linux-kernel - RE: [PATCH V2 2/3] perf protect LBR when Intel PT is enabled.

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Date:	Mon, 7 Jul 2014 13:57:16 +0000
From:	"Liang, Kan" <kan.liang@...el.com>
To:	Peter Zijlstra <peterz@...radead.org>,
	Andi Kleen <andi@...stfloor.org>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: RE: [PATCH V2 2/3] perf protect LBR when Intel PT is enabled.

> 
> On Thu, Jul 03, 2014 at 05:52:37PM +0200, Andi Kleen wrote:
> > > If there's active LBR users out there, we should refuse to enable PT
> > > and vice versa.
> >
> > This doesn't work, e.g. hardware debuggers can take over at any time.
> 
> Tough cookies. Hardware debuggers get to deal with whatever crap they
> cause.

If so, I think I may discard this patch (2/3). I will resubmit the other two patches as a patch set to only handle the KVM issue we found.
It checks the access of LBR and extra MSRs at the initialization time and set the flags. So we just need to check the flags at runtime and avoid the protection by _safe(). 

For Intel PT and LBR handling, since the PT codes haven't been integrated yet, I will try to implement another patch later.
The patch will add flags for LBR and PT.
When enabling PT, it checks LBR flag and update the PT flag. When enabling LBR, it checks PT flag and update the LBR flag. When disabling LBR/PT, we just update the related flags. we don't need to add _safe or extra rmsr in fast path. 

How do you think?

Thanks,
Kan
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