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Re: [Qemu-devel] [PATCH] hw/core/generic-loader: Fix PC overwriting

From: Julia Suvorova
Subject: Re: [Qemu-devel] [PATCH] hw/core/generic-loader: Fix PC overwriting
Date: Thu, 17 Jan 2019 13:58:37 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1

On 17.01.2019 13:13, Stefan Hajnoczi wrote:
On Wed, Jan 16, 2019 at 10:05:58PM +0300, Julia Suvorova via Qemu-devel wrote:
On 16.01.2019 0:51, Alistair Francis wrote:
On Tue, Jan 15, 2019 at 7:04 AM Julia Suvorova via Qemu-devel
<address@hidden> wrote:

If the memory is set using a file, and PC is specified on the command
line, it will be overwritten with the value 'entry'. This is not only
illogical, but also incorrect, because the load_ * functions do not take
into account the specifics of the ARM-M PC.

How does this come up?
I see that the value of entry will force overwrite the PC addr, but
doesn't force_raw fix that? Is there a common use case of loading an
ELF/uimage but having to manually specify a start address?

generic_loader_reset() is called after arm_cpu_reset() and damages PC
(it is wrong to call arm_cpu_set_pc() with entry to set ARM PC reset
value). Therefore, I tried to configure PC manually and ran into this

By the way, I do not know the right way to fix the original issue. Try
to replace generic_loader_reset() with the device reset function or
change the reset order or transfer PC reset value setting to a separate
function and associate it with cpu. What do you think about it?

generic_loader_reset() calls cpu_reset(s->cpu) followed by
CPUClass->set_pc(s->cpu, s->addr).

ARM's arm_cpu_set_pc() doesn't special-case the Thumb bit (that's only
done in arm_cpu_reset()) so we end up with an invalid PC for Thumb mode

Maybe the following arm_cpu_reset() code should be moved to

   env->regs[15] = initial_pc & ~1;
   env->thumb = initial_pc & 1;

Then arm_cpu_reset() can call arm_cpu_set_pc() instead of duplicating
this code.

No, set_pc() is called in cpu_tb_exec() to restore the PC value and
therefore should not be changed.

I haven't checked whether more special logic is needed in
arm_cpu_set_pc() aside from setting Thumb mode, but I think this should
do the trick?

The logic is a bit more complicated, but I think that it can be
transferred to a separate function, but not to arm_cpu_set_pc().

Best regards, Julia Suvorova.

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