Re: [PATCH v7 06/13] KVM: x86: Add Intel Processor Trace virtualization mode

[Paolo Bonzini]

On 03/05/2018 14:02, Alexander Shishkin wrote:
> I don't see why. System-wide tracing takes place while perf record is
> running. When it's done, it's done and we can un-disable the guest
> tracing, without requiring the user to kill all their VMs and reload
> modules.

Guest tracing can only be enabled at boot time, because the guest's
CPUID changes depending on whether it's enabled.  And likewise if perf
record can do system-wide tracing at any time during the guest's
execution, we need to know it at boot time in order to set the guest CPUID.

Paolo