Re: [Xen-devel] Branch Trace Storage for guests and VPMUinitialization

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 02/25/2015 10:12 AM, Kevin.Mayer@xxxxxxxx wrote:
> > -----UrsprÃngliche Nachricht-----
> > Von: Boris Ostrovsky [mailto:boris.ostrovsky@xxxxxxxxxx]
> > Gesendet: Dienstag, 24. Februar 2015 18:13
> > An: Mayer, Kevin; xen-devel@xxxxxxxxxxxxx
> > Betreff: Re: [Xen-devel] Branch Trace Storage for guests and VPMU
> > initialization
> >
> > On 02/24/2015 10:27 AM, Kevin.Mayer@xxxxxxxx wrote:
> > > Hi guys
> > >
> > > I`m trying to set up the BTS so that I can log the branches taken in
> > > the guest using Xen 4.4.1 with a WinXP SP3 guest on a Core i7 Sandy
> > > Bridge.
> > >
> > > I added the vpmu=bts boot parameter to my grub2 configuration and
> > > extended the libxl,libxc,domctl,â with an own command so that I can
> > > trigger the activation of the BTS whenever I want.
> > I am not sure why you are doing all these changes to Xen code. BTS is
> > supposed to be managed from the guest. For example, a Fedora HVM guest
> > will produce this:
> >
> > [root@dhcp-burlington7-2nd-B-east-10-152-55-140 ~]# perf record -e
> > branches:u -c 1 -d sleep 1 [ perf record: Woken up 3838 times to write data ] [
> > perf record: Captured and wrote 0.704 MB perf.data (~30756 samples) ]
> > [root@dhcp-burlington7-2nd-B-east-10-152-55-140 ~]# perf script -f
> > ip,addr,sym,dso,symoff --show-kernel-path
> >    ffffffff8167c347 native_irq_return_iret+0x0 (/proc/kcore) =>
> > 328c001590 [unknown] (/proc/kcore)
> >    ffffffff8167c347 native_irq_return_iret+0x0 (/proc/kcore) =>
> > 328c001590 [unknown] ([unknown])
> >          328c001593 [unknown] ([unknown]) =>       328c004b70 [unknown]
> > ([unknown])
> > ...
> I want to be able to log the taken branches (of the guest) without the need to 
> modify the guest at all.
> This means I have to do all the logic in the hypervisor, or am I wrong?
In that case, yes. But then you have to make sure that at least
 * you don't load guest's VPMU (or, at least, BTS-related registers) on 
context switch
 * You don't send the interrupt to the guest (meaning that you will 
need to somehow inform dom0 of the BTS interrupt)
and probably more.

Essentially, you want dom0 to profile the guest. I have been working on 
patches that would allow that but they are still under review.

> > > In this command I do the following:
> > >
> > > I set up the memory region for the BTS Buffer and the DS Buffer
> > > Management Area using xzalloc_bytes
> > I don't think you should be allocating BTS buffers in the hypervisor, they are
> > in guest's memory.
> I agree. As I said I think this is where my main problem is at the moment.
> Is there any way I can allocate memory in the hypervisor in a way the guest can 
> access it?
I am not sure this is what you want since you seem to *not* want the 
guest to process the samples, right?
But yes, you can. E.g. something like what map_vcpu_info() does. (I have 
no idea how you'd do this from Windows.)

> Of course the guest must not be able to use this memory in its normal 
> operations but just for BTS.
> Is this even possible? I am rather confused at the moment. :-D
>
> > > Then I write the pointer to the BTS Buffer into the DS Buffer
> > > Management Area at +0x0 and +0x8 (BTS Buffer Base and BTS Index)
> > >
> > > When I use vmx_msr_write_intercept to store the value in
> > > MSR_IA32_DS_AREA the host reboots (my idea is he tries to access a
> > > vpmu-struct that isnÂt there in the current vcpu and panics).

Who is trying to write to MSR_IA32_DS_AREA? The guest or dom0? I thought 
you said that you want dom0 to do sampling. Or are you trying to setup 
DS area from your guest and control it from dom0? I am somewhat confused.
> > >
> > Can you post hypervisor log? (hard to say how helpful it will be without
> > seeing your code changes though)
> Right after enabling the BTS I get a triple fault.
> hvm.c:1357:d2 Triple fault on VCPU0 - invoking HVM shutdown action 1.

That's not host reboot, this is your guest dying.


> > > When I use a modified version of vmx_msr_write_intercept I donât get
> > > any crashes as long as I donât enable BTS and TR in the
> > > GUEST_IA32_DEBUGCTL (BTR works). When I enable the BTS (and TR) the
> > > guest crashes. I suppose he gets killed by the hypervisor for
> > > accessing forbidden memory.
> > Possibly because DS area point to hypervisor memory.
> >
> >
> > Having said all this, I am not sure how well BTS works. You did notice
> > this in the hypervisor log:
> >
> > (XEN) ******************************************************
> > (XEN) ** WARNING: Emulation of BTS Feature is switched on **
> > (XEN) ** Using this processor feature in a virtualized **
> > (XEN) ** environment is not 100% safe. **
> > (XEN) ** Setting the DS buffer address with wrong values **
> > (XEN) ** may lead to hypervisor hangs or crashes. **
> > (XEN) ** It is NOT recommended for production use! **
> > (XEN) ******************************************************
> Yes, I saw that. It doesnât state that BTS is not working at all, just that it 
> is not that safe to use.
> As I understand it as long as I set the DS buffer address correctly I should be 
> fine, right?
Right. Except that I am not convinced you did set this buffer correctly, 
which is possibly why your hypervisor crashed (I am not sure I 
understood under what circumstances though).
-boris

> Since I donât want to use for production that is fine with me. At least for now.
>
>
> Kevin
> > -boris
> >
> >
> > > The modified version of vmx_msr_write_intercept takes a vcpu-struct as
> > > a parameter and uses this instead of the current vcpu.
> > >
> > > Instead of
> > >
> > > staticint vmx_msr_write_intercept(unsigned int msr, uint64_t
> > msr_content)
> > > {
> > >
> > >      struct vcpu *v = current;
> > >
> > > I just have
> > >
> > > staticint own_vmx_msr_write_intercept(unsigned int msr, uint64_t
> > > msr_content, struct vcpu *v)
> > >
> > > I get this vcpu by d->vcpu[0] as I have limited my guest domain to one
> > > vcpu atm.
> > >
> > > Of course I also use similarly modified version of the called
> > > functions(vpmu_do_wrmsr,â).
> > >
> > > IÂm pretty sure that my problem is with a wrong scope/usage of the
> > > vcpus/memory, but I have no idea how to fix this.
> > >
> > > I can see a potential problem with the memory allocation (in the host)
> > > into which the cpu in guest-mode is supposed to write.
> > >
> > > Or maybe I got the principle of a vcpu/vpmu all wrong.
> > >
> > > Since I couldnât find any project that uses the BTS for the guest, I
> > > am wondering if anyone has ever done this and if it is possible at all.
> > >
> > > Any input is welcome as I am pretty much stuck atmâ
> > >
> > > Cheers
> > >
> > > Kevin
> > >
> > >
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