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

From: Anthony Liguori
Subject: Re: [Qemu-devel] [PATCH 0/4] VGA optimization
Date: Mon, 10 Nov 2008 12:39:04 -0600
User-agent: Thunderbird (X11/20080925)

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.


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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