On Thu, 22 Apr 2010, Avi Kivity wrote:
On 04/22/2010 09:04 AM, Gerhard Wiesinger wrote:
On Wed, 21 Apr 2010, Avi Kivity wrote:
On 04/21/2010 09:50 PM, Gerhard Wiesinger wrote:
I don't think changing VGA window is a problem because there are
500.000-1Mio changes/s possible.
1MB/s, 500k-1M changes/s.... Coincidence? Is it taking a page fault
or trap on every write?
To clarify:
Memory Performance writing to segmen A000 is about 1MB/st.
That indicates a fault every write (assuming 8-16 bit writes). If you're
using 256 color vga and not switching banks, this indicates a bug.
Yes, 256 color VGA and no bank switches involved.
Calling INT 10 set/get window function with different windows (e.g.
toggling between window page 0 and 1) is about 500.000 to 1Mio function
calls per second.
That's suprisingly fast. I'd expect 100-200k/sec.
Sorry, I mixed up the numbers:
1.) QEMU-KVM: ~111k
2.) QEMU only: 500k-1Mio
Please run kvm_stat and report output for both tests to confirm.
See below. 2nd column is per second statistic when running the test.
efer_reload 0 0
exits 18470836 554582
fpu_reload 2147833 3469
halt_exits 2083 0
halt_wakeup 2047 0
host_state_reload 2148186 3470
hypercalls 0 0
insn_emulation 7688203 554244
This indicates that kvm is emulating instead of direct mapping. That's
probably a bug. If you fix it, performance will increase dramatically.
Where can I start here?
Any ideas how to?
One of my ideas: Move hw/vga.c functions
vga_mem_readb
vga_mem_readw
vga_mem_readl
vga_mem_writeb
vga_mem_writew
vga_mem_writel
to KVM to avoid switching from KVM to QEMU (I can write C code even
kernel but I'm not comfortable with KVM). Howto?
To get real good VGA performance both parameters should be:
About >50MB/s for writes to segment A000
~500.000 bank switches per second.
First should be doable easily, second is borderline.
I think this is very easy to distingish:
1.) VGA Segment A000 is legacy and should be handled through QEMU and not
through KVM (because it is much more faster). Also 16 color modes should
be fast enough there.
2.) All other flat PCI memory accesses should be handled through KVM
(there is a specialized driver loaded for that PCI device in the non
legacy OS).
Is that easily possible?
No. Code can run in either qemu or kvm, not both. You can switch between
them based on access statistics (early versions of qemu-kvm did that,
without the statistics part), but this isn't trivial.
Hmmm. Ok, 2 different opinions about the memory write performance:
Easily or not possible?
Switching between tcg and kvm is hard, but not needed. For 256 color modes,
direct map is possible and should yield good performance. Bank switching can
be improved perhaps 3x, but will never be fast.
Where can I start for KVM performance for the bank switching (256 color
mode)? (e.g. BIOS writes to VGA window I/O port to switch the bank)
Any ideas how to improve (architecture for the change)?
Thnx and sorry for long delay, was busy.
Ciao,
Gerhard
--
http://www.wiesinger.com/
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