much more work is needed
> > > > to fully remove that assumption.
> > >
> > > Just to add to this:
> > > Aff0 was never defined by the architecture to be the "core", it was
> > > just the smallest schedulable entity. The intent being that whether
> > > you had multiple hardware threads per core or not, you could just use
> > > the affinity to determine whether
> > > There is also a bit in the MPIDR to indicate whether the core *had*
> > > multiple hardware threads.
> > >
> > > In recent processors (without any change to the architecture), ARM
> > > thought it would be beneficial to keep software developers on their
> > > toes by starting to use the hyperthreading layout even for processors
> > > without hyperthreading support. I.e. Aff0 is always 0 even though MT
> > > is 0:
> > > https://developer.arm.com/documentation/100798/0301/Register-descriptions/AArch64-system-registers/MPIDR-EL1--Multiprocessor-Affinity-Register--EL1
> > > The justification being that an SoC might contain both processors
> > > with and without multiple hardware threads per core.
> > >
> > > Anyway, the point is that from at least Cortex-A76 onwards, Aff0 no
> > > longer maps to CoreId universally, and Aff1 no longer maps to
> > > ClusterId, for all non-threaded implementations.
> > > So we need to start cleaning up this use.
> > > This will possibly break some out-of-tree platforms, but I figure
> > > we're far enough from next stable tag for that not to matter too
> > > much.
> > >
> > > > Patch 2/2 implements the EFI_MP_SERVICES_PROTOCOL for DXE in Library/MpInitLib.
> > >
> > > Worth calling out in the cover letter that this is backed by PSCI.
> > >
> > > /
> > >     Leif
> > >
> > > > CpuDxe initializes the MP support, and as a result gains a dependency on
> > > > MpInitLib. ArmVirt.dsc.inc needs updated to add the new library, as will
> > > > all AARCH64 platforms.
> > > >
> > > > I sent out a patch for MdeModulePkg last week to add a corresponding test
> > > > application, MpServicesTest (see https://edk2.groups.io/g/devel/message/80878).
> > > >
> > > > Rebecca Cran (2):
> > > >   ArmPkg: Replace CoreId and ClusterId with Mpidr in ARM_CORE_INFO
> > > >     struct
> > > >   ArmPkg: Add Library/MpInitLib to support EFI_MP_SERVICES_PROTOCOL
> > > >
> > > >  ArmPkg/ArmPkg.dec                                              |    4 +
> > > >  ArmPkg/ArmPkg.dsc                                              |    4 +
> > > >  ArmPkg/Drivers/CpuDxe/AArch64/Arch.c                           |   22 +
> > > >  ArmPkg/Drivers/CpuDxe/Arm/Arch.c                               |   22 +
> > > >  ArmPkg/Drivers/CpuDxe/CpuDxe.c                                 |    2 +
> > > >  ArmPkg/Drivers/CpuDxe/CpuDxe.h                                 |   10 +
> > > >  ArmPkg/Drivers/CpuDxe/CpuDxe.inf                               |    6 +
> > > >  ArmPkg/Drivers/CpuDxe/CpuMpInit.c                              |  604 ++++++++
> > > >  ArmPkg/Include/Guid/ArmMpCoreInfo.h                            |    3 +-
> > > >  ArmPkg/Include/Library/ArmLib.h                                |    4 +
> > > >  ArmPkg/Include/Library/MpInitLib.h                             |  366 +++++
> > > >  ArmPkg/Library/MpInitLib/AArch64/MpFuncs.S                     |   61 +
> > > >  ArmPkg/Library/MpInitLib/DxeMpInitLib.inf                      |   53 +
> > > >  ArmPkg/Library/MpInitLib/DxeMpLib.c                            | 1498 ++++++++++++++++++++
> > > >  ArmPkg/Library/MpInitLib/InternalMpInitLib.h                   |  358 +++++
> > > >  ArmPlatformPkg/Library/ArmPlatformLibNull/ArmPlatformLibNull.c |    8 +-
> > > >  ArmPlatformPkg/PrePeiCore/MainMPCore.c                         |    2 +-
> > > >  ArmPlatformPkg/PrePi/MainMPCore.c                              |    2 +-
> > > >  ArmVirtPkg/ArmVirt.dsc.inc                                     |    3 +
> > > >  19 files changed, 3024 insertions(+), 8 deletions(-)
> > > >  create mode 100644 ArmPkg/Drivers/CpuDxe/AArch64/Arch.c
> > > >  create mode 100644 ArmPkg/Drivers/CpuDxe/Arm/Arch.c
> > > >  create mode 100644 ArmPkg/Drivers/CpuDxe/CpuMpInit.c
> > > >  create mode 100644 ArmPkg/Include/Library/MpInitLib.h
> > > >  create mode 100644 ArmPkg/Library/MpInitLib/AArch64/MpFuncs.S
> > > >  create mode 100644 ArmPkg/Library/MpInitLib/DxeMpInitLib.inf
> > > >  create mode 100644 ArmPkg/Library/MpInitLib/DxeMpLib.c
> > > >  create mode 100644 ArmPkg/Library/MpInitLib/InternalMpInitLib.h
> > > >
> > > > --
> > > > 2.31.1
> > > >