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

[Blue Swirl]

On Mon, Jun 18, 2012 at 6:57 AM, 陳韋任 (Wei-Ren Chen)
<address@hidden> wrote:
>> The idea looks nice, but instead of different TLB functions selected
>> at configure time, the optimization should be enabled by default.
>>
>> Maybe a 'call' instruction could be used to jump to the slow path,
>> that way the slow path could be shared.
>
>  I don't understand what "maybe a 'call' instruction could be used to jump to
> the slow path", could you elaborate on that? From YeongKyoon's cover letter 
> [1],
> the current flow is,
>
>  (1) TLB check
>  (2) If hit fall through, else jump to TLB miss case (5)
>  (3) TLB hit case: Load value from host memory
>  (4) Jump to next code (6)
>  (5) TLB miss case: call MMU helper
>  (6) ... (next code)

AFAICT, with the patches, the flow would become
(1) TLB check
(2) If hit fall through, else jump to TLB miss case (5)
(3) TLB hit case: Load value from host memory
(4) ... (next code)

Later generated:

(5) TLB miss case: call MMU helper
(6) Jump back to (4)

>
> Do you mean we directly call MMU helper ing step 2?

Not really. But the interface between the helper and the generated
code could be optimized, for example to get rid of the redundant TLB
check or pass around already calculated values.

>
> Regards,
> chenwj
>
> [1] http://www.mail-archive.com/address@hidden/msg91294.html
>
> --
> 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