Re: [PATCH v2] Virtual memory size detection for 64 bit MIPS CPUs

[David Daney <ddaney@caviumnetworks.com>]

Guenter Roeck wrote:
> On Mon, 2010-02-01 at 16:42 -0500, David Daney wrote:
>> Guenter Roeck wrote:
>> [...]
>>>  
>>> +static inline void cpu_set_vmbits(struct cpuinfo_mips *c)
>>> +{
>>> +	if (cpu_has_64bits) {
>>> +		unsigned long zbits;
>>> +
>>> +		asm volatile(".set mips64\n"
>>> +			     "and %0, 0\n"
>>> +			     "dsubu %0, 1\n"
>>> +			     "dmtc0 %0, $10, 0\n"
>>> +			     "dmfc0 %0, $10, 0\n"
>>> +			     "dsll %0, %0, 2\n"
>>> +			     "dsra %0, %0, 2\n"
>>> +			     "dclz %0, %0\n"
>>> +			     ".set mips0\n"
>>> +			     : "=r" (zbits));
>>> +		c->vmbits = 64 - zbits;
>>> +	} else
>>> +		c->vmbits = 32;
>>> +}
>>> +
>> It should be possible to express this in 'pure' C using 
>> read_c0_entryhi()/write_c0_entryhi(), also you need to be sure you are 
> 
> Sure, no problem.
> 
>> not writing 1s to any reserved bits of the register.
>>
> That may be tricky, since the upper bits are reserved in some
> architectures. For example, the 20Kc core specification says that bits
> 61:40 are reserved and must be written with 0.
> 
> I can write, say, 0x3fffffffffff0000 to avoid writing into lower
> reserved bits, but that won't help for any upper reserved bits. Would
> that be acceptable / better ?


The MIPS64® Privileged Resource Architecture manual might be a better 
reference.

I would set 1s only in bits 13-63 (the VPN2 and R fields).  We don't 
support and don't really care about VPN2X, the upper bits are really the 
only ones of interest here.

David Daney