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

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Date:   Mon, 25 Jul 2022 07:38:52 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jason Gunthorpe <jgg@...dia.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

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Thursday, July 21, 2022 7:51 PM
> 
> On Thu, Jul 21, 2022 at 08:54:39AM +0000, Tian, Kevin wrote:
> > > From: Jason Gunthorpe <jgg@...dia.com>
> > > Sent: Wednesday, July 20, 2022 4:08 AM
> > >
> > > On Tue, Jul 19, 2022 at 01:25:14PM -0600, Alex Williamson wrote:
> > >
> > > > > We don't really expect user space to hit this limit, the RAM in QEMU is
> > > > > divided today to around ~12 ranges as we saw so far in our evaluation.
> > > >
> > > > There can be far more for vIOMMU use cases or non-QEMU drivers.
> > >
> > > Not really, it isn't dynamic so vIOMMU has to decide what it wants to
> > > track up front. It would never make sense to track based on what is
> > > currently mapped. So it will be some small list, probably a big linear
> > > chunk of the IOVA space.
> >
> > How would vIOMMU make such decision when the address space
> > is managed by the guest? it is dynamic and could be sparse. I'm
> > curious about any example a vIOMMU can use to generate such small
> > list. Would it be a single range based on aperture reported from the
> > kernel?
> 
> 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?

> 
> All of these options are small lists.
> 
> Any vIOMMU maps that are created outside of what was asked to be
> tracked have to be made permanently dirty by qemu.
> 
> Jason