Re: [PATCH v4 1/1] vfio/nvgpu: Add vfio pci variant module for grace hopper

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

On Fri, 23 Jun 2023 10:26:05 -0300
Jason Gunthorpe <jgg@nvidia.com> wrote:

> On Wed, Jun 21, 2023 at 08:07:20PM -0700, ankita@nvidia.com wrote:
> > +			if (caps.size) {
> > +				info.flags |= VFIO_REGION_INFO_FLAG_CAPS;
> > +				if (info.argsz < sizeof(info) + caps.size) {
> > +					info.argsz = sizeof(info) + caps.size;
> > +					info.cap_offset = 0;  
> 
> Shouldn't this be an error if we can't fit the caps into the response?
> Silently discarding the caps seems wrong..

It's required for backwards compatibility.  If a userspace doesn't
support the info ioctl capabilities chain, it gets the basic
information successfully, while an enlightened userspace makes use of
the flags to know that a capability chain is available but unreported
due to an insufficient buffer size, with the required size being
provided in the return structure.
 
> > +static ssize_t nvgpu_vfio_pci_read(struct vfio_device *core_vdev,
> > +		char __user *buf, size_t count, loff_t *ppos)
> > +{
> > +	unsigned int index = VFIO_PCI_OFFSET_TO_INDEX(*ppos);
> > +
> > +	/*
> > +	 * Only the device memory present on the hardware is mapped, which may
> > +	 * not be power-of-2 aligned. A read to the BAR2 region implies an
> > +	 * access outside the available device memory on the hardware.
> > +	 */
> > +	if (index == VFIO_PCI_BAR2_REGION_INDEX)
> > +		return -EINVAL;  
> 
> What does the qemu do in this case? Crash the VM?

Yes, I don't think return -errno matches what we discussed for
returning -1 on read and dropping writes outside of the device memory.
Also see comments in my review that read/write should handle the
coherent memory area as well, the device should work with x-no-mmap=on.
Thanks,

Alex