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

[Alexander Graf]

On 22.08.2012, at 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 <address@hidden>
>>> ---
>>> 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().

I can only guess, but the code looks to me as if it wants to be a nop on ROM 
pages, while basically doing cpu_physical_memory_rw for RAM pages. Usually in 
QEMU, every non-RAM page gets treated as MMIO which might eventually lead to 
machine checks.


Alex