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 09:09:47 -0700
From: Alex Williamson <alex.williamson@...hat.com>
To: "Tian, Kevin" <kevin.tian@...el.com>
Cc: Ankit Agrawal <ankita@...dia.com>, Zhi Wang <zhi.wang.linux@...il.com>,
 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

On Thu, 8 Feb 2024 07:21:40 +0000
"Tian, Kevin" <kevin.tian@...el.com> wrote:

> > 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();
> 

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_core_device *nvdev,
        u64 offset = *ppos & VFIO_PCI_OFFSET_MASK;
        int ret;
 
+       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_core_device *nvdev,
        loff_t pos = *ppos & VFIO_PCI_OFFSET_MASK;
        int ret;
 
+       if (!mem_count)
+               return 0;
+
        ret = nvgrace_gpu_map_device_mem(index, nvdev);
        if (ret)
                return ret;