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Re: [Xen-devel] Branch Trace Storage for guestsandVPMUinitialization

On 02/26/2015 08:44 AM, Kevin.Mayer@xxxxxxxx wrote:

-----Ursprüngliche Nachricht-----
Von: Boris Ostrovsky [mailto:boris.ostrovsky@xxxxxxxxxx]
Gesendet: Mittwoch, 25. Februar 2015 23:20
An: Mayer, Kevin
Betreff: Re: AW: AW: [Xen-devel] Branch Trace Storage for guests

On 02/25/2015 01:23 PM, Kevin.Mayer@xxxxxxxx wrote:
-----Ursprüngliche Nachricht-----
Von: Boris Ostrovsky [mailto:boris.ostrovsky@xxxxxxxxxx]
Gesendet: Mittwoch, 25. Februar 2015 17:32
An: Mayer, Kevin
Cc: xen-devel@xxxxxxxxxxxxx
Betreff: Re: AW: [Xen-devel] Branch Trace Storage for guests and

On 02/25/2015 10:12 AM, Kevin.Mayer@xxxxxxxx wrote:
-----Ursprüngliche 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

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,… 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
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]

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.

Yes, this is exactly what I want to do.
Too bad that your patches are under review. Would have been pretty
helpful I think.

To be honest, I never tested them for BTS so they may not work in that
mode. In fact, as you will realize by reading what I said below, they probably
don't ;-(

Maybe I should point out that I´m a total noob with xen and I definitely
don’t understand all parts yet.
So there may be some dumb mistakes in my assumptions.

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
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.)
Right again. As you said my goal is to profile the guest from dom0. So
whenever the CPU is in guestmode and a branch is taken it should be stored
in the BTS, but not when the CPU is running dom0. My idea was basically to
set up the memory for the BTS and the GUEST_IA32_DEBUGCTL so when
there is a vmexit the logging stops and starts again when there is a vmenter.
As far as I understand the IA32_DEBUGCTL gets switched between the
dom0-value and the guest-value (stored in vmcs) when there is a
vmexit/vmenter, right?

Right. And now I am not longer sure whether your buffer should be in
hypervisor or guest's space: after VMENTER the hardware will load guest's
versions of IA32_DEBUGCTLMSR and MSR_IA32_DS_AREA. I don't know
whether you can prevent this from happening (need to look in the spec).
And if that's the case then you might be able to:

1. Map DS area and BTS buffer in both guest and hypervisor. I believe your
guest will have to have this mapped since these ares will be accessed via
guest's EPT. As I said, I don't know how you'd do this in Windows --- I know
nothing about programming there. I assume it can be done since there are
Windows PV drivers for Xen.
2. Have dom0 set appropriate bits in IA32_DEBUGCTLMSR to start tracing.
You will need to first pause your guest's VCPUs, then update appropriate
register in VMCS (bracketed with vmx_vmcs_enter/exit) and then unpause
3. If you program BTS to generate interrupts you may need to do something
about it in vpmu_interrupt() to prevent those interrupts from going into the
guest as this will likely confuse it and it will die (the interrupt I think 
will be an
NMI, making things real bad for the guest).
3. Now you should be able to read buffers from hypervisor.
Why should I prevent the loading of guest IA32_DEBUGCTLMSR and MSR_IA32_DS_AREA?

I was hoping that you might then keep the buffer in hypervisor space. But I don't think it's possible to make HW not virtualize those two registers (DS_AREA in particular).

The idea was to access/setup the guest IA32_DEBUGCTLMSR and MSR_IA32_DS_AREA 
when in dom0.
So when there is a VMENTER the guest registers get loaded and the BTS starts to 
And stops of course when there is an VMEXIT.

Regarding 1.
I`m not sure how I know at which address the BTS is located in this case.
Let´s say I setup the BTS in the guest at address x. To get this address x I 
need to
read the guest MSR_IA32_DS_AREA, right?

If the guest wrote this address to MSR_IA32_DS_AREA then yes.

For this I would need to access the vcpu->arch_vcpu-> hvm_vcpu->vpmu
used by the guest since the MSR_IA32_DS_AREA isn`t part of the vmcs
(and therefore cannot be accessed by the handy __vmread()).
Is there a good way to get this information during a vpmu_interrupt() (since I 
The BTINT will have to be handled there), or maybe a VMEXIT?

I believe the interrupt can only happen when the guest is running on the physical processor so you should be able to get to guest's VPMU as vcpu_vpmu(v)->context->ds_area. Same for VMEXIT.

2. I already use the vmx_vmcs_enter/exit but didn’t think about pausing the 
I will add that.

3. I didn’t look at the BTINT yet, but this sounds reasonable.

This would be "the guest is logging the branch traces", but it is setup and
controlled from the dom0. So more or less a hybrid I think.
Of course the guest must not be able to use this memory in its
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 isn´t 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.
The dom0 writes to MSR_IA32_DS_AREA. I want to do all the setup and
controlling from dom0 in a way that enables the guest to store branch
traces in the BTS (that was setup by the dom0)
I think I understand why you crash hypervisor now. I mentioned above that
writing into vmcs requires bracketing by vmx_vmcs_enter/exit. So, in
addition to having new vcpu parameter to vmx_msr_write_intercept(), you
need to add those two. See vmx_vlapic_msr_changed(), right above
vmx_msr_write_intercept(). And don't forget to pause guest's vcpu (I am
pretty sure you need that since your guest may be running somewhere else
at this time).

Sorry if my explanations are a bit confusing. I myself am confused about
this part of the Xen-code.
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 my own vmx_msr_write_intercept (which explicitly uses the
vcpu of my guest domain instead of the "current") and my own
core2_vpmu_do_wrmsr , core2_vpmu_msr_common_check I don’t get a
host reboot, but a dying guest when I try to enable BTS. As you said most
likely because the MSR_IA32_DS_AREA points to dom0-memory and the
hypervisor is not amused when a guest tries to write stuff there.
When I use the build in ones (which all use struct vcpu *v = current;) I get a
host reboot.
Maybe because of a missing vpmu-structs as I notice that only one vcpu_id
gets initialized in vpmu_initialise during boot.
So when using the build in vmx_msr_write_intercept the writing ends in
vpmu_do_wrmsr at if ( vpmu->arch_vpmu_ops && vpmu-
arch_vpmu_ops->do_wrmsr )
          return vpmu->arch_vpmu_ops->do_wrmsr(msr, msr_content); and
the host reboots.
Maybe I need some special kind of initialization before I call
Even with
struct vcpu *current_v=current;
return_value= vmx_msr_write_intercept(MSR_IA32_DS_AREA,
ds_buffer_management_area); I get an instant host reboot at the above
mentioned return vpmu->arch_vpmu_ops->do_wrmsr(msr, msr_content);
Right. Because you are trying to access VMCS from dom0 context. dom0
doesn't have VMCS as it is a PV guest.

I thought so, but isn’t the if clause
if ( vpmu->arch_vpmu_ops && vpmu->arch_vpmu_ops->do_wrmsr )
supposed to catch that?

Yes, it should. BTW, why are you explicitly calling vpmu_initialise()? It should be called during guest VCPU initialization.

I'd need to see 'xl dmesg' output at time of reboot, possibly with disassembly of code at RIP that presumably will be reported as causing the reboot.




When I use a modified version of vmx_msr_write_intercept I don’t
any crashes as long as I don’t enable BTS and TR in the
GUEST_IA32_DEBUGCTL (BTR works). When I enable the BTS (and TR)
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) ** 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! **
Yes, I saw that. It doesn’t state that BTS is not working at all, just that it 
not that safe to use.
As I understand it as long as I set the DS buffer address correctly I should
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).

We are thinking very much alike. I also am not convinced I set the buffer
correctly. ^^
But since I get a reboot as soon as
return vpmu->arch_vpmu_ops->do_wrmsr(msr, msr_content); gets called
I don’t think that the setup of the buffer is the problem (when using the
original vmx_msr_write_intercept), but rather something with the setup of
the vpmu.
When I use my own vmx_msr_write_intercept with the d->vcpu[0] instead
of current the writing succeeds but the guest crashes/gets killed when the
BTS is enabled.
So in this second case the setup of the buffer seems to be the problem.


Since I don’t want to use for production that is fine with me. At least for

The modified version of vmx_msr_write_intercept takes a vcpu-struct
a parameter and uses this instead of the current vcpu.

Instead of

staticint vmx_msr_write_intercept(unsigned int msr, uint64_t

       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

I´m 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 couldn’t 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 atm…



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