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Re: gdb vs. gdbserver with -mips3 / 32bitmode userspace

To: Johannes Stezenbach <js@linuxtv.org>, Daniel Jacobowitz <dan@debian.org>, "Maciej W. Rozycki" <macro@linux-mips.org>, linux-mips@linux-mips.org
Subject: Re: gdb vs. gdbserver with -mips3 / 32bitmode userspace
From: Ralf Baechle <ralf@linux-mips.org>
Date: Thu, 2 Feb 2006 13:43:29 +0000
In-reply-to: <20060201194443.GB21871@linuxtv.org>
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <20060131171508.GB6341@linuxtv.org> <Pine.LNX.4.64N.0601311724340.31371@blysk.ds.pg.gda.pl> <20060201164423.GA4891@nevyn.them.org> <20060201194443.GB21871@linuxtv.org>
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On Wed, Feb 01, 2006 at 08:44:43PM +0100, Johannes Stezenbach wrote:

> If I understand this correctly, gdbserver should check the
> register size supported by the OS, and communicate this to gdb?
> 
> I'm using a Linux kernel with CONFIG_32BIT, and if I understand
> the ptrace interface correctly, the registers as seen through
> ptrace are 32bit then, even though the CPU has 64bit registers.

Correct.  On 32-bit kernels Linux will largely forget about the 64-bitness
of the processor.  User processes will run with 64-bit (UX bit) disabled,
so they're never able to anything 64-bitish.  The kernel will only save
and restore the lower 32-bit of registers, so the upper 32-bit will be
lost.  That means only using the $zero register as 64-bit register is safe
and that's exploited by clear_page().  There are a few place where 64-bit
loads and stores are needed because particular hardware doesn't like being
talked to with 32-bit accesses; those accesses need make sure they're not
interrupted or bad things happen.  See the code for *readq() and write()
in <asm/io.h>.

Ages ago I tried leaving all the 64-bit functionality available as far as
possible even in 32-bit kernels.  It turned out to be example messy and I
was happy to have gotten rid of it, I think in 2.1.14.

> (I have no idea if the cp0 status suggested by others in this
> thread reflect CONFIG_32BIT vs. CONFIG_64BIT on Linux.)

On 32-bit kernels Linux will clear c0_status.kx, c0_status.ux and
c0_status.fr - on hardware were those bits exist at all, that is.

On 64-bit kernels Linux will set c0_status.kx.  It will also always set
c0_status.ux which means 64-bit operations are legal even for 32-bit
processes.  To my knowledge nobody is exploiting that; I guess it could
be exploited for a faster memcpy and similar.  Finally c0_status.fr which
will be set according to the ABI of the process, that is it'll be cleared
for o32 and set for N32 and N64 processes.

  Ralf

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