x86/debug: Support negative polarity DR6 bits
authorPeter Zijlstra <peterz@infradead.org>
Wed, 2 Sep 2020 13:26:01 +0000 (15:26 +0200)
committerThomas Gleixner <tglx@linutronix.de>
Fri, 4 Sep 2020 13:12:57 +0000 (15:12 +0200)
commitf4956cf83ed12271bdbd5b547f3378add72bbffb
tree38b50d0123e68a5accafd80d13a680383d506695
parent21d44be7b6ff4c254dc971e2c99d4082dd470afd
x86/debug: Support negative polarity DR6 bits

DR6 has a whole bunch of bits that have negative polarity; they were
architecturally reserved and defined to be 1 and are now getting used.
Since they're 1 by default, 0 becomes the signal value.

Handle this by xor'ing the read DR6 value by the reserved mask, this
will flip them around such that 1 is the signal value (positive
polarity).

Current Linux doesn't yet support any of these bits, but there's two
defined:

 - DR6[11] Bus Lock Debug Exception (ISEr39)
 - DR6[16] Restricted Transactional Memory (SDM)

Update ptrace_{set,get}_debugreg() to provide/consume the value in
architectural polarity. Although afaict ptrace_set_debugreg(6) is
pointless, the value is not consumed anywhere.

Change hw_breakpoint_restore() to alway write the DR6_RESERVED value
to DR6, again, no consumer for that write.

Suggested-by: Andrew Cooper <Andrew.Cooper3@citrix.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Daniel Thompson <daniel.thompson@linaro.org>
Link: https://lore.kernel.org/r/20200902133201.354220797@infradead.org
arch/x86/kernel/hw_breakpoint.c
arch/x86/kernel/ptrace.c
arch/x86/kernel/traps.c