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[Xen-devel] Re: [PATCH 2 of 3] xentrace: use tbuf_size for overflow chec

To: George Dunlap <george.dunlap@xxxxxxxxxx>
Subject: [Xen-devel] Re: [PATCH 2 of 3] xentrace: use tbuf_size for overflow check
From: Olaf Hering <olaf@xxxxxxxxx>
Date: Tue, 5 Apr 2011 12:19:51 +0200
Cc: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>
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On Fri, Apr 01, George Dunlap wrote:

> On Wed, 2011-03-30 at 19:04 +0100, Olaf Hering wrote:

> > xentrace: use tbuf_size for overflow check
> > 
> > The calculated number of per-cpu trace pages is stored in t_info and
> > shared with tools like xentrace. Since its an u16 the value may overflow
> > because the current check is based on u32.
> Hmm -- while this is true, it's possible this may change in the future.
> If we ever changed t_info.tbuf_size to be u32, then t_buf.prod/cons
> would again be the limiting factor.
> Should we perhaps add both checks?

I will update the patch to check for both.

> > Using the u16 means each cpu could in theory use up to 256MB as trace
> > buffer. However such a large allocation will currently fail on x86 due
> > to the MAX_ORDER limit.
> FWIW, I don't believe that there's any reason the allocations have to be
> contiguous any more.  I kept them contiguous to minimize the changes to
> the moving parts near a release.  But the new system has been pretty
> well tested now, so I think looking at non-contiguous allocations may be
> worthwhile.

This will be a bigger change I think. Added to my todo list.


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