Re: [PATCH v1 1/1] vfio/nvgrace-gpu: Convey kvm that the device is wc safe

[Jason Gunthorpe <jgg@nvidia.com>]

On Thu, Feb 29, 2024 at 08:56:39AM -0700, Alex Williamson wrote:
> On Wed, 28 Feb 2024 19:48:01 +0000
> <ankita@nvidia.com> wrote:
> 
> > From: Ankit Agrawal <ankita@nvidia.com>
> > 
> > The NVIDIA Grace Hopper GPUs have device memory that is supposed to be
> > used as a regular RAM. It is accessible through CPU-GPU chip-to-chip
> > cache coherent interconnect and is present in the system physical
> > address space. The device memory is split into two regions - termed
> > as usemem and resmem - in the system physical address space,
> > with each region mapped and exposed to the VM as a separate fake
> > device BAR [1].
> > 
> > Owing to a hardware defect for Multi-Instance GPU (MIG) feature [2],
> > there is a requirement - as a workaround - for the resmem BAR to
> > display uncached memory characteristics. Based on [3], on system with
> > FWB enabled such as Grace Hopper, the requisite properties
> > (uncached, unaligned access) can be achieved through a VM mapping (S1)
> > of NORMAL_NC and host mapping (S2) of MT_S2_FWB_NORMAL_NC.
> > 
> > KVM currently maps the MMIO region in S2 as MT_S2_FWB_DEVICE_nGnRE by
> > default. The fake device BARs thus displays DEVICE_nGnRE behavior in the
> > VM.
> > 
> > The following table summarizes the behavior for the various S1 and S2
> > mapping combinations for systems with FWB enabled [3].
> > S1           |  S2           | Result
> > NORMAL_WB    |  NORMAL_NC    | NORMAL_NC
> > NORMAL_WT    |  NORMAL_NC    | NORMAL_NC
> > NORMAL_NC    |  NORMAL_NC    | NORMAL_NC
> > NORMAL_WB    |  DEVICE_nGnRE | DEVICE_nGnRE
> > NORMAL_WT    |  DEVICE_nGnRE | DEVICE_nGnRE
> > NORMAL_NC    |  DEVICE_nGnRE | DEVICE_nGnRE
> > 
> > Recently a change was added that modifies this default behavior and
> > make KVM map MMIO as MT_S2_FWB_NORMAL_NC when a VMA flag
> > VM_ALLOW_ANY_UNCACHED is set. Setting S2 as MT_S2_FWB_NORMAL_NC
> > provides the desired behavior (uncached, unaligned access) for resmem.
> > 
> > Such setting is extended to the usemem as a middle-of-the-road
> > setting to take it closer to the desired final system memory
> > characteristics (cached, unaligned). This will eventually be
> > fixed with the ongoing proposal [4].
> > 
> > To use VM_ALLOW_ANY_UNCACHED flag, the platform must guarantee that
> > no action taken on the MMIO mapping can trigger an uncontained
> > failure. The Grace Hopper satisfies this requirement. So set
> > the VM_ALLOW_ANY_UNCACHED flag in the VMA.
> > 
> > Applied over next-20240227.
> > base-commit: 22ba90670a51
> > 
> > Link: https://lore.kernel.org/all/20240220115055.23546-4-ankita@nvidia.com/ [1]
> > Link: https://www.nvidia.com/en-in/technologies/multi-instance-gpu/ [2]
> > Link: https://developer.arm.com/documentation/ddi0487/latest/ section D8.5.5 [3]
> > Link: https://lore.kernel.org/all/20230907181459.18145-2-ankita@nvidia.com/ [4]
> > 
> > Cc: Alex Williamson <alex.williamson@redhat.com>
> > Cc: Kevin Tian <kevin.tian@intel.com>
> > Cc: Jason Gunthorpe <jgg@nvidia.com>
> > Cc: Vikram Sethi <vsethi@nvidia.com>
> > Cc: Zhi Wang <zhiw@nvidia.com>
> > Signed-off-by: Ankit Agrawal <ankita@nvidia.com>
> > ---
> >  drivers/vfio/pci/nvgrace-gpu/main.c | 18 ++++++++++++++++++
> >  1 file changed, 18 insertions(+)
> > 
> > diff --git a/drivers/vfio/pci/nvgrace-gpu/main.c b/drivers/vfio/pci/nvgrace-gpu/main.c
> > index 25814006352d..5539c9057212 100644
> > --- a/drivers/vfio/pci/nvgrace-gpu/main.c
> > +++ b/drivers/vfio/pci/nvgrace-gpu/main.c
> > @@ -181,6 +181,24 @@ static int nvgrace_gpu_mmap(struct vfio_device *core_vdev,
> >  
> >  	vma->vm_pgoff = start_pfn;
> >  
> > +	/*
> > +	 * The VM_ALLOW_ANY_UNCACHED VMA flag is implemented for ARM64,
> > +	 * allowing KVM stage 2 device mapping attributes to use Normal-NC
> > +	 * rather than DEVICE_nGnRE, which allows guest mappings
> > +	 * supporting write-combining attributes (WC). This also
> > +	 * unlocks memory-like operations such as unaligned accesses.
> > +	 * This setting suits the fake BARs as they are expected to
> > +	 * demonstrate such properties within the guest.
> > +	 *
> > +	 * ARM does not architecturally guarantee this is safe, and indeed
> > +	 * some MMIO regions like the GICv2 VCPU interface can trigger
> > +	 * uncontained faults if Normal-NC is used. The nvgrace-gpu
> > +	 * however is safe in that the platform guarantees that no
> > +	 * action taken on the MMIO mapping can trigger an uncontained
> > +	 * failure. Hence VM_ALLOW_ANY_UNCACHED is set in the VMA flags.
> > +	 */
> > +	vm_flags_set(vma, VM_ALLOW_ANY_UNCACHED);
> > +
> >  	return 0;
> >  }
> >  
> 
> The commit log sort of covers it, but this comment doesn't seem to
> cover why we're setting an uncached attribute to the usemem region
> which we're specifically mapping as coherent... did we end up giving
> this flag a really poor name if it's being used here to allow unaligned
> access?  Thanks,

Yeah, I sugged to fold that hunk into this:

        if (index == RESMEM_REGION_INDEX)
                vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot);

So it makes more sense. VM_ALLOW_ANY_UNCACHED shouldn't be used on the
cachable mapping. The comment should be more specific to this driver
and not so generic:

/*
 * nvgrace has no issue with uncontained failures on NORMAL_NC
 * access. Tell KVM to open up guest usage of NORMAL_NC for this mapping.
 */

Jason