Re: [Qemu-stable] [Qemu-devel] [PATCH] i386: mark the 'INTEL_PT' CPUID bit as unmigratable

[Eduardo Habkost]

On Fri, Dec 28, 2018 at 02:53:22PM +0100, Paolo Bonzini wrote:
> On 25/12/18 09:23, Kang, Luwei wrote:
> >> From: Qemu-devel [mailto:address@hidden On Behalf Of Paolo Bonzini
> >> Sent: Friday, December 21, 2018 8:44 PM
> >> To: address@hidden
> >> Cc: address@hidden; address@hidden
> >> Subject: [Qemu-devel] [PATCH] i386: mark the 'INTEL_PT' CPUID bit as 
> >> unmigratable
> >>
> >> Marshaling of processor tracing MSRs is not yet implemented in QEMU, mark 
> >> the feature as unmigratable.
> > 
> > Hi Paolo,
> >     I think Intel PT has supported live migration. I don't understand what 
> > you mean.
> > 
> >     commit b77146e9a129bcdb60edc23639211679ae846a92
> >     Author: Chao Peng <address@hidden>
> >     Date:   Mon Mar 5 00:48:36 2018 +0800
> >     i386: Add support to get/set/migrate Intel Processor Trace feature
> 
> Indeed.  I had forgotten this patch because it was committed so long
> before the kernel parts (which really should not happen, but Eduardo and
> I miscommunicated on it).  Can you check that it still works?

My mistake, sorry.  I should have checked the status of the
kernel code before merging the original series, or waited for
your review.

I'm re-reading the code now and I'm worried about two things:

The code will break if GET_SUPPORTED_CPUID returns INTEL_PT, but
the MSR emulation code is not present yet.  Maybe it won't be an
issue in practice because it happens only between the two Linux
commits (commit 86f5201df0d3 "KVM: x86: Add Intel Processor Trace
cpuid emulation" and commit bf8c55d8dc09 "KVM: x86: Implement
Intel PT MSRs read/write emulation") and shipping a kernel with
the CPUID code without the MSR commit seems pointless.

The kvm_arch_get_supported_cpuid() call inside kvm_get_msrs()
looks suspicious.  What should happen if we try to migrate to a
host that returns a smaller number on
kvm_arch_get_supported_cpuid(0x14, 1, R_EAX)?

-- 
Eduardo