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Subject: Re: [RFC][PATCH 0/6] perf: x86 RDPMC and RDTSC support
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Vince Weaver <vince@deater.net>
Cc: Vince Weaver <vweaver1@eecs.utk.edu>, mingo@elte.hu,
        William Cohen <wcohen@redhat.com>,
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        linux-kernel@vger.kernel.org
Date: Wed, 21 Dec 2011 23:32:59 +0100
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On Wed, 2011-12-21 at 17:19 -0500, Vince Weaver wrote:
> 
> Speaking of bit 48, does your implementation do anything useful about
> the 32/40/48 bit limitation that some processors have with rdpmc?
> Or is the answer that if you might get overflow, then you have to use slow 
> syscall-based read? 

Yeah, we program an overflow interrupt around there (47 bits iirc) and
consume the total into the u64 count value and update the mmap control
page's offset.