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

[Jan Kiszka <jan.kiszka@web.de>]

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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.

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.


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