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

From: Glauber Costa
Subject: Re: [Qemu-devel] [PATCH 0/4] VGA optimization
Date: Mon, 10 Nov 2008 18:17:02 -0200

On Mon, Nov 10, 2008 at 4:39 PM, Anthony Liguori <address@hidden> wrote:
> 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,

My proposal is to let kvm interface to be:
void kvm_physical_sync_dirty_bitmap(target_phys_addr_t start_addr,
target_phys_addr_t end_addr,
                                    ram_addr_t phys_offset)

to match, qemu then becomes:
void cpu_physical_sync_dirty_bitmap(target_phys_addr_t start_addr,
target_phys_addr_t end_addr,
                                    ram_addr_t phys_offset)

the caller does:

cpu_physical_sync_dirty_bitmap(s->map_addr, s->map_end, s->vram_offset);

This way, we tell kvm which particular area of the phys_ram_bitmap to
fill. I liked this
interface, because then we can have the behaviour of not synchronizing
back those bits
into qemu, by passing NULL as the third parameter.

I have a working version of it, will send shortly.

> 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.

Glauber  Costa.
"Free as in Freedom"

"The less confident you are, the more serious you have to act."

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