linux-kernel - Re: [PATCH v4 1/1] vfio/nvgpu: Add vfio pci variant module for grace hopper

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Message-ID: <ZJm7a1BpeLmQQ5Mj@nvidia.com>
Date:   Mon, 26 Jun 2023 13:23:07 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     ankita@...dia.com, aniketa@...dia.com, cjia@...dia.com,
        kwankhede@...dia.com, targupta@...dia.com, vsethi@...dia.com,
        acurrid@...dia.com, apopple@...dia.com, jhubbard@...dia.com,
        danw@...dia.com, kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/1] vfio/nvgpu: Add vfio pci variant module for grace
 hopper

On Mon, Jun 26, 2023 at 10:06:29AM -0600, Alex Williamson wrote:
> On Fri, 23 Jun 2023 10:26:05 -0300
> Jason Gunthorpe <jgg@...dia.com> wrote:
> 
> > On Wed, Jun 21, 2023 at 08:07:20PM -0700, ankita@...dia.com wrote:
> > > +			if (caps.size) {
> > > +				info.flags |= VFIO_REGION_INFO_FLAG_CAPS;
> > > +				if (info.argsz < sizeof(info) + caps.size) {
> > > +					info.argsz = sizeof(info) + caps.size;
> > > +					info.cap_offset = 0;  
> > 
> > Shouldn't this be an error if we can't fit the caps into the response?
> > Silently discarding the caps seems wrong..
> 
> It's required for backwards compatibility.  If a userspace doesn't
> support the info ioctl capabilities chain, it gets the basic
> information successfully, while an enlightened userspace makes use of
> the flags to know that a capability chain is available but unreported
> due to an insufficient buffer size, with the required size being
> provided in the return structure.

Hum, maybe this part should be pushed into the cap helpers..

Jason