linux-kernel - Re: [PATCH v5 1/1] vfio/nvgpu: Add vfio pci variant module for grace hopper

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Date:   Fri, 28 Jul 2023 09:21:44 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Ankit Agrawal <ankita@...dia.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, 28 Jul 2023 09:31:27 -0300
Jason Gunthorpe <jgg@...dia.com> wrote:

> 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.

Right, but also vfio_pci_core_read() doesn't know anything about this
virtual BAR2, so any in-range BAR2 accesses will error the same as
trying to access an unimplemented BAR.  It's not just the comment,
there's no code here to handle a read(2) from in-bound BAR2 accesses.
Thanks,

Alex