Re: [Qemu-devel] [PATCH 0/4] VGA optimization

[Anthony Liguori]

Glauber Costa wrote:
> hey guys,
>   

I gave you some bad advice that I think is causing the breakage I'm 
seeing now.  I suggested that you simply do a lookup to find the slot 
given a target_phys_addr_t but that isn't correct.  Let me explain why.

ram_addr_t represents a guest physical address.  From a ram_addr_t you 
can get a target_phys_addr_t.  Sometimes these are the same but they 
aren't always.

You can have multiple ram_addr_t's pointing to the same 
target_phys_addr_t.  This is ram aliasing and it happens for a variety 
of reasons.  In general, it's pretty expensive to map a ram_addr_t to a 
target_phys_addr_t because, among other things, for a range of 
(ram_addr_t, size_t), you may have many (target_phys_addr_t, size) 
tuples that you have to deal with.

vga_common_init() takes a target_phys_addr_t (well, it really takes an 
unsigned long, but that's a bug).  It takes this as an optimization.  It 
avoids having to do the conversion and ensures that it's one big linear 
region.

For dirty tracking, we have a bitmap indexed by target_phys_addr_t in 
QEMU.  This means that we can happily set dirty bits based on 
target_phys_addr_t's.  We don't have to worry about what ram_addr_t it 
came from because they all map to the same bits.

Since KVM uses a slot API, and that API is indexed in ram_addr_t's, we 
need to enable dirty tracking on the ram_addr_t's.  We don't have a 
ram_addr_t in the VGA code.

The solution is pretty simple.  We need to keep track of the 
ram_addr_t's in the VGA code and enable dirty tracking on the 
appropriate ram_addr_ts.

Regards,

Anthony Liguori

> I hope this is the last version (Of course, once this is merged,
> the optimizations of the optimization can start ;-) )
>
> I split it in 4 patches. The first two ones are just moving
> things out of the way, and then #3 and #4 do the real thing.
> #3 kvm-side, #4 overall qemu.
>
> They merge most of the suggestion Anthony and Stefano's sent
> on last iteration.
>
> Hope you like it.
>  
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