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

[Jan Kiszka <jan.kiszka@siemens.com>]

Avi Kivity wrote:
> Jan Kiszka wrote:
>> 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).
>>   
> 
> Note that kvm suffers from something similar (to a smaller magnitude) as
> well: if a guest pages in its page tables, kvm knows nothing about it
> and will thus have outdated shadows.  To date we haven't encountered a
> problem with it, but it's conceivable.  I think Windows can page its
> page tables, but maybe it's disabled by default, or maybe it doesn't dma
> directly into the page tables.

Can't follow, always thought that kernel space gets informed when some
I/O operation handled by user space modified an "interesting" page.

> 
> Not sure how to fix.  Maybe write protect the host page tables when we

You mean guest page table?

> shadow a page table, and get an mmu notifier to tell us when its made
> writable?  Seems expensive.  Burying head in sand is much easier.
> 

Does this still apply to nested paging? I guess (hope) not...

Jan

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