Re: [Qemu-devel] [PATCH] memory: Reintroduce dirty flag to optimize changes on disabled regions

[Jan Kiszka]

On 2012-11-04 20:21, Avi Kivity wrote:
> On 11/04/2012 10:30 AM, Jan Kiszka wrote:
>> From: Jan Kiszka <address@hidden>
>>
>> Cirrus is triggering this, e.g. during Win2k boot: Changes only on
>> disabled regions require no topology update when transaction depth drops
>> to 0 again.
> 
> 817dcc5368988b0 (pci: give each device its own address space) mad this
> much worse by multiplying the number of address spaces.  Each change is
> now evaluated N+2 times, where N is the number of PCI devices.  It also
> causes a corresponding expansion in memory usage.

I know... But this regression predates your changes, is already visible
right after 02e2b95fb4.

> 
> I want to address this by caching AddressSpaceDispatch trees with the
> key being the contents of the FlatView for that address space.  This
> will drop the number of distinct trees to 2-4 (3 if some devices have
> PCI_COMMAND_MASTER disabled, 4 if the PCI address space is different
> from the cpu memory address space) but will fail if we make each address
> space different (for example filtering out the device's own BARs).
> 
> If this change also improves cpu usage sufficiently, then it will be
> better than your patch, which doesn't recognize changes in an enabled
> region inside a disabled or hidden region.

True, though the question is how common such scenarios are. This one
(cirrus with win2k) is already special.

>  In other words, your patch
> fits the problem at hand but isn't general.  On the other hand my
> approach doesn't eliminate render_memory_region(), just the exec.c stuff
> and listener updates.  So we need to understand where the slowness comes
> from.

I would just like to have some even intermediate solution for 1.3. We
can still make it more perfect later on if required.

Jan

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