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

[Alex Williamson <alex.williamson@redhat.com>]

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  
> > >> > +    * 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;