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

[David Matlack <dmatlack@google.com>]

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?

> +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.

> +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().

> +
> +	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.

> +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_*()