Re: [Qemu-arm] [RFC v3 00/15] ARM virt: PCDIMM/NVDIMM at 2TB

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Auger Eric (eric.auger@redhat.com) wrote:
> Hi,
> 
> On 7/3/18 9:19 AM, Eric Auger wrote:
> > This series aims at supporting PCDIMM/NVDIMM intantiation in
> > machvirt at 2TB guest physical address.
> > 
> > This is achieved in 3 steps:
> > 1) support more than 40b IPA/GPA
> > 2) support PCDIMM instantiation
> > 3) support NVDIMM instantiation
> 
> While respinning this series I have some general questions that raise up
> when thinking about extending the RAM on mach-virt:
> 
> At the moment mach-virt offers 255GB max initial RAM starting at 1GB
> ("-m " option).
> 
> This series does not touch this initial RAM and only targets to add
> device memory (usable for PCDIMM, NVDIMM, virtio-mem, virtio-pmem) in
> 3.1 machine, located at 2TB. 3.0 address map top currently is at 1TB
> (legacy aarch32 LPAE limit) so it would leave 1TB for IO or PCI. Is it OK?

Is there a reason not to make this configurable?
It sounds a perfectly reasonable number, but you wouldn't be too
surprised if someone came along with a pile of huge GPUs.

> - Putting device memory at 2TB means only ARMv8/aarch64 would get
> benefit of it. Is it an issue? ie. no device memory for ARMv7 or
> ARMv8/aarch32. Do we need to put effort supporting more memory and
> memory devices for those configs? there is less than 256GB free in the
> existing 1TB mach-virt memory map anyway.

They can always explicitly specify an address on a pc-dimm's addr
property can't they?

> - is it OK to rely only on device memory to extend the existing 255 GB
> RAM or would we need additional initial memory? device memory usage
> induces a more complex command line so this puts a constraint on upper
> layers. Is it acceptable though?

Check with a libvirt person?

> - I revisited the series so that the max IPA size shift would get
> automatically computed according to the top address reached by the
> device memory, ie. 2TB + (maxram_size - ramsize). So we would not need
> any additional kvm-type or explicit vm-phys-shift option to select the
> correct max IPA shift (or any CPU phys-bits as suggested by Dave). This
> also assumes we don't put anything beyond the device memory. It is OK?

Generically that probably sounds OK; be careful about how complex that
calculation gets, otherwise it might turn into a complex thing you have
to be careful of the effect of changing it (and eg if changing it causes
migration issues).

> - Igor told me we was concerned about the split-memory RAM model as it
> caused a lot of trouble regarding compat/migration on PC machine. After
> having studied the pc machine code I now wonder if we can compare the PC
> compat issues with the ones we could encounter on ARM with the proposed
> split memory model.
> 
> On PC there are many knobs to tune the RAM layout
> - max_ram_below_4g option tunes how much RAM we want below 4G
> - gigabyte_align to force 3GB versus 3.5GB lowmem limit if ram_size >
> max_ram_below_4g
> - plus the usual ram_size which affects the rest of the initial ram
> - plus the maxram_size, slots which affect the size of the device memory
> - the device memory is just behind the initial RAM, aligned to 1GB
> 
> Note the inital RAM and the device memory may be disjoint due to
> misalignment of the initial ram size against 1GB
> 
> On ARM, we would have 3.0 virt machine supporting only initial RAM from
> 1GB to 256 GB. 3.1 (or beyond ;-)) virt machine would support the same
> initial RAM + device memory from 2TB to 4TB.
> 
> With that memory split and the different machine type, I don't see any
> major hurdle with respect to migration. Do I miss something?

A lot of those knobs are there to keep migration compatibility due to
keeping behaviour the same for migration.

Dave

> Alternative to have a split model is having a floating RAM base for a
> contiguous initial + device memory (contiguity actually depends on
> initial RAM size alignment too). This requires significant changes in FW
> and also potentially impacts the legacy virt address map as we need to
> pass the RAM floating base address in some way (using an SRAM at 1GB) or
> using fw_cfg. Is it worth the effort? Also, Peter/Laszlo mentioned their
> reluctance to move the RAM earlier
> (https://lists.gnu.org/archive/html/qemu-devel/2017-10/msg03172.html).
> 
> Your feedbacks on those points are really welcome!
> 
> Thanks
> 
> Eric
> 
> > 
> > This series reuses/rebases patches initially submitted by Shameer in [1]
> > and Kwangwoo in [2].
> > 
> > I put all parts all together for consistency and due to dependencies
> > however as soon as the kernel dependency is resolved we can consider
> > upstreaming them separately.
> > 
> > Support more than 40b IPA/GPA [ patches 1 - 5 ]
> > -----------------------------------------------
> > was "[RFC 0/6] KVM/ARM: Dynamic and larger GPA size"
> > 
> > At the moment the guest physical address space is limited to 40b
> > due to KVM limitations. [0] bumps this limitation and allows to
> > create a VM with up to 52b GPA address space.
> > 
> > With this series, QEMU creates a virt VM with the max IPA range
> > reported by the host kernel or 40b by default.
> > 
> > This choice can be overriden by using the -machine kvm-type=<bits>
> > option with bits within [40, 52]. If <bits> are not supported by
> > the host, the legacy 40b value is used.
> > 
> > Currently the EDK2 FW also hardcodes the max number of GPA bits to
> > 40. This will need to be fixed.
> > 
> > PCDIMM Support [ patches 6 - 11 ]
> > ---------------------------------
> > was "[RFC 0/5] ARM virt: Support PC-DIMM at 2TB"
> > 
> > We instantiate the device_memory at 2TB. Using it obviously requires
> > at least 42b of IPA/GPA. While its max capacity is currently limited
> > to 2TB, the actual size depends on the initial guest RAM size and
> > maxmem parameter.
> > 
> > Actual hot-plug and hot-unplug of PC-DIMM is not suported due to lack
> > of support of those features in baremetal.
> > 
> > NVDIMM support [ patches 12 - 15 ]
> > ----------------------------------
> > 
> > Once the memory hotplug framework is in place it is fairly
> > straightforward to add support for NVDIMM. the machine "nvdimm" option
> > turns the capability on.
> > 
> > Best Regards
> > 
> > Eric
> > 
> > References:
> > 
> > [0] [PATCH v3 00/20] arm64: Dynamic & 52bit IPA support
> > https://www.spinics.net/lists/kernel/msg2841735.html
> > 
> > [1] [RFC v2 0/6] hw/arm: Add support for non-contiguous iova regions
> > http://patchwork.ozlabs.org/cover/914694/
> > 
> > [2] [RFC PATCH 0/3] add nvdimm support on AArch64 virt platform
> > https://lists.gnu.org/archive/html/qemu-devel/2016-07/msg04599.html
> > 
> > Tests:
> > - On Cavium Gigabyte, a 48b VM was created.
> > - Migration tests were performed between kernel supporting the
> >   feature and destination kernel not suporting it
> > - test with ACPI: to overcome the limitation of EDK2 FW, virt
> >   memory map was hacked to move the device memory below 1TB.
> > 
> > This series can be found at:
> > https://github.com/eauger/qemu/tree/v2.12.0-dimm-2tb-v3
> > 
> > History:
> > 
> > v2 -> v3:
> > - fix pc_q35 and pc_piix compilation error
> > - kwangwoo's email being not valid anymore, remove his address
> > 
> > v1 -> v2:
> > - kvm_get_max_vm_phys_shift moved in arch specific file
> > - addition of NVDIMM part
> > - single series
> > - rebase on David's refactoring
> > 
> > v1:
> > - was "[RFC 0/6] KVM/ARM: Dynamic and larger GPA size"
> > - was "[RFC 0/5] ARM virt: Support PC-DIMM at 2TB"
> > 
> > Best Regards
> > 
> > Eric
> > 
> > 
> > Eric Auger (9):
> >   linux-headers: header update for KVM/ARM KVM_ARM_GET_MAX_VM_PHYS_SHIFT
> >   hw/boards: Add a MachineState parameter to kvm_type callback
> >   kvm: add kvm_arm_get_max_vm_phys_shift
> >   hw/arm/virt: support kvm_type property
> >   hw/arm/virt: handle max_vm_phys_shift conflicts on migration
> >   hw/arm/virt: Allocate device_memory
> >   acpi: move build_srat_hotpluggable_memory to generic ACPI source
> >   hw/arm/boot: Expose the pmem nodes in the DT
> >   hw/arm/virt: Add nvdimm and nvdimm-persistence options
> > 
> > Kwangwoo Lee (2):
> >   nvdimm: use configurable ACPI IO base and size
> >   hw/arm/virt: Add nvdimm hot-plug infrastructure
> > 
> > Shameer Kolothum (4):
> >   hw/arm/virt: Add memory hotplug framework
> >   hw/arm/boot: introduce fdt_add_memory_node helper
> >   hw/arm/boot: Expose the PC-DIMM nodes in the DT
> >   hw/arm/virt-acpi-build: Add PC-DIMM in SRAT
> > 
> >  accel/kvm/kvm-all.c                            |   2 +-
> >  default-configs/arm-softmmu.mak                |   4 +
> >  hw/acpi/aml-build.c                            |  51 ++++
> >  hw/acpi/nvdimm.c                               |  28 ++-
> >  hw/arm/boot.c                                  | 123 +++++++--
> >  hw/arm/virt-acpi-build.c                       |  10 +
> >  hw/arm/virt.c                                  | 330 ++++++++++++++++++++++---
> >  hw/i386/acpi-build.c                           |  49 ----
> >  hw/i386/pc_piix.c                              |   8 +-
> >  hw/i386/pc_q35.c                               |   8 +-
> >  hw/ppc/mac_newworld.c                          |   2 +-
> >  hw/ppc/mac_oldworld.c                          |   2 +-
> >  hw/ppc/spapr.c                                 |   2 +-
> >  include/hw/acpi/aml-build.h                    |   3 +
> >  include/hw/arm/arm.h                           |   2 +
> >  include/hw/arm/virt.h                          |   7 +
> >  include/hw/boards.h                            |   2 +-
> >  include/hw/mem/nvdimm.h                        |  12 +
> >  include/standard-headers/linux/virtio_config.h |  16 +-
> >  linux-headers/asm-mips/unistd.h                |  18 +-
> >  linux-headers/asm-powerpc/kvm.h                |   1 +
> >  linux-headers/linux/kvm.h                      |  16 ++
> >  target/arm/kvm.c                               |   9 +
> >  target/arm/kvm_arm.h                           |  16 ++
> >  24 files changed, 597 insertions(+), 124 deletions(-)
> > 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK