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Re: [Qemu-devel] qemu log function to print out the registers of the gue
From: |
Max Filippov |
Subject: |
Re: [Qemu-devel] qemu log function to print out the registers of the guest |
Date: |
Sun, 26 Aug 2012 00:41:32 +0400 |
On Sat, Aug 25, 2012 at 9:20 PM, Steven <address@hidden> wrote:
> On Tue, Aug 21, 2012 at 3:18 AM, Max Filippov <address@hidden> wrote:
>> On Tue, Aug 21, 2012 at 9:40 AM, Steven <address@hidden> wrote:
>>> Hi, Max,
>>> I wrote a small program to verify your patch could catch all the load
>>> instructions from the guest. However, I found some problem from the
>>> results.
>>>
>>> The guest OS and the emulated machine are both 32bit x86. My simple
>>> program in the guest declares an 1048576-element integer array,
>>> initialize the elements, and load them in a loop. It looks like this
>>> int array[1048576];
>>> initialize the array;
>>>
>>> /* region of interests */
>>> int temp;
>>> for (i=0; i < 1048576; i++) {
>>> temp = array[i];
>>> }
>>> So ideally, the path should catch the guest virtual address of in the
>>> loop, right?
>>> In addition, the virtual address for the beginning and end
>>> of the array is 0xbf68b6e0 and 0xbfa8b6e0.
>>> What i got is as follows
>>>
>>> __ldl_mmu, vaddr=bf68b6e0
>>> __ldl_mmu, vaddr=bf68b6e4
>>> __ldl_mmu, vaddr=bf68b6e8
>>> .....
>>> These should be the virtual address of the above loop. The
>>> results look good because the gap between each vaddr is 4 bypte, which
>>> is the length of each element.
>>> However, after certain address, I got
>>>
>>> __ldl_mmu, vaddr=bf68bffc
>>> __ldl_mmu, vaddr=bf68c000
>>> __ldl_mmu, vaddr=bf68d000
>>> __ldl_mmu, vaddr=bf68e000
>>> __ldl_mmu, vaddr=bf68f000
>>> __ldl_mmu, vaddr=bf690000
>>> __ldl_mmu, vaddr=bf691000
>>> __ldl_mmu, vaddr=bf692000
>>> __ldl_mmu, vaddr=bf693000
>>> __ldl_mmu, vaddr=bf694000
>>> ...
>>> __ldl_mmu, vaddr=bf727000
>>> __ldl_mmu, vaddr=bf728000
>>> __ldl_mmu, vaddr=bfa89000
>>> __ldl_mmu, vaddr=bfa8a000
>>> So the rest of the vaddr I got has a different of 4096 bytes, instead
>>> of 4. I repeated the experiment for several times and got the same
>>> results. Is there anything wrong? or could you explain this? Thanks.
>>
>> I see two possibilities here:
>> - maybe there are more fast path shortcuts in the QEMU code?
>> in that case output of qemu -d op,out_asm would help.
>> - maybe your compiler had optimized that sample code?
>> could you try to declare array in your sample as 'volatile int'?
> After adding the "volatile" qualifier, the results are correct now.
> So your patch can trap all the guest memory data load access, no
> matter slow path or fast path.
>
> However, I found some problem when I try understanding the instruction
> access. So I run the VM with "-d in_asm" to see program counter of
> each guest code. I got
>
> __ldl_cmmu,ffffffff8102ff91
> __ldl_cmmu,ffffffff8102ff9a
> ----------------
> IN:
> 0xffffffff8102ff8a: mov 0x8(%rbx),%rax
> 0xffffffff8102ff8e: add 0x790(%rbx),%rax
> 0xffffffff8102ff95: xor %edx,%edx
> 0xffffffff8102ff97: mov 0x858(%rbx),%rcx
> 0xffffffff8102ff9e: cmp %rcx,%rax
> 0xffffffff8102ffa1: je 0xffffffff8102ffb0
> .....
>
> __ldl_cmmu,00000000004005a1
> __ldl_cmmu,00000000004005a6
> ----------------
> IN:
> 0x0000000000400594: push %rbp
> 0x0000000000400595: mov %rsp,%rbp
> 0x0000000000400598: sub $0x20,%rsp
> 0x000000000040059c: mov %rdi,-0x18(%rbp)
> 0x00000000004005a0: mov $0x1,%edi
> 0x00000000004005a5: callq 0x4004a0
>
> From the results, I see that the guest virtual address of the pc is
> slightly different between the __ldl_cmmu and the tb's pc(below IN:).
> Could you help to understand this? Which one is the true pc memory
> access? Thanks.
Guest code is accessed at the translation time by C functions and
I guess there are other layers of address translation caching. I wouldn't
try to interpret these _cmmu printouts and would instead instrument
[cpu_]ld{{u,s}{b,w},l,q}_code macros.
--
Thanks.
-- Max
- Re: [Qemu-devel] qemu log function to print out the registers of the guest, (continued)
- Re: [Qemu-devel] qemu log function to print out the registers of the guest, Laurent Desnogues, 2012/08/16
- Re: [Qemu-devel] qemu log function to print out the registers of the guest, Steven, 2012/08/16
- Re: [Qemu-devel] qemu log function to print out the registers of the guest, Wei-Ren Chen, 2012/08/17
- Re: [Qemu-devel] qemu log function to print out the registers of the guest, Max Filippov, 2012/08/17
- Re: [Qemu-devel] qemu log function to print out the registers of the guest, Wei-Ren Chen, 2012/08/19
- Re: [Qemu-devel] qemu log function to print out the registers of the guest, Steven, 2012/08/21
- Re: [Qemu-devel] qemu log function to print out the registers of the guest, Max Filippov, 2012/08/21
- Message not available
- Re: [Qemu-devel] qemu log function to print out the registers of the guest,
Max Filippov <=
- Re: [Qemu-devel] qemu log function to print out the registers of the guest, Steven, 2012/08/27
- Re: [Qemu-devel] qemu log function to print out the registers of the guest, Wei-Ren Chen, 2012/08/27
- Re: [Qemu-devel] qemu log function to print out the registers of the guest, Steven, 2012/08/27
- Re: [Qemu-devel] qemu log function to print out the registers of the guest, Max Filippov, 2012/08/28