Re: [RFC] [PATCH 1/7] User Space Breakpoint Assistance Layer (UBP)

From: Avi Kivity
Date: Mon Jan 18 2010 - 08:34:17 EST

On 01/18/2010 03:15 PM, Peter Zijlstra wrote:
On Mon, 2010-01-18 at 14:37 +0200, Avi Kivity wrote:
On 01/18/2010 02:14 PM, Peter Zijlstra wrote:
Well, the alternatives are very unappealing. Emulation and
single-stepping are going to be very slow compared to a couple of jumps.

With CPL2 or RPL on user segments the protection issue seems to be
manageable for running the instructions from kernel space.

CPL2 gives unrestricted access to the kernel address space; and RPL does
not affect page level protection. Segment limits don't work on x86-64.
But perhaps I missed something - these things are tricky.
So setting RPL to 3 on the user segments allows access to kernel pages
just fine? How useful.. :/

The further we stay away from segmentation, the better. Thankfully AMD removed hardware task switching from x86-64 so we can't even think about that.

It should be possible to translate the instruction into an address space
check, followed by the action, but that's still slower due to privilege
level switches.
Well, if you manage to do the address validation you don't need the priv
level switch anymore, right?


Are the ins encodings sane enough to recognize mem parameters without
needing to know the actual ins?

No. You need to know whether the instruction accesses memory or not.

Look at the tables at the beginning of arch/x86/kvm/emulate.c. Opcodes marked with ModRM, BitOp, MemAbs, String, Stack are all different styles of memory instructions. You need to know the operand size for the edge cases. And there are probably a few special cases in the code.

How about using a hw-breakpoint to close the gap for the inline single
step? You could even re-insert the int3 lazily when you need the
hw-breakpoint again. It would consume one hw-breakpoint register for
each task/cpu that has probes though..

If you have more than four threads, it breaks, no? And you need an IPI each time you hit the breakpoint.

Ultimately I'd like to see the breakpoint avoided as well, use a jump to the XOL area and trace in ~20 cycles instead of ~1000.

error compiling committee.c: too many arguments to function

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