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[Qemu-devel] Re: POLL: Why do you use kqemu?
From: |
Jan Kiszka |
Subject: |
[Qemu-devel] Re: POLL: Why do you use kqemu? |
Date: |
Mon, 08 Jun 2009 15:44:12 +0200 |
User-agent: |
Mozilla/5.0 (X11; U; Linux i686 (x86_64); de; rv:1.8.1.12) Gecko/20080226 SUSE/2.0.0.12-1.1 Thunderbird/2.0.0.12 Mnenhy/0.7.5.666 |
Avi Kivity wrote:
> Jan Kiszka wrote:
>> 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 and will thus have outdated shadows.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.
>>
>
> It doesn't. Host userspace has unrestricted access to guest memory.
>
>>> Not sure how to fix. Maybe write protect the host page tables when we
>>>
>>
>> You mean guest page table?
>>
>
> Both :)
>
> When kvm write protects a guest page table in the shadow page table
> entries pointing to that guest page, it should also write protect the
> guest page table in the host page table entries to the same guest page.
Ah, now I got it. What do other hypervisors do?
>
>>> 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...
>>
>
> No, nested paging brings cancer and cures world peace. Or something.
>
Well, then it's probably not worth bothering, at least until a real
guest problem is explainable with this limitation. Are there any
suspicious reports floating around (maybe not only about Windows)?
Jan
--
Siemens AG, Corporate Technology, CT SE 2
Corporate Competence Center Embedded Linux
- Re: [Qemu-devel] POLL: Why do you use kqemu?, (continued)
- Re: [Qemu-devel] POLL: Why do you use kqemu?, Jamie Lokier, 2009/06/07
- Re: [Qemu-devel] POLL: Why do you use kqemu?, Avi Kivity, 2009/06/08
- Re: [Qemu-devel] POLL: Why do you use kqemu?, Jamie Lokier, 2009/06/08
- Re: [Qemu-devel] POLL: Why do you use kqemu?, Avi Kivity, 2009/06/08
- Re: [Qemu-devel] POLL: Why do you use kqemu?, Jamie Lokier, 2009/06/08
- Re: [Qemu-devel] POLL: Why do you use kqemu?, Avi Kivity, 2009/06/08
- [Qemu-devel] Re: POLL: Why do you use kqemu?, Jan Kiszka, 2009/06/08
- [Qemu-devel] Re: POLL: Why do you use kqemu?, Avi Kivity, 2009/06/08
- [Qemu-devel] Re: POLL: Why do you use kqemu?, Jan Kiszka, 2009/06/08
- [Qemu-devel] Re: POLL: Why do you use kqemu?, Avi Kivity, 2009/06/08
- [Qemu-devel] Re: POLL: Why do you use kqemu?,
Jan Kiszka <=
- [Qemu-devel] Re: POLL: Why do you use kqemu?, Avi Kivity, 2009/06/08
- [Qemu-devel] Re: POLL: Why do you use kqemu?, Jan Kiszka, 2009/06/08
- Re: [Qemu-devel] POLL: Why do you use kqemu?, Lennart Sorensen, 2009/06/08
Re: [Qemu-devel] POLL: Why do you use kqemu?, Andreas Färber, 2009/06/06
- Re: [Qemu-devel] POLL: Why do you use kqemu?, Avi Kivity, 2009/06/06
- Re: [Qemu-devel] POLL: Why do you use kqemu?, Blue Swirl, 2009/06/06
- [Qemu-devel] Re: POLL: Why do you use kqemu?, Jan Kiszka, 2009/06/06
- [Qemu-devel] Re: POLL: Why do you use kqemu?, Blue Swirl, 2009/06/06
- [Qemu-devel] Re: POLL: Why do you use kqemu?, Jan Kiszka, 2009/06/06
- Re: [Qemu-devel] Re: POLL: Why do you use kqemu?, Andreas Färber, 2009/06/06