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Subject[RFC][PATCH 0/12] KVM, x86, ppc, asm-generic: moving dirty bitmaps to user space
Hi, sorry for sending from my personal account.
The following series are all from me:

From: Takuya Yoshikawa <>

The 3rd version of "moving dirty bitmaps to user space".

From this version, we add x86 and ppc and asm-generic people to CC lists.

[To KVM people]

Sorry for being late to reply your comments.

- I've wrote an answer to your question in patch 5/12: drivers/vhost/vhost.c .

- I've considered to change the set_bit_user_non_atomic to an inline function,
but did not change because the other helpers in the uaccess.h are written as
macros. Anyway, I hope that x86 people will give us appropriate suggestions
about this.

- I thought that documenting about making bitmaps 64-bit aligned will be
written when we add an API to register user-allocated bitmaps. So probably
in the next series.

Avi, Alex,
- Could you check the ia64 and ppc parts, please? I tried to keep the logical
changes as small as possible.

I personally tried to build these with cross compilers. For ia64, I could check
build success with my patch series. But book3s, even without my patch series,
it failed with the following errors:

arch/powerpc/kvm/book3s_paired_singles.c: In function 'kvmppc_emulate_paired_single':
arch/powerpc/kvm/book3s_paired_singles.c:1289: error: the frame size of 2288 bytes is larger than 2048 bytes
make[1]: *** [arch/powerpc/kvm/book3s_paired_singles.o] Error 1
make: *** [arch/powerpc/kvm] Error 2

About changelog: there are two main changes from the 2nd version:
1. I changed the treatment of clean slots (see patch 1/12).
This was already applied today, thanks!
2. I changed the switch API. (see patch 11/12).

To show this API's advantage, I also did a test (see the end of this mail).

[To x86 people]

Hi, Thomas, Ingo, Peter,

Please review the patches 4,5/12. Because this is the first experience for
me to send patches to x86, please tell me if this lacks anything.

[To ppc people]

Hi, Benjamin, Paul, Alex,

Please see the patches 6,7/12. I first say sorry for that I've not tested these
yet. In that sense, these may not be in the quality for precise reviews. But I
will be happy if you would give me any comments.

Alex, could you help me? Though I have a plan to get PPC box in the future,
currently I cannot test these.

[To asm-generic people]

Hi, Arnd,

Please review the patch 8/12. This kind of macro is acceptable?

[Performance test]

We measured the tsc needed to the ioctl()s for getting dirty logs in

Test environment

AMD Phenom(tm) 9850 Quad-Core Processor with 8GB memory

1. GUI test (running Ubuntu guest in graphical mode)

sudo qemu-system-x86_64 -hda dirtylog_test.img -boot c -m 4192 -net ...

We show a relatively stable part to compare how much time is needed
for the basic parts of dirty log ioctl. get.opt switch.opt

slots[7].len=32768 278379 66398 64024
slots[8].len=32768 181246 270 160
slots[7].len=32768 263961 64673 64494
slots[8].len=32768 181655 265 160
slots[7].len=32768 263736 64701 64610
slots[8].len=32768 182785 267 160
slots[7].len=32768 260925 65360 65042
slots[8].len=32768 182579 264 160
slots[7].len=32768 267823 65915 65682
slots[8].len=32768 186350 271 160

At a glance, we know our optimization improved significantly compared
to the original get dirty log ioctl. This is true for both get.opt and
switch.opt. This has a really big impact for the personal KVM users who
drive KVM in GUI mode on their usual PCs.

Next, we notice that switch.opt improved a hundred nano seconds or so for
these slots. Although this may sound a bit tiny improvement, we can feel
this as a difference of GUI's responses like mouse reactions.

To feel the difference, please try GUI on your PC with our patch series!

2. Live-migration test (4GB guest, write loop with 1GB buf)

We also did a live-migration test. get.opt switch.opt

slots[0].len=655360 797383 261144 222181
slots[1].len=3757047808 2186721 1965244 1842824
slots[2].len=637534208 1433562 1012723 1031213
slots[3].len=131072 216858 331 331
slots[4].len=131072 121635 225 164
slots[5].len=131072 120863 356 164
slots[6].len=16777216 121746 1133 156
slots[7].len=32768 120415 230 278
slots[8].len=32768 120368 216 149
slots[0].len=655360 806497 194710 223582
slots[1].len=3757047808 2142922 1878025 1895369
slots[2].len=637534208 1386512 1021309 1000345
slots[3].len=131072 221118 459 296
slots[4].len=131072 121516 272 166
slots[5].len=131072 122652 244 173
slots[6].len=16777216 123226 99185 149
slots[7].len=32768 121803 457 505
slots[8].len=32768 121586 216 155
slots[0].len=655360 766113 211317 213179
slots[1].len=3757047808 2155662 1974790 1842361
slots[2].len=637534208 1481411 1020004 1031352
slots[3].len=131072 223100 351 295
slots[4].len=131072 122982 436 164
slots[5].len=131072 122100 300 503
slots[6].len=16777216 123653 779 151
slots[7].len=32768 122617 284 157
slots[8].len=32768 122737 253 149

For slots other than 0,1,2 we can see the similar improvement.

Considering the fact that switch.opt does not depend on the bitmap length
except for kvm_mmu_slot_remove_write_access(), this is the cause of some
usec to msec time consumption: there might be some context switches.

But note that this was done with the workload which dirtied the memory
endlessly during the live-migration.

In usual workload, the number of dirty pages varies a lot for each iteration
and we should gain really a lot for relatively clean cases.

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