linux-kernel - RE: [PATCH v17 3/3] vfio/nvgrace-gpu: Add vfio pci variant module for grace hopper

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Date: Thu, 8 Feb 2024 07:21:40 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: Ankit Agrawal <ankita@...dia.com>, Alex Williamson
	<alex.williamson@...hat.com>, Zhi Wang <zhi.wang.linux@...il.com>
CC: Jason Gunthorpe <jgg@...dia.com>, Yishai Hadas <yishaih@...dia.com>,
	"mst@...hat.com" <mst@...hat.com>, "shameerali.kolothum.thodi@...wei.com"
	<shameerali.kolothum.thodi@...wei.com>, "clg@...hat.com" <clg@...hat.com>,
	"oleksandr@...alenko.name" <oleksandr@...alenko.name>, "K V P, Satyanarayana"
	<satyanarayana.k.v.p@...el.com>, "eric.auger@...hat.com"
	<eric.auger@...hat.com>, "brett.creeley@....com" <brett.creeley@....com>,
	"horms@...nel.org" <horms@...nel.org>, Rahul Rameshbabu
	<rrameshbabu@...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>, "Currid,
 Andy" <acurrid@...dia.com>, Alistair Popple <apopple@...dia.com>, "John
 Hubbard" <jhubbard@...dia.com>, Dan Williams <danw@...dia.com>, "Anuj
 Aggarwal (SW-GPU)" <anuaggarwal@...dia.com>, Matt Ochs <mochs@...dia.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "virtualization@...ts.linux-foundation.org"
	<virtualization@...ts.linux-foundation.org>
Subject: RE: [PATCH v17 3/3] vfio/nvgrace-gpu: Add vfio pci variant module for
 grace hopper

> From: Ankit Agrawal <ankita@...dia.com>
> Sent: Thursday, February 8, 2024 3:13 PM
> >> > +    * Determine how many bytes to be actually read from the
> >> > device memory.
> >> > +    * Read request beyond the actual device memory size is
> >> > filled with ~0,
> >> > +    * while those beyond the actual reported size is skipped.
> >> > +    */
> >> > +   if (offset >= memregion->memlength)
> >> > +           mem_count = 0;
> >>
> >> If mem_count == 0, going through nvgrace_gpu_map_and_read() is not
> >> necessary.
> >
> > Harmless, other than the possibly unnecessary call through to
> > nvgrace_gpu_map_device_mem().  Maybe both
> nvgrace_gpu_map_and_read()
> > and nvgrace_gpu_map_and_write() could conditionally return 0 as their
> > first operation when !mem_count.  Thanks,
> >
> >Alex
> 
> 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 planning to
> leave this code as it is.

a slight difference. if mem_count==0 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 subtle
difference when reading non-exist range.

but regarding to readability it's still clearer:

if (mem_count)
	nvgrace_gpu_map_and_read();