linux-kernel - Re: [patch 02/11] x86 architecture implementation of Hardware Breakpoint interfaces

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Date:	Wed, 11 Mar 2009 14:10:07 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	"K.Prasad" <prasad@...ux.vnet.ibm.com>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Roland McGrath <roland@...hat.com>
Subject: Re: [patch 02/11] x86 architecture implementation of Hardware
	Breakpoint interfaces

* K.Prasad <prasad@...ux.vnet.ibm.com> wrote:

> Even if #3 was implemented as described, we would still retain 
> a majority of the complexity in balance_kernel_vs_user() to 
> check newer tasks with requests for breakpoint registers.

Not if it's implemented in a really simple way:

Kernel gets debug registers in db4..db3..db2..db1 order, and its 
allocation is essentially hardcoded - i.e. we dont try to be 
fancy.

User-space (gdb) on the other hand will try to allocate in the 
db1..db2..db3..db4 order.

Maintain a 'max debug register index' value driven by ptrace and 
maintain a 'min debug register index' driven by kernel-space 
hw-breakpoint allocations.

If they meet somewhere inbetween then we have overcommit which 
we dont allow. In all other cases (which i proffer covers 100% 
of the sane cases) they will mix amicably.

Sure, user-space can in principle do db4..db3..db2..db1 
allocations as well, but it would be silly and GDB does not do 
that.

So there's no real overlap between register usage - hence no 
need for any complex scheduling smarts. Keep it simple first, 
and only add complexity when it's justified.

[ for the special case of an architecture having just a single 
  debug register this will bring the expected behavior of either 
  allowing gdb to use the breakpoint or allowing the kernel to 
  use it. ]

	Ingo
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