Re: [PATCH v19 0/3] vfio/nvgrace-gpu: Add vfio pci variant module for grace hopper

[Alex Williamson <alex.williamson@redhat.com>]

On Tue, 20 Feb 2024 17:20:52 +0530
<ankita@nvidia.com> wrote:

> From: Ankit Agrawal <ankita@nvidia.com>
> 
> NVIDIA's upcoming Grace Hopper Superchip provides a PCI-like device
> for the on-chip GPU that is the logical OS representation of the
> internal proprietary chip-to-chip cache coherent interconnect.
> 
> The device is peculiar compared to a real PCI device in that whilst
> there is a real 64b PCI BAR1 (comprising region 2 & region 3) on the
> device, it is not used to access device memory once the faster
> chip-to-chip interconnect is initialized (occurs at the time of host
> system boot). The device memory is accessed instead using the
> chip-to-chip interconnect that is exposed as a contiguous physically
> addressable region on the host. Since the device memory is cache
> coherent with the CPU, it can be mmaped into the user VMA with a
> cacheable mapping and used like a regular RAM. The device memory is
> not added to the host kernel, but mapped directly as this reduces
> memory wastage due to struct pages.
> 
> There is also a requirement of a minimum reserved 1G uncached region
> (termed as resmem) to support the Multi-Instance GPU (MIG) feature [1].
> This is to work around a HW defect. Based on [2], the requisite properties
> (uncached, unaligned access) can be achieved through a VM mapping (S1)
> of NORMAL_NC and host (S2) mapping with MemAttr[2:0]=0b101. To provide
> a different non-cached property to the reserved 1G region, it needs to
> be carved out from the device memory and mapped as a separate region
> in Qemu VMA with pgprot_writecombine(). pgprot_writecombine() sets
> the Qemu VMA page properties (pgprot) as NORMAL_NC.
> 
> Provide a VFIO PCI variant driver that adapts the unique device memory
> representation into a more standard PCI representation facing userspace.
> 
> The variant driver exposes these two regions - the non-cached reserved
> (resmem) and the cached rest of the device memory (termed as usemem) as
> separate VFIO 64b BAR regions. This is divergent from the baremetal
> approach, where the device memory is exposed as a device memory region.
> The decision for a different approach was taken in view of the fact that
> it would necessiate additional code in Qemu to discover and insert those
> regions in the VM IPA, along with the additional VM ACPI DSDT changes to
> communiate the device memory region IPA to the VM workloads. Moreover,
> this behavior would have to be added to a variety of emulators (beyond
> top of tree Qemu) out there desiring grace hopper support.
> 
> Since the device implements 64-bit BAR0, the VFIO PCI variant driver
> maps the uncached carved out region to the next available PCI BAR (i.e.
> comprising of region 2 and 3). The cached device memory aperture is
> assigned BAR region 4 and 5. Qemu will then naturally generate a PCI
> device in the VM with the uncached aperture reported as BAR2 region,
> the cacheable as BAR4. The variant driver provides emulation for these
> fake BARs' PCI config space offset registers.
> 
> The hardware ensures that the system does not crash when the memory
> is accessed with the memory enable turned off. It synthesis ~0 reads
> and dropped writes on such access. So there is no need to support the
> disablement/enablement of BAR through PCI_COMMAND config space register.
> 
> The memory layout on the host looks like the following:
>                devmem (memlength)
> |--------------------------------------------------|
> |-------------cached------------------------|--NC--|
> |                                           |
> usemem.memphys                              resmem.memphys
> 
> PCI BARs need to be aligned to the power-of-2, but the actual memory on the
> device may not. A read or write access to the physical address from the
> last device PFN up to the next power-of-2 aligned physical address
> results in reading ~0 and dropped writes. Note that the GPU device
> driver [6] is capable of knowing the exact device memory size through
> separate means. The device memory size is primarily kept in the system
> ACPI tables for use by the VFIO PCI variant module.
> 
> Note that the usemem memory is added by the VM Nvidia device driver [5]
> to the VM kernel as memblocks. Hence make the usable memory size memblock
> (MEMBLK_SIZE) aligned. This is a hardwired ABI value between the GPU FW and
> VFIO driver. The VM device driver make use of the same value for its
> calculation to determine USEMEM size.
> 
> Currently there is no provision in KVM for a S2 mapping with
> MemAttr[2:0]=0b101, but there is an ongoing effort to provide the same [3].
> As previously mentioned, resmem is mapped pgprot_writecombine(), that
> sets the Qemu VMA page properties (pgprot) as NORMAL_NC. Using the
> proposed changes in [3] and [4], KVM marks the region with
> MemAttr[2:0]=0b101 in S2.
> 
> If the device memory properties are not present, the driver registers the
> vfio-pci-core function pointers. Since there are no ACPI memory properties
> generated for the VM, the variant driver inside the VM will only use
> the vfio-pci-core ops and hence try to map the BARs as non cached. This
> is not a problem as the CPUs have FWB enabled which blocks the VM
> mapping's ability to override the cacheability set by the host mapping.
> 
> This goes along with a qemu series [6] to provides the necessary
> implementation of the Grace Hopper Superchip firmware specification so
> that the guest operating system can see the correct ACPI modeling for
> the coherent GPU device. Verified with the CUDA workload in the VM.
> 
> [1] https://www.nvidia.com/en-in/technologies/multi-instance-gpu/
> [2] section D8.5.5 of https://developer.arm.com/documentation/ddi0487/latest/
> [3] https://lore.kernel.org/all/20240211174705.31992-1-ankita@nvidia.com/
> [4] https://lore.kernel.org/all/20230907181459.18145-2-ankita@nvidia.com/
> [5] https://github.com/NVIDIA/open-gpu-kernel-modules
> [6] https://lore.kernel.org/all/20231203060245.31593-1-ankita@nvidia.com/
> 
> Applied over v6.8-rc5.
> 
> Signed-off-by: Aniket Agashe <aniketa@nvidia.com>
> Signed-off-by: Ankit Agrawal <ankita@nvidia.com>
> ---
[snip]
> Ankit Agrawal (3):
>   vfio/pci: rename and export do_io_rw()
>   vfio/pci: rename and export range_intersect_range
>   vfio/nvgrace-gpu: Add vfio pci variant module for grace hopper
> 
>  MAINTAINERS                           |  16 +-
>  drivers/vfio/pci/Kconfig              |   2 +
>  drivers/vfio/pci/Makefile             |   2 +
>  drivers/vfio/pci/nvgrace-gpu/Kconfig  |  10 +
>  drivers/vfio/pci/nvgrace-gpu/Makefile |   3 +
>  drivers/vfio/pci/nvgrace-gpu/main.c   | 879 ++++++++++++++++++++++++++
>  drivers/vfio/pci/vfio_pci_config.c    |  42 ++
>  drivers/vfio/pci/vfio_pci_rdwr.c      |  16 +-
>  drivers/vfio/pci/virtio/main.c        |  72 +--
>  include/linux/vfio_pci_core.h         |  10 +-
>  10 files changed, 993 insertions(+), 59 deletions(-)
>  create mode 100644 drivers/vfio/pci/nvgrace-gpu/Kconfig
>  create mode 100644 drivers/vfio/pci/nvgrace-gpu/Makefile
>  create mode 100644 drivers/vfio/pci/nvgrace-gpu/main.c
> 

Applied to vfio next branch for v6.9.  Thanks,

Alex