Paul Mackerras <pau...@samba.org> writes:
> On Mon, Jul 09, 2012 at 06:43:33PM +0530, Aneesh Kumar K.V wrote:
>> From: "Aneesh Kumar K.V" <aneesh.ku...@linux.vnet.ibm.com>
>>
>> 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
77guccfav....@skywalker.in.ibm.com
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
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