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Subject: Re: [PATCH v1 01/10] powerpc/pseries/iommu: Replace hard-coded page
 shift
To: Leonardo Bras <leobras.c@gmail.com>, Oliver O'Halloran <oohall@gmail.com>
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From: Alexey Kardashevskiy <aik@ozlabs.ru>
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Date: Thu, 3 Sep 2020 14:26:12 +1000
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On 02/09/2020 07:38, Leonardo Bras wrote:
> On Mon, 2020-08-31 at 13:48 +1000, Alexey Kardashevskiy wrote:
>>>>> Well, I created this TCE_RPN_BITS = 52 because the previous mask was a
>>>>> hardcoded 40-bit mask (0xfffffffffful), for hard-coded 12-bit (4k)
>>>>> pagesize, and on PAPR+/LoPAR also defines TCE as having bits 0-51
>>>>> described as RPN, as described before.
>>>>>
>>>>> IODA3 Revision 3.0_prd1 (OpenPowerFoundation), Figure 3.4 and 3.5.
>>>>> shows system memory mapping into a TCE, and the TCE also has bits 0-51
>>>>> for the RPN (52 bits). "Table 3.6. TCE Definition" also shows it.
>>>>> In fact, by the looks of those figures, the RPN_MASK should always be a
>>>>> 52-bit mask, and RPN = (page >> tceshift) & RPN_MASK.
>>>>
>>>> I suspect the mask is there in the first place for extra protection
>>>> against too big addresses going to the TCE table (or/and for virtial vs
>>>> physical addresses). Using 52bit mask makes no sense for anything, you
>>>> could just drop the mask and let c compiler deal with 64bit "uint" as it
>>>> is basically a 4K page address anywhere in the 64bit space. Thanks,
>>>
>>> Assuming 4K pages you need 52 RPN bits to cover the whole 64bit
>>> physical address space. The IODA3 spec does explicitly say the upper
>>> bits are optional and the implementation only needs to support enough
>>> to cover up to the physical address limit, which is 56bits of P9 /
>>> PHB4. If you want to validate that the address will fit inside of
>>> MAX_PHYSMEM_BITS then fine, but I think that should be done as a
>>> WARN_ON or similar rather than just silently masking off the bits.
>>
>> We can do this and probably should anyway but I am also pretty sure we
>> can just ditch the mask and have the hypervisor return an error which
>> will show up in dmesg.
> 
> Ok then, ditching the mask.


Well, you could run a little experiment and set some bits above that old 
mask and see how phyp reacts :)


> Thanks!
> 

-- 
Alexey