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Re: [Qemu-devel] How to measure guest memory access (qemu_ld/qemu_st) ti

From: Blue Swirl
Subject: Re: [Qemu-devel] How to measure guest memory access (qemu_ld/qemu_st) time?
Date: Sun, 24 Jun 2012 06:13:37 +0000

On Fri, Jun 22, 2012 at 9:58 AM, Xin Tong <address@hidden> wrote:
> It is a pity that QEMU does not outline the TLB lookup code. I do not
> know how much impact the inlined TLB code has due to icache misses...

With a test case the impact could be measured. Maybe it could be just
a program performing loads in a loop, executing under a user emulator.

> Another benefit one gets from outlined TLB code is that it is much
> easier to gather the amount of time spent in the TLB. one can just
> profile QEMU and count up how many ticks happened in the outlined TLB
> translation code.

There's also a possible benefit that the code generation buffer does
not fill as fast.

> In fact, i do not think outlining QEMU inlined TLB lookup is too hard
> to implement. one can still keep most of the original inlined TLB code
> and use call/ret to get a TLB translation. of course, one needs to
> come up with a new linkage.

If it can be shown with a test case and statistics that outlining does
not make things worse, we can switch.

> Xin
> On Wed, Jun 20, 2012 at 3:57 AM, 陳韋任 (Wei-Ren Chen)
> <address@hidden> wrote:
>>  CC'ed to the mailing list.
>> --
>> Wei-Ren Chen (陳韋任)
>> Computer Systems Lab, Institute of Information Science,
>> Academia Sinica, Taiwan (R.O.C.)
>> Tel:886-2-2788-3799 #1667
>> Homepage: http://people.cs.nctu.edu.tw/~chenwj
>> ---------- Forwarded message ----------
>> From: Orit Wasserman <address@hidden>
>> To: "\"陳韋任 (Wei-Ren Chen)\"" <address@hidden>
>> Cc:
>> Date: Tue, 19 Jun 2012 12:01:08 +0300
>> Subject: Re: [Qemu-devel] How to measure guest memory access 
>> (qemu_ld/qemu_st) time?
>> On 06/19/2012 11:49 AM, 陳韋任 (Wei-Ren Chen) wrote:
>>>   Mind me CC this to ML? :)
>> sure I will read the threads to understand more.
>> Orit
>>>> Well it was a while back (2008-9) ,the company was acquired by IBM a year 
>>>> later :
>>>> http://www.linux-kvm.org/wiki/images/9/98/KvmForum2008%24kdf2008_2.pdf
>>>> I think stefan Hanjoczi worked there ...
>>>> The company used the technology for cross platform guest support but claim 
>>>> to get speedup too
>>>> (for ppc) don't think the speedup was related to mmu but more to the 
>>>> instruction stream.
>>>> I hope this is helpful.
>>>   Thanks.
>>>> Do you have performance result for the cost of the address translation ?
>>>> If I understand you are concentrating on ARM ?
>>>   The whole discussion thread is on [1], and you can get some feel about
>>> the cost of address translation here [2]. Yes, ARM is our target right now,
>>> but I think we are not limit to it.
>>> Regards,
>>> chenwj
>>> [1] http://www.mail-archive.com/address@hidden/msg116159.html
>>> [2] http://www.mail-archive.com/address@hidden/msg116404.html

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