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

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Date:   Thu, 21 Jul 2022 09:05:18 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     Yishai Hadas <yishaih@...dia.com>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.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 Thu, Jul 21, 2022 at 08:45:10AM +0000, Tian, Kevin wrote:

> > + * will format its internal logging to match the reporting page size, possibly
> > + * by replicating bits if the internal page size is lower than requested.
> 
> what's the purpose of this? I didn't quite get why an user would want to
> start tracking in one page size and then read back the dirty bitmap in
> another page size...

There may be multiple kernel trackers that are working with different
underlying block sizes, so the concept is userspace decides what block
size it wants to work in and the kernel side transparently adapts. The
math is simple so putting it in the kernel is convenient.

Effectively the general vision is that qemu would allocate one
reporting buffer and then invoke these IOCTLs in parallel on all the
trackers then process the single bitmap. Forcing qemu to allocate
bitmaps per tracker page size is just inefficient.

Jason