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Re: [Qemu-ppc] [PATCH] spapr-rtas: reset top 4 bits in parameters addres

From: Alexander Graf
Subject: Re: [Qemu-ppc] [PATCH] spapr-rtas: reset top 4 bits in parameters address
Date: Thu, 5 Sep 2013 12:21:34 +0200

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.


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