Re: [Qemu-devel] [PATCH] spapr-rtas: reset top 4 bits in parameters address

[Alexander Graf]

Am 06.09.2013 um 07:04 schrieb Alexey Kardashevskiy <address@hidden>:

> On 09/06/2013 12:24 AM, Alexey Kardashevskiy wrote:
>> On 09/05/2013 11:08 PM, Alexander Graf wrote:
>>> 
>>> On 05.09.2013, at 14:49, Alexey Kardashevskiy wrote:
>>> 
>>>> On 09/05/2013 10:16 PM, Alexander Graf wrote:
>>>>> 
>>>>> On 05.09.2013, at 14:04, Alexey Kardashevskiy wrote:
>>>>> 
>>>>>> On 09/05/2013 08:21 PM, Alexander Graf wrote:
>>>>>>> 
>>>>>>> On 05.09.2013, at 12:17, Alexey Kardashevskiy wrote:
>>>>>>> 
>>>>>>>> On 09/05/2013 07:27 PM, Alexander Graf wrote:
>>>>>>>>> 
>>>>>>>>> On 05.09.2013, at 09:40, Alexey Kardashevskiy wrote:
>>>>>>>>> 
>>>>>>>>>> On 09/05/2013 05:08 PM, Alexander Graf wrote:
>>>>>>>>>>> 
>>>>>>>>>>> 
>>>>>>>>>>> Am 05.09.2013 um 07:58 schrieb Alexey Kardashevskiy 
>>>>>>>>>>> <address@hidden>:
>>>>>>>>>>> 
>>>>>>>>>>>> On the real hardware, RTAS is called in real mode and therefore
>>>>>>>>>>>> ignores top 4 bits of the address passed in the call.
>>>>>>>>>>> 
>>>>>>>>>>> Shouldn't we ignore the upper 4 bits for every memory access in 
>>>>>>>>>>> real mode, not just that one parameter?
>>>>>>>>>> 
>>>>>>>>>> We probably should but I just do not see any easy way of doing this. 
>>>>>>>>>> Yet
>>>>>>>>>> another "Ignore N bits on the top" memory region type? No idea.
>>>>>>>>> 
>>>>>>>>> Well, it already works for code that runs inside of guest context, 
>>>>>>>>> because there the softmmu code for real mode strips the upper 4 bits.
>>>>>>>>> 
>>>>>>>>> I basically see 2 ways of fixing this "correctly":
>>>>>>>> 
>>>>>>>>> 1) Don't access memory through cpu_physical_memory_rw or ldx_phys but
>>>>>>>>> instead through real mode wrappers that strip the upper 4 bits, 
>>>>>>>>> similar
>>>>>>>>> to how we handle virtual memory differently from physical memory
>>>>>>>> 
>>>>>>>> But there is no a ready wrapper for this, correct? I could not find 
>>>>>>>> any. I
>>>>>>>> would rather do this, looks nicer than 2).
>>>>>>>> 
>>>>>>>> 
>>>>>>>>> 2) Create 15 aliases to system_memory at the upper 4 bits of address
>>>>>>>>> space. That should at the end of the day give you the same effect
>>>>>>>> 
>>>>>>>> Wow. Is not that too much?
>>>>>>>> Ooor since I am normally making bad decisions, I should do this :)
>>>>>>>> 
>>>>>>>> 
>>>>>>>>> The fix as you're proposing it wouldn't work for indirect memory
>>>>>>>>> descriptors. Imagine you have an "address" parameter that gives you a
>>>>>>>>> pointer to a struct in memory that again contains a pointer. You still
>>>>>>>>> want that pointer be interpreted correctly, no?
>>>>>>>> 
>>>>>>>> Yes I do. I just think that having non zero bits at the top is a bug 
>>>>>>>> and I
>>>>>>>> would not want the guest to continue sending bad addresses to the 
>>>>>>>> host. Or
>>>>>>>> at least I want to know if it still happening.
>>>>>>>> 
>>>>>>>> Now we know that the only occasion of this misbehaviour is the 
>>>>>>>> "stop-self"
>>>>>>>> call and others works just fine. If something new comes up (what is 
>>>>>>>> pretty
>>>>>>>> unlikely, otherwise we would have noticed this issue a loong time ago 
>>>>>>>> AND
>>>>>>>> Paul already made&posted a patch for the host to fix __pa() so it is 
>>>>>>>> not
>>>>>>>> going to happen on new kernels either), ok, we will think of fixing 
>>>>>>>> this.
>>>>>>>> 
>>>>>>>> Doing in QEMU what the hardware does is a good thing but here I would 
>>>>>>>> think
>>>>>>>> twice.
>>>>>>> 
>>>>>>> Well, the idea behind RTAS is that everything RTAS does is usually run 
>>>>>>> in IR=0 DR=0 inside of guest context, so that's the view of the world 
>>>>>>> we should expose.
>>>>>>> 
>>>>>>> Which makes me think.
>>>>>> 
>>>>>>> Couldn't we just set IR=0 DR=0 when getting an RTAS call and use the
>>>>>>> virtual memory access functions? Those will already strip the upper 4
>>>>>>> bits.
>>>>>> 
>>>>>> Ok. We reached the border where my ignorance starts :) Never could
>>>>>> understand the concept of the guest virtual memory in QEMU.
>>>>>> 
>>>>>> So we clear IR/DR and call what API? This is not address_space_rw() and
>>>>>> company, right?
>>>>> 
>>>>> Nono, we basically route things through the same accesses that 
>>>>> instructions inside of guest context would call. Something like
>>>>> 
>>>>> cpu_ldl_data()
>>>>> 
>>>>> for example. IIRC there is also an #ifdef that allows you to just run 
>>>>> ldl().
>>>> 
>>>> cpu_ldl_data() is defined for CONFIG_USER_ONLY. But ok, it is defined
>>>> simply as ldl_p():
>>>> 
>>>> #define cpu_ldl_data(env, addr) ldl_raw(addr)
>>>> #define g2h(x) ((void *)((unsigned long)(target_ulong)(x) + GUEST_BASE))
>>>> #define laddr(x) g2h(x)
>>>> #define ldl_raw(p) ldl_p(laddr((p)))
>>>> 
>>>> static inline int ldl_p(const void *ptr)
>>>> {
>>>>   int32_t r;
>>>>   memcpy(&r, ptr, sizeof(r));
>>>>   return r;
>>>> }
>>>> 
>>>> So it tries accessing memory @ptr (which is the guest physical) and -
>>>> crashes :) So I need an address converter which is not there.
>>>> 
>>>> What do I miss? Thanks.
>>> 
>>> It should be defined through a bunch of macros and incomprehensible 
>>> #include's and glue()'s for softmmu too. Just try and see if it works for 
>>> you.
>> 
>> 
>> Hm. I was not clear. I tried. It crashed in ldl_p() and I explained why
>> exactly. I understand what you expected but it should be different set of
>> macros than the one you proposed.
> 
> Oh. Figured it out, that actually works. I just looked at wrong definition
> (which does not use CPU state) of cpu_ldl_data() because cscope and grep
> just could not the correct one.
> 
> I had to put a breakpoint in ppc_hash64_handle_mmu_fault() to find a
> cpu_ldl_code, then I tried to define the _data versions of cpu_lXX_code via
> exec/exec-all.h (this is where the _code versions are defined) but it
> turned out that they are already defined in "exec/softmmu_exec.h" :-/
> 
> The glue() macro is a pure, refined evil, there should be at least a
> comment saying what those wonderful macros define :(

Yeah :).

With this change we might need to do a cpu_register_sync on every RTAS call 
however which might incur bad performance penalties. Unless we manually define 
msr.dr=0.

But I'd certainly prefer to reuse the existing real mode special casing code.

Also, keep in mind that we might need something to handle this in the in-kernel 
rtas handlers too.

Alex

> 
> 
> -- 
> Alexey