Re: [virtio-comment] Re: [PATCH V2 6/6] virtio-pci: implement dirty page tracking

["Zhu, Lingshan" <lingshan.zhu@intel.com>]

On 11/6/2023 6:33 PM, Michael S. Tsirkin wrote:
> On Mon, Nov 06, 2023 at 11:58:03AM +0800, Zhu, Lingshan wrote:
> > On 11/6/2023 12:12 AM, Michael S. Tsirkin wrote:
> > > On Fri, Nov 03, 2023 at 03:47:34PM +0000, Parav Pandit wrote:
> > > > > > [1]
> > > > > > https://lists.oasis-open.org/archives/virtio-comment/202310/msg00475.h
> > > > > > tml
> > > > > you still need to explain why this does not work for pass-through.
> > > > It does not work for following reasons.
> > > > 1. Because all the fields that put on the member device are not in direct control of the hypervisor.
> > > > The device is directly controlled by the guest including the device status and when it resets the device all the things stored in the device are lost.
> > > I think the idea is that when this gateway is in the device then
> > > device reset has to trap. At a high level, ok. But then what?
> > No, when device reset, the device is expected to forget everything and
> > re-intialize.
> That's a problem then - memory that was already written will not be
> detected as such.
Two cases:
1) memory written and has sent an interrupt. Then CPU has acknowledged 
the data, it should process the data
2) memory written but no interrupt, this should be lost as expected.
> > > Is a full scan of all memory required until device reset is complete?
> > Who scan the memory? The device tracks its own dirty pages.
> yes but reset erases this information.
should be, or the hypervisor will copy invalid information, cause a 
conflict.

I remember Jason as given an example on this.
> > > Drivers currently tend to busy poll the reset register,
> > > if this takes very long we might start seeing soft lockup
> > > messages. What is the idea then? Maybe for this we need a separate
> > > weaker reset that does not touch this capability?
> > When reset, how can we expect the LM progress continue running.
> >
> > For example, when the device DMA writes something, then reset before sending
> > an interrupt,
> > the DMA-ed pages should be lost as expected, right?
> interrupt is going to guest, has nothing to do with it.
>
> device writes data into memory
> device sends interrupt
> driver sees data
At this point, the guest CPU owns the dirty pages, owns the data
> driver sends reset
At this point, the device should forget everything
> meanwhile hypervisor did not see any dirty pages
>
> now what? hypervisor must apparently retrieve all
> dirty page data before it can reset the device.
It is not the hypervisor, once an interrupt has sent,
the guest take over the pages.
>