linux-kernel - Re: [PATCH] tools lib traceevent: update KVM plugin

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Date:	Tue, 20 Oct 2015 11:26:48 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Arnaldo Carvalho de Melo <acme@...hat.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org, acme@...nel.org
Subject: Re: [PATCH] tools lib traceevent: update KVM plugin

On Tue, 20 Oct 2015 17:19:12 +0200
Paolo Bonzini <pbonzini@...hat.com> wrote:

> 
> 
> On 20/10/2015 16:44, Steven Rostedt wrote:
> > What happens if you run new perf on an older kernel. Is this new plugin
> > going to be screwed up? Plugins should be backward compatible.
> 
> If you run new perf on older kernel, the new plugin will print the 
> "role" field (see kvm_mmu_print_role) slightly incorrectly.  That said, 
> the existing plugin was _also_ printing the role in a wildly wrong 
> format, like 2.6.35 vintage; the glevels field was removed by commit 
> 5b7e0102ae74, "KVM: MMU: Replace role.glevels with role.cr4_pae", in 
> April 2010.

Can we add a check if glevels field exists, and do the old format if it
does? I'm very strict on making sure event-parse works with all
incarnations of kernels.

> 
> Going forward it's really unlikely that the role will change apart from 
> adding new bits.  These can be added to the plugin while keeping it 
> backwards-compatible.  Addition to the role happen when you implement 
> new virtual MMU features such as SMEP, SMAP or SMM.  That's once per year
> or less.
> 
> > Is the plugin even still needed? I'm looking at some of the kvm events
> > and they seem to be mostly self sufficient. What ones need a plugin
> > today?
> 
> Yes, most of them are.  It's only needed for kvm_mmu_get_page, 
> kvm_mmu_prepare_zap_page and kvm_emulate_insn.  The latter is only 
> interesting if you install the disassembler library, and I wouldn't 
> really care if it went away.
> 
> kvm_mmu_get_page and kvm_mmu_prepare_zap_page, however, have output like
> 
>     kvm_mmu_get_page: [FAILED TO PARSE] mmu_valid_gen=0x2 gfn=786432 role=1923 root_count=0 unsync=0 created=1
> 
> without the plugin vs.
> 
>     kvm_mmu_get_page: new sp gfn c0000 3 q0 direct rwx !pae !nxe !wp root 0 sync
> 
> with the plugin.

Thanks for the update.

-- Steve
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