Re: [Qemu-devel] [PATCH 0/2] exec: alternative fix for master abort woes

[Paolo Bonzini <pbonzini@redhat.com>]

Il 07/11/2013 19:54, Michael S. Tsirkin ha scritto:
> On Thu, Nov 07, 2013 at 06:29:40PM +0100, Paolo Bonzini wrote:
>> Il 07/11/2013 17:47, Michael S. Tsirkin ha scritto:
>>> That's on kvm with 52 bit address.
>>> But where I would be concerned is systems with e.g. 36 bit address
>>> space where we are doubling the cost of the lookup.
>>> E.g. try i386 and not x86_64.
>>
>> Tried now...
>>
>>                 P_L2_LEVELS pre-patch           post-patch
>>    i386         3                               6
>>    x86_64       4                               6
>>
>> I timed the inl_from_qemu test of vmexit.flat with both KVM and TCG.  With
>> TCG there's indeed a visible penalty of 20 cycles for i386 and 10 for x86_64
>> (you can extrapolate to 30 cycles for TARGET_PHYS_ADDR_SPACE_BITS=32 targets).
>> These can be more or less entirely ascribed to phys_page_find:
>>
>>                                  TCG             |      KVM
>>                            pre-patch  post-patch |  pre-patch   post-patch
>> phys_page_find(i386)          13%         25%    |     0.6%         1%
>> inl_from_qemu cycles(i386)    153         173    |   ~12000      ~12000
> 
> I'm a bit confused by the numbers above. The % of phys_page_find has
> grown from 13% to  25% (almost double, which is kind of expected
> give we have twice the # of levels).

Yes.

> But overhead in # of cycles only went from 153 to
> 173?

new cycles / old cycles = 173 / 153 = 113%

% outside phys_page_find + % in phys_page_find*2 = 87% + 13%*2 = 113%

> Maybe the test is a bit wrong for tcg - how about unrolling the
> loop in kvm unit test?

Done that already. :)

>> Also, compiling with "-fstack-protector" instead of "-fstack-protector-all",
>> as suggested a while ago by rth, is already giving a savings of 20 cycles.
> 
> Is it true that with TCG this affects more than just MMIO
> as phys_page_find will also sometimes run on CPU accesses to memory?

Yes.  I tried benchmarking with perf the boot of a RHEL guest, which has

                         TCG               |             KVM
               pre-patch      post-patch   | pre-patch        post-patch
                  3%             5.8%      |    0.9%             1.7%

This is actually higher than usual for KVM because there are many VGA
access during GRUB.

>> And of course, if this were a realistic test, KVM's 60x penalty would
>> be a severe problem---but it isn't, because this is not a realistic setting.
> 
> Well, for this argument to carry the day we'd need to design
> a realistic test which isn't easy :)

Yes, I guess the number that matters is the extra 2% penalty for TCG
(the part that doesn't come from MMIO).

Paolo