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

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] [PATCH] hw/core/generic-loader: Fix PC overwriting
Date: Thu, 17 Jan 2019 10:13:09 +0000
User-agent: Mutt/1.10.1 (2018-07-13)

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
> problem.
> 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.

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?


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