MIME-Version: 1.0
In-Reply-To: <20121015111443.GA2006@linaro.org>
References: <1350242593-17761-1-git-send-email-rabin@rab.in>
 <1350242593-17761-9-git-send-email-rabin@rab.in> <20121015111443.GA2006@linaro.org>
From: Rabin Vincent <rabin@rab.in>
Date: Mon, 15 Oct 2012 13:44:55 +0200
Message-ID: <CAH+eYFC1fr0vpK2T4hz9LK=Nz5bzQBQ4O-o0f9tsnj-_D9ZEJg@mail.gmail.com>
Subject: Re: [PATCH 9/9] ARM: add uprobes support
To: Dave Martin <dave.martin@linaro.org>
Cc: linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        Peter Zijlstra <peterz@infradead.org>,
        Srikar Dronamraju <srikar@linux.vnet.ibm.com>, oleg@redhat.com,
        Tixy <tixy@linaro.org>
Content-Type: text/plain; charset=ISO-8859-1
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1945
Lines: 46

2012/10/15 Dave Martin <dave.martin@linaro.org>:
> On Sun, Oct 14, 2012 at 09:23:13PM +0200, Rabin Vincent wrote:
>> Add basic uprobes support for ARM.
>>
>> perf probe --exec and SystemTap's userspace probing work.  The ARM
>> kprobes test code has also been run in a userspace harness to test the
>> uprobe instruction decoding.
>
> The assumption that the target code is ARM appears to be buried all over
> the place.

Right, as stated:

>> Caveats:
>>  - Thumb is not supported

> Certainly this code as currently written must depend on !THUMB2_KERNEL.

Why?  It currently works for ARM userspace even if the kernel is
Thumb-2.

> However, there's an underlying problem here which we'd need to solve.
> The kprobes code can take advantage of the fact that the kernel is all
> ARM or (almost) all Thumb code.  So there is no support for kprobes
> supporting ARM and Thumb at the same time.
>
> With userspace, we don't have this luxury.  With Debian armhf, Ubuntu
> and Linaro building Thumb-2 userspaces, it may be an increasingly common
> configuration independently of whether the kernel is built in Thumb-2
> or not.
>
> Furthermode, there is no such thing as a pure Thumb-2 system in practice:
> PLTs are always ARM, for example.  For uprobes to work well in userspace,
> both should be supported together.

Right.  I don't think it's difficult to support both of them together,
my thought was just to have userspace tell us if they want to probe a
Thumb instruction via the usual 0th-bit set convention, and then take it
from there.  I didn't do the Thumb handling currently because I didn't
really look into modifying the kprobes Thumb instruction decoding and
the IT handling for uprobes.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/