Subject: Re: [PATCH -V3 03/11] arch/powerpc: Convert virtual address to vpn

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Mon, 23 Jul 2012 11:24:37 +0530

Paul Mackerras <[email-masked]> writes:

> On Mon, Jul 09, 2012 at 06:43:33PM +0530, Aneesh Kumar K.V wrote:
>> From: "Aneesh Kumar K.V" <[email-masked]>
>> 
>> This patch convert different functions to take virtual page number
>> instead of virtual address. Virtual page number is virtual address
>> shifted right by VPN_SHIFT (12) bits. This enable us to have an
>> address range of upto 76 bits.
>
> Some comments inline below...
>
>> +/*
>> + * encode page number shift.
>> + * Inorder to fit the 78 bit va in a 64 bit variable we shift the va by
>       ^ "in order"
>
>> + * 12 bits. This enable us to address upto 76 bit va.
>                                          ^ "up to"
>
>> + * For hpt hash from a va we can ignore the page size bits of va and for
>> + * hpte encoding we ignore upto 23 bits of va. So ignoring lower 12 bits ensure
>> + * we work in all cases including 4k page size.
>> + */
>> +#define VPN_SHIFT	12
>
> This can't be more than 12 bits because we sometimes use 4k pages even
> in a kernel configured for 64k pages (e.g. with the subpage_protection
> system call).

Yes, I wanted to make that explicit, ie this is not a virtual page
number and hence all these BUG_ON. I actually updated few of them
to BUILD_BUG_ON() based of earlier review.

>
>> +static inline unsigned long hpte_encode_avpn(unsigned long vpn, int psize,
>> +					     int ssize)
>> +{
>> +	unsigned long v;
>> +	/*
>> +	 * The AVA field omits the low-order 23 bits of the 78 bits VA.
>> +	 * These bits are not needed in the PTE, because the
>> +	 * low-order b of these bits are part of the byte offset
>> +	 * into the virtual page and, if b < 23, the high-order
>> +	 * 23-b of these bits are always used in selecting the
>> +	 * PTEGs to be searched
>> +	 */
>> +	BUG_ON(VPN_SHIFT > 23);
>
> I don't think we need this.  If VPN_SHIFT was computed by some complex
> expression whose value is not obvious, then BUG_ON (or BUILD_BUG_ON)
> would be appropriate, but since it's just a #define, a comment at the
> site of the definition will suffice.
>
>>  static inline unsigned long hpt_hash(unsigned long va, unsigned int shift,
>>  				     int ssize)
>>  {
>> +	int mask;
>>  	unsigned long hash, vsid;
>>  
>> +	BUG_ON(shift < VPN_SHIFT);
>
> So VPN_SHIFT can be at most 12, since 12 is the smallest shift value
> possible here.
>
>> -static inline void __tlbiel(unsigned long va, int psize, int ssize)
>> +static inline void __tlbiel(unsigned long vpn, int psize, int ssize)
>>  {
>> +	unsigned long va;
>>  	unsigned int penc;
>>  
>> +	BUG_ON((77 - 65) > VPN_SHIFT);
>> +	va = vpn << VPN_SHIFT;
>
> So VPN_SHIFT has to be at least 12.  What is the significance of 77
> and 65 here?
>

It is the other way around . I updated that in [email-masked]

       BUILD_BUG_ON(VPN_SHIFT > (77 - 65));

tlbiel says we can only ignore bits above 65 of va. hence the math of 77- 65. 

-aneesh