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

[Kang, Luwei]

> > 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.

Yes, This series will expose all the HW capabilities to KVM guest if the Intel 
PT is supported in the guest.

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

I don't want to drop intel-pt live migration feature. As discussed with you 
before, the Intel PT feature includes some sub-features and may be different on 
each HW platform. Expose all the capabilities to the guest can't make live 
migration safe. Do you have any new proposals?

Thanks,
Luwei Kang

> 
> >
> >     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