netdev - Re: [PATCH V2 vfio 03/11] vfio: Introduce DMA logging uAPIs

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Date:   Fri, 29 Jul 2022 11:11:46 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     Alex Williamson <alex.williamson@...hat.com>,
        Yishai Hadas <yishaih@...dia.com>,
        "saeedm@...dia.com" <saeedm@...dia.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "Martins, Joao" <joao.m.martins@...cle.com>,
        "leonro@...dia.com" <leonro@...dia.com>,
        "maorg@...dia.com" <maorg@...dia.com>,
        "cohuck@...hat.com" <cohuck@...hat.com>
Subject: Re: [PATCH V2 vfio 03/11] vfio: Introduce DMA logging uAPIs

On Fri, Jul 29, 2022 at 03:01:51AM +0000, Tian, Kevin wrote:
> > From: Jason Gunthorpe <jgg@...dia.com>
> > Sent: Thursday, July 28, 2022 8:06 PM
> > 
> > On Thu, Jul 28, 2022 at 04:05:04AM +0000, Tian, Kevin wrote:
> > 
> > > > I think this is not correct, just because we made it discoverable does
> > > > not absolve the kernel of compatibility. If we change the limit, eg to
> > > > 1, and a real userspace stops working then we still broke userspace.
> > >
> > > iiuc Alex's suggestion doesn't conflict with the 'try and fail' model.
> > > By using the reserved field of vfio_device_feature_dma_logging_control
> > > to return the limit of the specified page_size from a given tracker,
> > > the user can quickly retry and adapt to that limit if workable.
> > 
> > Again, no it can't. The marshalling limit is not the device limit and
> > it will still potentially fail. Userspace does not gain much
> > additional API certainty by knowing this internal limit.
> 
> Why cannot the tracker return device limit here?

Because it is "complicated". mlx5 doesn't really have a limit in
simple terms of the number of ranges - it has other limits based on
total span and page size.

Jason