Re: [PATCH v12 4/4] selftests/vfio: Add NVIDIA Falcon driver for DMA testing

[Alex Williamson <alex@shazbot.org>]

On Mon, 6 Apr 2026 23:46:53 +0000
David Matlack <dmatlack@google.com> wrote:

> On 2026-04-03 04:44 PM, Rubin Du wrote:
> > Add a new VFIO PCI driver for NVIDIA GPUs that enables DMA testing
> > via the Falcon (Fast Logic Controller) microcontrollers. This driver
> > extracts and adapts the DMA test functionality from NVIDIA's
> > gpu-admin-tools project and integrates it into the existing VFIO
> > selftest framework.
> > 
> > Falcons are general-purpose microcontrollers present on NVIDIA GPUs
> > that can perform DMA operations between system memory and device
> > memory. By leveraging Falcon DMA, this driver allows NVIDIA GPUs to
> > be tested alongside Intel IOAT and DSA devices using the same
> > selftest infrastructure.
> > 
> > The driver is named 'nv_falcon' to reflect that it specifically
> > controls the Falcon microcontrollers for DMA operations, rather
> > than exposing general GPU functionality.
> > 
> > Reference implementation:
> > https://github.com/NVIDIA/gpu-admin-tools
> > 
> > Signed-off-by: Rubin Du <rubind@nvidia.com>  
> 
> Will anyone from Nvidia be able to help review the correctness of the
> driver?

You can tell by the versioning snafu that we've seen a bunch of
versions of this internally, so we think it's correct.  We also have
the reference implementation in the above link if anyone else wants to
compare.
 
> > +static int gpu_poll_register(struct vfio_pci_device *device,
> > +			     const char *name, u32 offset,
> > +			     u32 expected, u32 mask, u32 timeout_ms)
> > +{
> > +	struct gpu_device *gpu = to_gpu_device(device);
> > +	struct timespec start, now;
> > +	u64 elapsed_ms;
> > +	u32 value;
> > +
> > +	clock_gettime(CLOCK_MONOTONIC, &start);
> > +
> > +	for (;;) {
> > +		value = gpu_read32(gpu, offset);
> > +		if ((value & mask) == expected)
> > +			return 0;
> > +
> > +		clock_gettime(CLOCK_MONOTONIC, &now);
> > +		elapsed_ms = (now.tv_sec - start.tv_sec) * 1000
> > +			     + (now.tv_nsec - start.tv_nsec) / 1000000;
> > +
> > +		if (elapsed_ms >= timeout_ms)
> > +			break;
> > +
> > +		usleep(1000);
> > +	}
> > +
> > +	dev_err(device,
> > +		"Timeout polling %s (0x%x): value=0x%x expected=0x%x mask=0x%x after %llu ms\n",
> > +		name, offset, value, expected, mask,
> > +		(unsigned long long)elapsed_ms);  
> 
> nit: You can replace ll with l in the format string and drop the cast to
> unsigned long long.
> 
> We only support VFIO selftests on 64-bit architectures, and that matches
> what existing printfs for u64 use in VFIO selftests.

Yup

> > +static bool fsp_check_ofa_dma_support(struct vfio_pci_device *device)
> > +{
> > +	struct gpu_device *gpu = to_gpu_device(device);
> > +	u32 val = gpu_read32(gpu, NV_OFA_DMA_SUPPORT_CHECK_REG);
> > +
> > +	return (val >> 16) != 0xbadf;
> > +}
> > +
> > +static int size_to_dma_encoding(u64 size)
> > +{
> > +	size = min_t(u64, size, NV_FALCON_DMA_MAX_TRANSFER_SIZE);  
> 
> It should be impossible for size to be greater than
> NV_FALCON_DMA_MAX_TRANSFER_SIZE. This should be dropped or converted
> into a VFIO_ASSERT_LE().

Ok, I think the below tests can also become asserts.

> > +
> > +	if (!size || (size & 0x3))
> > +		return -EINVAL;
> > +
> > +	return ffs(size) - 3;  
> 
> Sashiko pointed out that this can lead to partial memcpys:
> 
> . If a non-power-of-two size is passed (for example, 24 bytes), ffs(size) will
> . return the lowest set bit (bit 4, yielding an encoding for 8 bytes).
> .
> . Because nv_gpu_memcpy_wait() performs exactly one chunk transfer and does not
> . loop over the remainder, could this silently drop the remaining bytes and
> . cause a partial data copy? Should this check if the size is a power of 2, or
> . should the caller loop to handle remainders?
> 
> https://sashiko.dev/#/patchset/20260403234444.350867-1-rubind%40nvidia.com?part=4
> 
> I think this nuance needs to be handled within the nv_falcon driver.
> vfio_pci_driver_memcpy_start() just guarantees that size <=
> NV_FALCON_DMA_MAX_TRANSFER_SIZE.
> 
> Perhaps this is why you had the loop in an earlier version of the
> patchset, in which case it was my mistake to ask you to remove it!
> 
> When you add the loop back please add a comment to the loop to explain
> that it is necessary since the region needs to be sliced up into
> power-of-2 sizes for Falcon.

Yes, shouldn't have been removed.  We can't do arbitrary sizes, only
power-of-2 down to 4-bytes.  We can make this more self documenting and
assert on memcpy_start for the size alignment requirement.

> > +const struct vfio_pci_driver_ops nv_falcon_ops = {
> > +	.name = "nv_falcon",
> > +	.probe = nv_gpu_probe,
> > +	.init = nv_gpu_init,
> > +	.remove = nv_gpu_remove,
> > +	.memcpy_start = nv_gpu_memcpy_start,
> > +	.memcpy_wait = nv_gpu_memcpy_wait,
> > +};  
> 
> Any particular reason these functions are named nv_gpu_*() instead of
> nv_falcon_*()

Yes, these can be made more consistent.  Thanks,

Alex