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

[Paolo Bonzini]

Il 11/11/2013 17:43, 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).
> 
> So how did you measure this exactly?

I mention extrapolation because x86 is TARGET_PHYS_ADDR_SPACE_BITS=36,
not 32.

Paolo