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

[Alexander Shishkin]

On Thu, May 03, 2018 at 01:50:39PM +0200, Paolo Bonzini wrote:
> On 03/05/2018 13:32, Alexander Shishkin wrote:
> >>  
> >> +/* Default is SYSTEM mode. */
> >> +static int __read_mostly pt_mode = PT_MODE_SYSTEM;
> >> +module_param(pt_mode, int, S_IRUGO);
> > So, it's an explicit module parameter? One apparent problem with this
> > is that one would need to reload kvm module(s) to be able to use PT,
> > which is not ideal.
> 
> If you want to do tracing system-wide, that by definition must disable
> guest tracing,

Sure.

> so I think the module parameter is appropriate.

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.

Regards,
--
Alex