Re: [Qemu-devel] [PATCH RFC 0/5] making address spaces 64 bit wide

[Paolo Bonzini]

Il 11/11/2013 17:40, Michael S. Tsirkin ha scritto:
> At the moment, exec ignores high bits in each address,
> for efficiency.
> This is incorrect: devices can do full 64 bit DMA, it's
> only the CPU that is limited by target address space.
> Using full 64 bit addresses was clocked at 12% performance
> hit on a microbenchmark.
> To solve, teach pagetables to skip bits at any level
> and not just the lowest level.
> 
> This should solve the performance problem (only one line
> of code changed on the data path).
> I'm still trying to figure out how to measure speed
> properly with TCG, sending this out for early feedback and flames.

I used this:

x86_64-softmmu/qemu-system-x86_64 -kernel
../../kvm-unit-tests/x86/vmexit.flat  -serial stdio -device
isa-debug-exit,iobase=0xf4

with only one test enabled (I tried both inl_from_qemu and
inl_from_pmtimer) and with roughly the same inlining of the "inb %dx,
%al" instruction that you suggested earlier on the mailing list.

Paolo