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

From: Alexey Kardashevskiy
Subject: Re: [Qemu-devel] [PATCH] spapr-rtas: reset top 4 bits in parameters address
Date: Thu, 05 Sep 2013 22:49:00 +1000
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130625 Thunderbird/17.0.7

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 automatically uses the current virtual layout the same way that the 
> instruction emulator would do it - which is pretty much what we want.

> IIRC you also have to enter RTAS calls with DR=0, so we wouldn't even
> need to flip any MSR bits when emulating RTAS calls, right?

Probably. Right now cpu->env.msr==0x0 in rtas handler but not sure that I
see the real value.


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