[Qemu-devel] Re: Another SIGFPE in display code, now in cirrus

[Avi Kivity <avi@redhat.com>]

On 05/10/2010 10:41 AM, Avi Kivity wrote:
> On 05/06/2010 11:07 PM, Michael Tokarev wrote:
>> There was a bug recently fixed in vnc code.  Apparently
>> there's something similar in the cirrus emulation as well.
>> Here it triggers _always_ (including old versions of kvm)
>> when running windows NT and hitting "test" button in its
>> display resolution dialog.  Here's what gdb is to say:
>>
>> Program received signal SIGFPE, Arithmetic exception.
>> [Switching to Thread 0xf76cab70 (LWP 580)]
>> 0x080c5e45 in cirrus_do_copy (s=0x86134dc, dst=960000, src=0, w=2, h=9)
>>     at hw/cirrus_vga.c:687
>> 687        sx = (src % ABS(s->cirrus_blt_srcpitch)) / depth;
>> (gdb) p depth
>> $1 = 2
>> (gdb) p s->cirrus_blt_srcpitch
>> $2 = 0
>>
>>
>> This qemu-kvm-0.12.3 - actually a debian package of it,
>> but there's no patches relevant to video applied.
>>
>> Anything can be done with it?
>
> Well, it's trivial to check for the condition, but how to handle it?
>

That code is wrong.  It shouldn't be using the bitblt source pitch, but 
the actual display pitch.  If the two are different, then the blt 
doesn't actually affect a rectangular region, but instead a parallelogram.

Further:

>
>     if (notify)
>         qemu_console_copy(s->vga.ds,
>               sx, sy, dx, dy,
>               s->cirrus_blt_width / depth,
>               s->cirrus_blt_height);
>
>     /* we don't have to notify the display that this portion has
>        changed since qemu_console_copy implies this */
>
>     cirrus_invalidate_region(s, s->cirrus_blt_dstaddr,
>                 s->cirrus_blt_dstpitch, s->cirrus_blt_width,
>                 s->cirrus_blt_height);


Shouldn't we avoid the invalidate if notify != 0, as the comment says?

31c05501c says this breaks bitblt, but I can't see why this is true.  
The copy should update the display.  This is probably due to a 
miscalculation of the affected region, and now we have two invalidates 
instead of one, reducing performance.

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