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SubjectRe: [patch 02/11] x86 architecture implementation of Hardware Breakpoint interfaces

* K.Prasad <> 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

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

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. ]


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