Re: [PATCH kernel v7 20/20] vfio_pci: Add NVIDIA GV100GL [Tesla V100 SXM2] subdriver

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 21/12/2018 03:46, Alex Williamson wrote:
> On Thu, 20 Dec 2018 19:23:50 +1100
> Alexey Kardashevskiy <aik@ozlabs.ru> wrote:
> 
>> POWER9 Witherspoon machines come with 4 or 6 V100 GPUs which are not
>> pluggable PCIe devices but still have PCIe links which are used
>> for config space and MMIO. In addition to that the GPUs have 6 NVLinks
>> which are connected to other GPUs and the POWER9 CPU. POWER9 chips
>> have a special unit on a die called an NPU which is an NVLink2 host bus
>> adapter with p2p connections to 2 to 3 GPUs, 3 or 2 NVLinks to each.
>> These systems also support ATS (address translation services) which is
>> a part of the NVLink2 protocol. Such GPUs also share on-board RAM
>> (16GB or 32GB) to the system via the same NVLink2 so a CPU has
>> cache-coherent access to a GPU RAM.
>>
>> This exports GPU RAM to the userspace as a new VFIO device region. This
>> preregisters the new memory as device memory as it might be used for DMA.
>> This inserts pfns from the fault handler as the GPU memory is not onlined
>> until the vendor driver is loaded and trained the NVLinks so doing this
>> earlier causes low level errors which we fence in the firmware so
>> it does not hurt the host system but still better be avoided; for the same
>> reason this does not map GPU RAM into the host kernel (usual thing for
>> emulated access otherwise).
>>
>> This exports an ATSD (Address Translation Shootdown) register of NPU which
>> allows TLB invalidations inside GPU for an operating system. The register
>> conveniently occupies a single 64k page. It is also presented to
>> the userspace as a new VFIO device region. One NPU has 8 ATSD registers,
>> each of them can be used for TLB invalidation in a GPU linked to this NPU.
>> This allocates one ATSD register per an NVLink bridge allowing passing
>> up to 6 registers. Due to the host firmware bug (just recently fixed),
>> only 1 ATSD register per NPU was actually advertised to the host system
>> so this passes that alone register via the first NVLink bridge device in
>> the group which is still enough as QEMU collects them all back and
>> presents to the guest via vPHB to mimic the emulated NPU PHB on the host.
>>
>> In order to provide the userspace with the information about GPU-to-NVLink
>> connections, this exports an additional capability called "tgt"
>> (which is an abbreviated host system bus address). The "tgt" property
>> tells the GPU its own system address and allows the guest driver to
>> conglomerate the routing information so each GPU knows how to get directly
>> to the other GPUs.
>>
>> For ATS to work, the nest MMU (an NVIDIA block in a P9 CPU) needs to
>> know LPID (a logical partition ID or a KVM guest hardware ID in other
>> words) and PID (a memory context ID of a userspace process, not to be
>> confused with a linux pid). This assigns a GPU to LPID in the NPU and
>> this is why this adds a listener for KVM on an IOMMU group. A PID comes
>> via NVLink from a GPU and NPU uses a PID wildcard to pass it through.
>>
>> This requires coherent memory and ATSD to be available on the host as
>> the GPU vendor only supports configurations with both features enabled
>> and other configurations are known not to work. Because of this and
>> because of the ways the features are advertised to the host system
>> (which is a device tree with very platform specific properties),
>> this requires enabled POWERNV platform.
>>
>> The V100 GPUs do not advertise any of these capabilities via the config
>> space and there are more than just one device ID so this relies on
>> the platform to tell whether these GPUs have special abilities such as
>> NVLinks.
>>
>> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
>> ---
>> Changes:
>> v6.1:
>> * fixed outdated comment about VFIO_REGION_INFO_CAP_NVLINK2_LNKSPD
>>
>> v6:
>> * reworked capabilities - tgt for nvlink and gpu and link-speed
>> for nvlink only
>>
>> v5:
>> * do not memremap GPU RAM for emulation, map it only when it is needed
>> * allocate 1 ATSD register per NVLink bridge, if none left, then expose
>> the region with a zero size
>> * separate caps per device type
>> * addressed AW review comments
>>
>> v4:
>> * added nvlink-speed to the NPU bridge capability as this turned out to
>> be not a constant value
>> * instead of looking at the exact device ID (which also changes from system
>> to system), now this (indirectly) looks at the device tree to know
>> if GPU and NPU support NVLink
>>
>> v3:
>> * reworded the commit log about tgt
>> * added tracepoints (do we want them enabled for entire vfio-pci?)
>> * added code comments
>> * added write|mmap flags to the new regions
>> * auto enabled VFIO_PCI_NVLINK2 config option
>> * added 'tgt' capability to a GPU so QEMU can recreate ibm,npu and ibm,gpu
>> references; there are required by the NVIDIA driver
>> * keep notifier registered only for short time
>> ---
>>  drivers/vfio/pci/Makefile           |   1 +
>>  drivers/vfio/pci/trace.h            | 102 ++++++
>>  drivers/vfio/pci/vfio_pci_private.h |  14 +
>>  include/uapi/linux/vfio.h           |  37 +++
>>  drivers/vfio/pci/vfio_pci.c         |  27 +-
>>  drivers/vfio/pci/vfio_pci_nvlink2.c | 482 ++++++++++++++++++++++++++++
>>  drivers/vfio/pci/Kconfig            |   6 +
>>  7 files changed, 667 insertions(+), 2 deletions(-)
>>  create mode 100644 drivers/vfio/pci/trace.h
>>  create mode 100644 drivers/vfio/pci/vfio_pci_nvlink2.c
>>
> ...
>> diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
>> index 8131028..5562587 100644
>> --- a/include/uapi/linux/vfio.h
>> +++ b/include/uapi/linux/vfio.h
>> @@ -353,6 +353,21 @@ struct vfio_region_gfx_edid {
>>  #define VFIO_DEVICE_GFX_LINK_STATE_DOWN  2
>>  };
>>  
>> +/*
>> + * 10de vendor sub-type
>> + *
>> + * NVIDIA GPU NVlink2 RAM is coherent RAM mapped onto the host address space.
>> + */
>> +#define VFIO_REGION_SUBTYPE_NVIDIA_NVLINK2_RAM	(1)
>> +
>> +/*
>> + * 1014 vendor sub-type
>> + *
>> + * IBM NPU NVlink2 ATSD (Address Translation Shootdown) register of NPU
>> + * to do TLB invalidation on a GPU.
>> + */
>> +#define VFIO_REGION_SUBTYPE_IBM_NVLINK2_ATSD	(1)
>> +
>>  /*
>>   * The MSIX mappable capability informs that MSIX data of a BAR can be mmapped
>>   * which allows direct access to non-MSIX registers which happened to be within
>> @@ -363,6 +378,28 @@ struct vfio_region_gfx_edid {
>>   */
>>  #define VFIO_REGION_INFO_CAP_MSIX_MAPPABLE	3
>>  
>> +/*
>> + * Capability with compressed real address (aka SSA - small system address)
>> + * where GPU RAM is mapped on a system bus. Used by a GPU for DMA routing.
>> + */
>> +#define VFIO_REGION_INFO_CAP_NVLINK2_SSATGT	4
>> +
>> +struct vfio_region_info_cap_nvlink2_ssatgt {
>> +	struct vfio_info_cap_header header;
>> +	__u64 tgt;
>> +};
>> +
>> +/*
>> + * Capability with an NVLink link speed.
>> + */
> 
> I was really hoping for something more like SSATGT above indicating the
> intended users and purpose, and an update to SSATGT since it's now used
> by both the GPU and NPU2.  This comment is correct, but it's basically
> useless, it doesn't provide any information that isn't readily apparent
> from the structure definition.  AIUI, SSATGT is used not only for the
> GPU to determine where its RAM is mapped on the system bus, but also by
> the NPU2 to associate itself to a GPU, right?

Correct. It could be improved by

diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
index 5562587..ff238ef9c 100644
--- a/include/uapi/linux/vfio.h
+++ b/include/uapi/linux/vfio.h
@@ -380,7 +380,8 @@ struct vfio_region_gfx_edid {

 /*
  * Capability with compressed real address (aka SSA - small system address)
- * where GPU RAM is mapped on a system bus. Used by a GPU for DMA routing.
+ * where GPU RAM is mapped on a system bus. Used by a GPU for DMA routing
+ * and by the userspace to associate a NVLink bridge with a GPU.
  */
 #define VFIO_REGION_INFO_CAP_NVLINK2_SSATGT    4



> And the link speed here
> is consumed by the NPU2 in order to fill in DT information for the
> guest for compatibility and possibly routing optimizations?


It is just some speed number, 8 or 9, one works and the other does not,
depending on the actual system. The NVIDIA driver handles it in the
binary blob. The existing comment is not much use but I am really not
sure what other comment could be useful in here.






> 
> Alex
> 
>> +#define VFIO_REGION_INFO_CAP_NVLINK2_LNKSPD	5
>> +
>> +struct vfio_region_info_cap_nvlink2_lnkspd {
>> +	struct vfio_info_cap_header header;
>> +	__u32 link_speed;
>> +	__u32 __pad;
>> +};
>> +

-- 
Alexey