Re: [PATCH v1 0/3] Remove the limitation of Intel PT CPUID info

[Eduardo Habkost]

On Mon, Sep 21, 2020 at 07:49:22AM +0000, Kang, Luwei wrote:
> Hi Eduardo,
>     This patch set will remove some limitations of Intel PT CPUID information.
>     1. The "IP payloads" feature will disable the Intel PT in guests and it 
> will be coming soon.
>     2. To make the live migration safe, we set the Intel PT CPUID as a 
> constant value(Icelake server CPUID). It will mask off the new feature of 
> Intel PT.

Isn't this series doing the opposite of 2?  It replaces all
constant CPUID values with kvm_arch_get_supported_cpuid(), making
the feature unavailable in migration-safe mode.

Does it mean the plan is to drop intel-pt migration support
entirely?

> 
>     About this issue https://bugzilla.redhat.com/show_bug.cgi?id=1853972, 
> Intel PT is disabled in the guest by default, we should use "-cpu 
> Icelake-Server,+intel-pt" to enable the Intel PT.

That's correct.  The point of the BZ is that libvirt
mode=host-model was expected to include intel-pt automatically
when available.  With this series, the request in the BZ stops
making sense (because intel-pt won't be migration-safe anymore),
but I'm not sure yet that's really the plan.


> 
> Thanks,
> Luwei Kang
> 
> > -----Original Message-----
> > From: Eduardo Habkost <ehabkost@redhat.com>
> > Sent: Saturday, September 19, 2020 6:03 AM
> > To: Kang, Luwei <luwei.kang@intel.com>
> > Cc: pbonzini@redhat.com; rth@twiddle.net; qemu-devel@nongnu.org; Strong,
> > Beeman <beeman.strong@intel.com>; Jiri Denemark
> > <jdenemar@redhat.com>; Robert Hoo <robert.hu@linux.intel.com>
> > Subject: Re: [PATCH v1 0/3] Remove the limitation of Intel PT CPUID info
> > 
> > Hi Luwei Kang,
> > 
> > I was looking for info on intel-pt and just saw this series, and it was 
> > never
> > reviewed or merged (sorry for missing it!).  Is this still the approach we 
> > want to
> > follow for intel-pt?
> > 
> > I'm CCing Jiri Denemark because this might be relevant for a libvirt issue 
> > related
> > to intel-pt we were investigating[1].
> > 
> > [1] https://bugzilla.redhat.com/show_bug.cgi?id=1853972
> > 
> > 
> > On Mon, Mar 30, 2020 at 09:56:09AM +0000, Kang, Luwei wrote:
> > > > -----Original Message-----
> > > > From: Kang, Luwei <luwei.kang@intel.com>
> > > > Sent: Tuesday, February 25, 2020 5:38 AM
> > > > To: pbonzini@redhat.com; rth@twiddle.net; ehabkost@redhat.com
> > > > Cc: qemu-devel@nongnu.org; Strong, Beeman
> > <beeman.strong@intel.com>;
> > > > Kang, Luwei <luwei.kang@intel.com>
> > > > Subject: [PATCH v1 0/3] Remove the limitation of Intel PT CPUID info
> > > >
> > > > The Intel PT feature includes some
> > > > sub-features(CPUID.(EAX=14H,ECX=0H))
> > > > and these sub-features are different on different HW platforms. To
> > > > make the live migration safety(get the same CPUID info with same cpu
> > > > model on different HW platform), the current Intel PT CPUID
> > > > information is set to a constant value(from ICELAKE Server).
> > > >
> > > > It will block the new feature in the later HW platform. what's more,
> > > > the support of "IP payloads" will disable the Intel PT in KVM
> > > > guest(patch 1) but it will come soon.
> > > >
> > > > This patchset remove this limitation and expose all the capabilities
> > > > to KVM guest. As it will break the live migration safe, Intel PT
> > > > will be masked as unmigratable.
> > >
> > > Ping.
> > >
> > > Thanks,
> > > Luwei Kang
> > >
> > > >
> > > > Luwei Kang (3):
> > > >   i386: Remove the limitation of IP payloads for Intel PT
> > > >   i386: Remove the CPUID limitation of Intel PT
> > > >   i386: Mark the 'INTEL_PT' CPUID bit as unmigratable
> > > >
> > > >  target/i386/cpu.c | 69
> > > > ++++---------------------------------------------------
> > > >  1 file changed, 5 insertions(+), 64 deletions(-)
> > > >
> > > > --
> > > > 1.8.3.1
> > >
> > 
> > --
> > Eduardo
> 

-- 
Eduardo