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

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Message-ID:
 <SA1PR12MB7199A9470EC2562C5BCC2FAFB0442@SA1PR12MB7199.namprd12.prod.outlook.com>
Date: Thu, 8 Feb 2024 07:12:57 +0000
From: Ankit Agrawal <ankita@...dia.com>
To: Alex Williamson <alex.williamson@...hat.com>, Zhi Wang
	<zhi.wang.linux@...il.com>
CC: Jason Gunthorpe <jgg@...dia.com>, Yishai Hadas <yishaih@...dia.com>,
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 Rameshbabu <rrameshbabu@...dia.com>, Aniket Agashe <aniketa@...dia.com>, Neo
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	<linux-kernel@...r.kernel.org>, "virtualization@...ts.linux-foundation.org"
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Subject: Re: [PATCH v17 3/3] vfio/nvgrace-gpu: Add vfio pci variant module for
 grace hopper

>> > +/* Memory size expected as non cached and reserved by the VM driver
>> > */ +#define RESMEM_SIZE 0x40000000
>> > +#define MEMBLK_SIZE 0x20000000
>> > +
>>
>> Maybe use SZ_* definitions in linux/size.h
>
> Good suggestion.

Ack.

>>
>> Better move this part to the place between vfio_pci_core_enable() and
>> vfio_pci_core_finish_enable() like others for respecting the expected
>> device initialization sequence of life cycle.
>>
>> It doesn't bite something right now, but think about when someone
>> changes the behavior of vfio_pci_core_finish_enable() in the future,
>> they have to propose a patch for this.
>
> Agree.

Good point, will move it.

>>
>> Wouldn't it be better to do the map in the open path?
>
> AIUI the device would typically be used exclusively through the mmap of
> these ranges, these mappings are only for pread/pwrite type accesses,
> so I think it makes sense to map them on demand.

That's right, agree with Alex to keep it on-demand.


>> > +    * 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.