Re: [Qemu-devel] passing translated address out in QEMU

[Xin Tong <xerox.time.tech@gmail.com>]

On Thu, Aug 23, 2012 at 9:06 PM, 陳韋任 (Wei-Ren Chen)
<chenwj@iis.sinica.edu.tw> wrote:
>> That might be difficult. what i did was that i disabled inlined
>> translated and push the virt/phys address into 2 new fields in the cpu
>> structure in the call out lookup. because in the callout lookup we
>> have a handle to the cpu env.
>
>   What you mean by "disabled inlined translated"? You mean apply Max's
> patch so that all guest memory access go through the slow path without
> looking software tlb? Since you said you're running arm on x86 host,
> I guess what you did might be,
>
> int cpu_arm_handle_mmu_fault (CPUARMState *env, target_ulong address,
>                               int access_type, int mmu_idx)
> {
>     ...
>
>     ret = get_phys_addr(env, address, access_type, is_user, &phys_addr, &prot,
>                         &page_size);
>
>     // store phys_addr into env->cpu_last_paddr
>
>     ...
> }
>
>> not too sure how much impact inlined lookup has on the performance.
>> since i disabled it, next step i would just get rid of that piece of
>> generated assembly, as it is no good for icache ( generated for every
>> memory operation).
>
>   You can run a benchmark inside your guest. I guess if you run a
> long-running benchmark, you can see performance degradation. If software
> tlb hit, you can get the value of guest memory in the code cache
> with a few host instructions. Disabling software tlb lookup, every guest
> memory access will call a helper function which takes a lot of time.
> What you mean by "get rid of that piece of generated assembly"?

every inlined TLB lookup has ~10 instructions .

Xin

>
> Regards,
> chenwj
>
> --
> 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