Re: [Qemu-devel] [PATCH 1/2] Move graphic-related coalesced MMIO flushes to affected device models

[Avi Kivity <avi@redhat.com>]

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On 10/18/2011 09:50 PM, Jan Kiszka wrote:
> On 2011-10-18 19:34, Avi Kivity wrote:
> > On 10/18/2011 06:49 PM, Jan Kiszka wrote:
> >> On 2011-10-18 18:40, Avi Kivity wrote:
> >>> On 10/18/2011 04:30 PM, Avi Kivity wrote:
> >>>> This takes a while to reproduce, let me talk to gdb for a bit.
> >>>>
> >>>
> >>> a vcpu exit causes kvm_flush_coalesced_mmio_buffer() to run, which does
> >>> a bitblt, which is cirrus_do_copy(), which goes to vga_hw_update, which
> >>
> >> Why does it have to do vga_hw_update? Why can't it set some flag for the
> >> next requested screen update or so? Just thinking, haven't looked at the
> >> code yet.
> >
> > Maybe it's a remnant from the days where it asked the host hardware to
> > do the blt.
>
> If it's no longer needed - drop it? Already for other reasons like
> efficiency.

I think it actually is needed - it calls qemu_console_copy() to do the
copy.  Which incidentally means the the coalesced flush, had it worked,
would be a bug: it would bring pending mmio writes in front of a
currently executing bitblt.  I don't think we can regard my hack as a
fix for that.  Maybe we need to revert the original patch.  Or make sure
the flush only happens from the display thread.

>
>
> >
> >> Do you think that only cirrus is affected by this pattern?
> >
> > It's also possible for hotunplug:
> >
> > - hotunplug
> > - unregister coalesced regions
> > - flush mmios
> > - call back into same device
>
> Which device triggers hotunplug via a coalesced mmio region?

Er, none.

> Anyway, if we want to avoid other surprises like that, better make
> kvm_flush_coalesced_mmio_buffer reentrance-safe. If we think that this
> remains an odd scenario, issue a warning to the console that some device
> may require fixing.
>

- -- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.
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