Re: [PATCH v13 08/12] KVM: x86: Add Intel PT context switch for each vcpu

[Alexander Shishkin]

Paolo Bonzini <pbonzini@xxxxxxxxxx> writes:

> On 30/10/2018 11:00, Thomas Gleixner wrote:
>> So at least we need a way for perf on the host to programmatically detect,
>> that 'guest traces itself' is enabled, so it can inject that information
>> into the host data and post processing can tell that. W/o something like
>> that it's going to be a FAQ.
>
> In guest-tracing mode there will be already a TIP.PGD and TIP.PGE packet
> respectively before vmentry and after vmexit, caused by the RTIT_CTL
> WRMSRs in pt_guest_enter and pt_guest_exit.  The target IP of the
> packets will come from kvm-intel.ko.

Most people aren't tracing the kernel, so they'd just get a PGD with no
address and a PGE after the kvm is done without any indication of what
happened in between.

> In system mode instead you get a Paging Information Packet on
> vmentry/vmexit, with bit 0 set in the third byte.  You won't get it if
> guest-side tracing is on (because tracing has been disabled by
> pt_guest_enter and won't be re-enabled until pt_guest_exit).  I don't
> think it's correct to "fake" the PIP in guest-tracing mode, because
> TIP.PGD should be followed immediately by TIP.PGE.

Indeed, we should most definitely not fake PIP. Perf has RECORD_AUX,
which already has PARTIAL flag that was introduced specifically because
of kvm.

Regards,
--
Alex