[Qemu-devel] Re: POLL: Why do you use kqemu?

[Jan Kiszka]

Avi Kivity wrote:
> Jamie Lokier wrote:
>> Is there a reason to duplicate QEMU's task switch emulation, instead
>> of trapping out to QEMU?  Modern OSes don't use x86 task switching
>> (because it's slow on real CPUs) except for ring stack switches, so
>> it's hardly a performance requirement.  Accurate task switch support
>> is fiddly to get right.  Think of all the exceptions including
>> paging/segment exceptions in the middle of reading the TSS block.
>>   
> 
> kvm is designed to be useful without full emulation in userspace.
> 

And the fact that kqemu has to use tcg in order to achieve a reasonable
performance is rather a disadvantage. The complexity and overhead for
synchronizing tcg with the in-kernel accelerator is enormous. If there
were a feasible way to overcome this with kqemu, it would benefit a lot.
But unfortunately there is none (given you don't want to invest
reasonable efforts).

Jan

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