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Message-ID: <ZMO1H/uepDTtAaet@nvidia.com>
Date: Fri, 28 Jul 2023 09:31:27 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Ankit Agrawal <ankita@...dia.com>
Cc: Alex Williamson <alex.williamson@...hat.com>,
Aniket Agashe <aniketa@...dia.com>, Neo Jia <cjia@...dia.com>,
Kirti Wankhede <kwankhede@...dia.com>,
"Tarun Gupta (SW-GPU)" <targupta@...dia.com>,
Vikram Sethi <vsethi@...dia.com>,
Andy Currid <acurrid@...dia.com>,
Alistair Popple <apopple@...dia.com>,
John Hubbard <jhubbard@...dia.com>,
Dan Williams <danw@...dia.com>,
"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 1/1] vfio/nvgpu: Add vfio pci variant module for grace
hopper
On Fri, Jul 28, 2023 at 04:36:05AM +0000, Ankit Agrawal wrote:
>
> >> +static ssize_t nvgrace_gpu_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);
> >> + struct nvgrace_gpu_vfio_pci_core_device *nvdev = container_of(
> >> + core_vdev, struct nvgrace_gpu_vfio_pci_core_device, core_device.vdev);
> >> + u64 offset = *ppos & VFIO_PCI_OFFSET_MASK;
> >> + u8 val = 0xFF;
> >> + size_t i;
> >> +
> >> + /*
> >> + * 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.
> >> + */
> >
> > This is not true, userspace has no requirement to only access BAR2 via
> > mmap. This should support reads from within the coherent memory area.
>
> Just to confirm, the ask is to just update the comment to reflect the behavior,
> right? (I missed to do that in this posting). Because we do redirect the call to
> vfio_pci_core_read() here which will perform the read that is within the device
> region. The read response to synthesize -1 is only for the range that is outside
> the device memory region.
This doesn't seem right, vfio_pci_core_read() will use pci_iomap() to
get a mapping which will be a DEVICE mapping, this will make the
access incoherent with any cachable mappings.
Jason
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