netdev - Re: [PATCH V2 vfio 06/11] vfio: Introduce the DMA logging feature support

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Message-ID: <20220725143706.GD3747@nvidia.com>
Date:   Mon, 25 Jul 2022 11:37:06 -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 06/11] vfio: Introduce the DMA logging feature
 support

On Mon, Jul 25, 2022 at 07:38:52AM +0000, Tian, Kevin wrote:

> > Yes. qemu has to select a static aperture at start.
> > 
> >  The entire aperture is best, if that fails
> > 
> >  A smaller aperture and hope the guest doesn't use the whole space, if
> >  that fails,
> > 
> >  The entire guest physical map and hope the guest is in PT mode
> 
> That sounds a bit hacky... does it instead suggest that an interface
> for reporting the supported ranges on a tracker could be helpful once
> trying the entire aperture fails?

It is the "try and fail" approach. It gives the driver the most
flexability in processing the ranges to try and make them work. If we
attempt to describe all the device constraints that might exist we
will be here forever.

Eg the driver might be able to do the entire aperture, but it has to
use 2M pages or something.

Jason