On Thu, 8 Feb 2024 07:21:40 +0000
"Tian, Kevin" [off-list ref] wrote:

quoted

From: Ankit Agrawal <ankita@nvidia.com>
Sent: Thursday, February 8, 2024 3:13 PM  

quoted

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+    * 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()  

quoted

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;