Re: [PATCH v1 01/10] powerpc/pseries/iommu: Replace hard-coded page shift

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 31/08/2020 11:41, Oliver O'Halloran wrote:
> On Mon, Aug 31, 2020 at 10:08 AM Alexey Kardashevskiy <aik@ozlabs.ru> wrote:
>>
>> On 29/08/2020 05:55, Leonardo Bras wrote:
>>> On Fri, 2020-08-28 at 12:27 +1000, Alexey Kardashevskiy wrote:
>>>>
>>>> On 28/08/2020 01:32, Leonardo Bras wrote:
>>>>> Hello Alexey, thank you for this feedback!
>>>>>
>>>>> On Sat, 2020-08-22 at 19:33 +1000, Alexey Kardashevskiy wrote:
>>>>>>> +#define TCE_RPN_BITS             52              /* Bits 0-51 represent RPN on TCE */
>>>>>>
>>>>>> Ditch this one and use MAX_PHYSMEM_BITS instead? I am pretty sure this
>>>>>> is the actual limit.
>>>>>
>>>>> I understand this MAX_PHYSMEM_BITS(51) comes from the maximum physical memory addressable in the machine. IIUC, it means we can access physical address up to (1ul << MAX_PHYSMEM_BITS).
>>>>>
>>>>> This 52 comes from PAPR "Table 9. TCE Definition" which defines bits
>>>>> 0-51 as the RPN. By looking at code, I understand that it means we may input any address < (1ul << 52) to TCE.
>>>>>
>>>>> In practice, MAX_PHYSMEM_BITS should be enough as of today, because I suppose we can't ever pass a physical page address over
>>>>> (1ul << 51), and TCE accepts up to (1ul << 52).
>>>>> But if we ever increase MAX_PHYSMEM_BITS, it doesn't necessarily means that TCE_RPN_BITS will also be increased, so I think they are independent values.
>>>>>
>>>>> Does it make sense? Please let me know if I am missing something.
>>>>
>>>> The underlying hardware is PHB3/4 about which the IODA2 Version 2.4
>>>> 6Apr2012.pdf spec says:
>>>>
>>>> "The number of most significant RPN bits implemented in the TCE is
>>>> dependent on the max size of System Memory to be supported by the platform".
>>>>
>>>> IODA3 is the same on this matter.
>>>>
>>>> This is MAX_PHYSMEM_BITS and PHB itself does not have any other limits
>>>> on top of that. So the only real limit comes from MAX_PHYSMEM_BITS and
>>>> where TCE_RPN_BITS comes from exactly - I have no idea.
>>>
>>> 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.


-- 
Alexey