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Date: Tue, 13 Sep 2016 18:57:04 +0200
From: Greg KH <gregkh@linuxfoundation.org>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Matt Fleming <matt@codeblueprint.co.uk>, linux-kernel@vger.kernel.org,
        stable@vger.kernel.org
Subject: Re: KVM patches applied in weird order in -stable
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On Tue, Sep 13, 2016 at 06:26:12PM +0200, Paolo Bonzini wrote:
> 
> 
> On 13/09/2016 16:58, Greg KH wrote:
> > [adding stable@ as this is a stable issue, not a 'normal' issue]
> > 
> > On Tue, Sep 13, 2016 at 03:51:00PM +0100, Matt Fleming wrote:
> >> Folks,
> >>
> >> While hunting down a performance issue involving KVM I was surprised
> >> to see "native_set_debugreg()" as the first entry in `perf top`.
> >>
> >> Digging deeper, it looks as though the following patches were applied
> >> in the wrong order in -stable. This is the order as they appear in
> >> Linus' tree,
> >>
> >>  [0] commit 4e422bdd2f84 ("KVM: x86: fix missed hardware breakpoints")
> >>  [1] commit 172b2386ed16 ("KVM: x86: fix missed hardware breakpoints")
> >>  [2] commit 70e4da7a8ff6 ("KVM: x86: fix root cause for missed hardware breakpoints")
> >>
> >> but this is the order for linux-4.4.y
> >>
> >>  [1] commit fc90441e728a ("KVM: x86: fix missed hardware breakpoints")
> >>  [2] commit 25e8618619a5 ("KVM: x86: fix root cause for missed hardware breakpoints")
> >>  [0] commit 0f6e5e26e68f ("KVM: x86: fix missed hardware breakpoints")
> >>
> >> The upshot is that KVM_DEBUGREG_RELOAD is always set when returning
> >> from kvm_arch_vcpu_load() in stable, but not in Linus' tree.
> > 
> > How would applying these in a different order cause breakage?
> 
> [2] is reverting [0]+[1].  Stable is not due to the different order.

Really?  Are you sure that [0] and [1] isn't just the same commit?  It
looks like that to me.

> > And if this is a problem, can you please send me a patch to fix it up?
> 
> Yup, on the way.

thanks,

greg k-h