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Date: Thu, 8 Feb 2024 09:09:47 -0700
From: Alex Williamson <alex.williamson@redhat.com>
To: "Tian, Kevin" <kevin.tian@intel.com>
Cc: Ankit Agrawal <ankita@nvidia.com>, Zhi Wang <zhi.wang.linux@gmail.com>,
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Subject: Re: [PATCH v17 3/3] vfio/nvgrace-gpu: Add vfio pci variant module
 for grace hopper
Message-ID: <20240208090947.1254e72c.alex.williamson@redhat.com>
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On Thu, 8 Feb 2024 07:21:40 +0000
"Tian, Kevin" <kevin.tian@intel.com> wrote:

> > From: Ankit Agrawal <ankita@nvidia.com>
> > Sent: Thursday, February 8, 2024 3:13 PM =20
> > >> > +=C2=A0=C2=A0=C2=A0 * Determine how many bytes to be actually read=
 from the
> > >> > device memory.
> > >> > +=C2=A0=C2=A0=C2=A0 * Read request beyond the actual device memory=
 size is
> > >> > filled with ~0,
> > >> > +=C2=A0=C2=A0=C2=A0 * while those beyond the actual reported size =
is skipped.
> > >> > +=C2=A0=C2=A0=C2=A0 */
> > >> > +=C2=A0=C2=A0 if (offset >=3D memregion->memlength)
> > >> > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 mem_=
count =3D 0; =20
> > >>
> > >> If mem_count =3D=3D 0, going through nvgrace_gpu_map_and_read() is n=
ot
> > >> necessary. =20
> > >
> > > Harmless, other than the possibly unnecessary call through to
> > > nvgrace_gpu_map_device_mem().=C2=A0 Maybe both =20
> > nvgrace_gpu_map_and_read() =20
> > > and nvgrace_gpu_map_and_write() could conditionally return 0 as their
> > > first operation when !mem_count.=C2=A0 Thanks,
> > >
> > >Alex =20
> >=20
> > IMO, this seems like adding too much code to reduce the call length for=
 a
> > very specific case. If there aren't any strong opinion on this, I'm pla=
nning to
> > leave this code as it is. =20
>=20
> a slight difference. if mem_count=3D=3D0 the result should always succeed
> no matter nvgrace_gpu_map_device_mem() succeeds or not. Of course
> if it fails it's already a big problem probably nobody cares about the su=
btle
> difference when reading non-exist range.
>=20
> but regarding to readability it's still clearer:
>=20
> if (mem_count)
> 	nvgrace_gpu_map_and_read();
>=20

The below has better flow imo vs conditionalizing the call to
map_and_read/write and subsequent error handling, but I don't think
either adds too much code.  Thanks,

Alex

--- a/drivers/vfio/pci/nvgrace-gpu/main.c
+++ b/drivers/vfio/pci/nvgrace-gpu/main.c
@@ -429,6 +429,9 @@ nvgrace_gpu_map_and_read(struct nvgrace_gpu_vfio_pci_co=
re_device *nvdev,
        u64 offset =3D *ppos & VFIO_PCI_OFFSET_MASK;
        int ret;
=20
+       if (!mem_count)
+               return 0;
+
        /*
         * Handle read on the BAR regions. Map to the target device memory
         * physical address and copy to the request read buffer.
@@ -547,6 +550,9 @@ nvgrace_gpu_map_and_write(struct nvgrace_gpu_vfio_pci_c=
ore_device *nvdev,
        loff_t pos =3D *ppos & VFIO_PCI_OFFSET_MASK;
        int ret;
=20
+       if (!mem_count)
+               return 0;
+
        ret =3D nvgrace_gpu_map_device_mem(index, nvdev);
        if (ret)
                return ret;