Re: [PATCH] memory: synchronize dirty bitmap before unmapping a range

Avi Kivity
2011-08-01T09:45:53Z

On 08/01/2011 12:05 PM, Jan Kiszka wrote:
> On 2011-08-01 10:16, Avi Kivity wrote:
> > On 08/01/2011 10:52 AM, Jan Kiszka wrote:
> >> On 2011-08-01 09:34, Jan Kiszka wrote:
> >> > On 2011-07-31 21:47, Avi Kivity wrote:
> >> >> When a range is being unmapped, ask accelerators (e.g. kvm) to
> >> synchronize the
> >> >> dirty bitmap to avoid losing information forever.
> >> >>
> >> >> Fixes grub2 screen update.
> >> >
> >> > I does.
> >> >
> >> > But something is still broken. As I reported before, the
> >> performance of
> >> > grub2 startup is an order of magnitude slower than with the existing
> >> > code. According to ftrace, we are getting tons of additional
> >> > EPT_MISCONFIG exits over the 0xA0000 segment. But I haven't spot the
> >> > difference yet. The effective slot setup as communicated to kvm looks
> >> > innocent.
> >>
> >> I take it back: We obviously once in a while resume the guest with the
> >> vga segment unmapped. And that, of course, ends up doing mmio instead of
> >> plain ram accesses.
> >>
> >
> > qemu-kvm.git 6b5956c573 and its predecessor fix the issue (and I think
> > they're even faster than upstream, but perhaps I'm not objective).
> >
>
> Just updated to the latest memory-region branch - how did you test it?
> It does not link here due to forgotten rwhandler in Makefile.target.
>
> Anyway, that commit has no impact on the issue I'm seeing. I'm also
> carrying transaction changes for cirrus here, but they have no
> noticeable impact. That indicates that the new API is not actually slow,
> it likely just has some bug.

Here's the log of range changes while in grub2:

adding a0000-affff offset 40000 ram 40040000
dropping a0000-affff
adding a0000-affff offset 30000 ram 40040000
dropping a0000-affff
adding a0000-affff offset 40000 ram 40040000
dropping a0000-affff
adding a0000-affff offset 30000 ram 40040000
dropping a0000-affff
adding a0000-affff offset 40000 ram 40040000
dropping a0000-affff
adding a0000-affff offset 30000 ram 40040000
dropping a0000-affff
adding a0000-affff offset 40000 ram 40040000
dropping a0000-affff
adding a0000-affff offset 30000 ram 40040000
dropping a0000-affff
adding a0000-affff offset 40000 ram 40040000
dropping a0000-affff
adding a0000-affff offset 30000 ram 40040000
dropping a0000-affff
adding a0000-affff offset 40000 ram 40040000
dropping a0000-affff
adding a0000-affff offset 30000 ram 40040000
dropping a0000-affff
adding a0000-affff offset 40000 ram 40040000
dropping a0000-affff
adding a0000-affff offset 30000 ram 40040000
dropping a0000-affff
adding a0000-affff offset 40000 ram 40040000
dropping a0000-affff
adding a0000-affff offset 30000 ram 40040000
dropping a0000-affff
adding a0000-affff offset 40000 ram 40040000
dropping a0000-affff
adding a0000-affff offset 30000 ram 40040000
dropping a0000-affff
adding a0000-affff offset 40000 ram 40040000
dropping a0000-affff
adding a0000-affff offset 30000 ram 40040000
dropping a0000-affff
adding a0000-affff offset 40000 ram 40040000
dropping a0000-affff
adding a0000-affff offset 20000 ram 40040000
dropping a0000-affff
adding a0000-affff offset 30000 ram 40040000

Note that drop/add is always paired (i.e. the guest never sees an
unmapped area), and we always map the full 64k even though cirrus code
manages each 32k bank individually. It looks optimal... we're probably
not testing the same thing (either qemu or guest code).

--
error compiling committee.c: too many arguments to function