Re: [Qemu-devel] [PATCH] cpu_physical_memory_write_rom() needs to do TB invalidates

[David Gibson <dwg@au1.ibm.com>]

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On Wed, Aug 22, 2012 at 09:05:52AM +0200, Jan Kiszka wrote:
> On 2012-08-22 08:47, Jan Kiszka wrote:
> > On 2012-08-22 07:57, David Gibson wrote:
> >> On Wed, Aug 22, 2012 at 07:55:31AM +0200, Alexander Graf wrote:
> >>>
> >>> On 22.08.2012, at 06:59, David Gibson wrote:
> >>>
> >>>> cpu_physical_memory_write_rom(), despite the name, can also be used to
> >>>> write images into RAM - and will often be used that way if the machine
> >>>> uses load_image_targphys() into RAM addresses.
> >>>>
> >>>> However, cpu_physical_memory_write_rom(), unlike cpu_physical_memory_rw()
> >>>> does invalidate any cached TBs which might be affected by the region
> >>>> written.
> >>>>
> >>>> This was breaking reset (under full emu) on the pseries machine - we loaded
> >>>> our firmware image into RAM, and while executing it rewrite the code at
> >>>> the entry point (correctly causing a TB invalidate/refresh).  When we
> >>>> reset the firmware image was reloaded, but the TB from the rewrite was
> >>>> still active and caused us to get an illegal instruction trap.
> >>>>
> >>>> This patch fixes the bug by duplicating the tb invalidate code from
> >>>> cpu_physical_memory_rw() in cpu_physical_memory_write_rom().
> >>>>
> >>>> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> >>>> ---
> >>>> exec.c |    7 +++++++
> >>>> 1 file changed, 7 insertions(+)
> >>>>
> >>>> diff --git a/exec.c b/exec.c
> >>>> index 5834766..eff40d7 100644
> >>>> --- a/exec.c
> >>>> +++ b/exec.c
> >>>> @@ -3523,6 +3523,13 @@ void cpu_physical_memory_write_rom(target_phys_addr_t addr,
> >>>>             /* ROM/RAM case */
> >>>>             ptr = qemu_get_ram_ptr(addr1);
> >>>>             memcpy(ptr, buf, l);
> >>>> +            if (!cpu_physical_memory_is_dirty(addr1)) {
> >>>> +                /* invalidate code */
> >>>> +                tb_invalidate_phys_page_range(addr1, addr1 + l, 0);
> >>>> +                /* set dirty bit */
> >>>> +                cpu_physical_memory_set_dirty_flags(
> >>>> +                    addr1, (0xff & ~CODE_DIRTY_FLAG));
> >>>> +            }
> >>>
> >>> Can't we just call cpu_physical_memory_rw in the RAM case? The
> >>> function only tries to not do MMIO accesses on ROM pages, right?
> >>
> >> Maybe.  It's not clear at all to me what cases
> >> cpu_physical_memory_write_rom() is supposed to be for, as opposed to
> >> just using cpu_physical_memory_rw().
> > 
> > write_rom ignores write protection - that you usually find on ROMs. That
> > makes no difference under KVM so far as there we lack read-only
> > sections. But that will be fixed soon, patches are on the list.
> 
> In fact, it does make a difference also for KVM mode as
> cpu_physical_memory_rw works from userspace while the limitation only
> affects guest code running under KVM control.

Ok, so IIUC, that means we do need the cpu_physical_memory_write_rom()
version for load_image_targphys(), and so my original patch is
basically the right fix.

> Jan
> 
> PS: I'm still facing a bogus Mail-Followup-To tag in your postings,
> David, thus you easily fall from the To list on reply.

Ah, yes, thanks for the reminder.  I think I finally found the right
option to stop mutt from doing that.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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