[Xen-devel] RE: Question: Dynamic code in x86_64 Xen

["Santos, Jose Renato G" <joserenato.santos@xxxxxx>]

Thanks, Keir. That explains it ...
Glad to know this is not a xenoprof bug :)

Regards

Renato 

>> -----Original Message-----
>> From: Keir Fraser [mailto:keir@xxxxxxxxxxxxx] 
>> Sent: Tuesday, July 10, 2007 3:01 PM
>> To: Santos, Jose Renato G
>> Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
>> Subject: Re: Question: Dynamic code in x86_64 Xen
>> 
>> On 10/7/07 19:47, "Santos, Jose Renato G" 
>> <joserenato.santos@xxxxxx> wrote:
>> 
>> > Could you please help me understand some Xen code profile that I 
>> > cannot explain?
>> > When running netperf on x86_64 Xen, oprofile reports that 
>> 2% of the PC 
>> > samples are not recognized (i.e. they correspond to code 
>> outside the 
>> > ".text" section reported on xen image file). I added some 
>> > instrumentation in oprofile and observed that these PC samples are 
>> > located in:
>> > 1) for dom0: in Xen BSS section (more specifically on cpu0_stack)
>> > 2) for domU: outside any section specified in Xen image 
>> file  (i.e. 
>> > dynamically allocated memory)
>> 
>> Yes, these are executions on the stack. This is expected 
>> behaviour for x86_64. The syscall instruction enters Xen via 
>> a stack trampoline. This is because syscall does not switch 
>> %rsp for us. Hence using a stack trampoline cunningly lets 
>> us compute %rsp from %rip.
>> 
>> If you see 2% of your samples in the syscall trampoline, 
>> this probably indicates mainly that the processor is 
>> spending ~2% of its time doing a syscall transition (and 
>> then the NMI occurs on teh very first instruction executed in Xen).
>> 
>>  -- Keir
>> 
>> > I suspect case 2 is also a stack for a different CPU which is 
>> > dynamically allocated (and used in the CPU that the guest is 
>> > executing) but I am not sure...
>> >  
>> > Anyway, why whould Xen execute any code from the stack? Is this 
>> > expected Xen behavior or this is a bug somewhere (probably in 
>> > xenoprofile)?
>> > I did not see this behavior in the past when I was using 
>> x86_32 Xen.
>> > Could you please shed some light on this ...
>> 
>> 

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