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Date: Tue, 18 Oct 2011 18:49:57 +0200
From: Jan Kiszka <jan.kiszka@siemens.com>
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Subject: Re: [Qemu-devel] [PATCH 1/2] Move graphic-related coalesced MMIO
 flushes to affected device models
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To: Avi Kivity <avi@redhat.com>
Cc: Anthony Liguori <aliguori@us.ibm.com>, qemu-devel <qemu-devel@nongnu.org>, David Gibson <david@gibson.dropbear.id.au>

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.

Do you think that only cirrus is affected by this pattern?

> does vga_update_display(), which calls
> qemu_flush_coalesced_mmio_buffer(), which is not reentrant.
> 
> It's easy to make qemu_flush_coalesced_mmio_buffer reentrant:
> 
>   if (s->coalesced_flush_in_progress) {
>       return;
>   }
> 
> it isn't very pretty and is also a lie.  Other ideas?
> 
> I'll probably commit this soon to avoid the regression, to be replaced
> by a better fix when we find it.
> 

Agreed. Unless we can avoid that recursion at devices level, there is
likely no alternative.

Jan

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