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

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Date:   Thu, 21 Jul 2022 08:54:39 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>,
        Alex Williamson <alex.williamson@...hat.com>
CC:     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: 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?