Re: [Qemu-devel] Re: QEMU-KVM and video performance

Gerhard Wiesinger
2010-04-22T05:37:53Z

On Wed, 21 Apr 2010, Avi Kivity wrote:

> On 04/21/2010 09:14 PM, Gerhard Wiesinger wrote:
>>
>> Can you explain which code files/functions of KVM is involved in handling
>> VGA memory window and page switching through the port write to the VGA
>> window register (or is that part handled through QEMU), so a little bit
>> architecture explaination would be nice?
>
> qemu hw/vga.c and hw/cirrus_vga.c. Boring functions like
> vbe_ioport_write_data() and vga_ioport_write().
>

Yes, I was already in that code part and that are very simple functions as
already explained and are therefore in QEMU only very fast. But I ment:
How is the calling path from KVM guest OS to hw/vga.c for memory and I/O
accesses, and which parts are done in hardware directly (to understand the
speed gap and maybe to find a solution)?

>>
>> BTW: In which KVM code parts is decided where "direct code" or an "emulated
>> device code" is used?
>>
>
> Same place. Look for calls to cpu_register_physical_memory(). If the last
> argument was obtained by a call to cpu_register_io_memory(), then all writes
> trap. Otherwise, it was obtained by qemu_ram_alloc() and writes will not
> trap (except the first write to a page in a 30ms window, used to note that
> the page is dirty and needs redrawing).

Ok, that finally ends in:
cpu_register_physical_memory_offset()
...
// 0.12.3
if (kvm_enabled())
kvm_set_phys_mem(start_addr, size, phys_offset);
// KVM
cpu_notify_set_memory(start_addr, size, phys_offset);
...

I/O is always done through:
cpu_register_io_memory => cpu_register_io_memory_fixed
cpu_register_io_memory_fixed()
...
No call to KVM?
...

Where is the trap from KVM to QEMU?

Thnx.

Ciao,
Gerhard

--