Re: [PATCH v5 02/18] mm: Define __pte_leaf_size() to also take a PMD entry

[LEROY Christophe]

Le 11/06/2024 à 18:10, Oscar Salvador a écrit :
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> 
> On Tue, Jun 11, 2024 at 11:20:01AM -0400, Peter Xu wrote:
>> On Tue, Jun 11, 2024 at 05:08:45PM +0200, Oscar Salvador wrote:
>>> The problem is that we do not have spare bits for 8xx to mark these ptes
>>> as cont-ptes or mark them pte as 8MB, so I do not see a clear path on how
>>> we could remove huge_ptep_get for 8xx.
>>
>> Right, I remember I thought about this too when I initially looked at one
>> previous version of the series, I didn't come up yet with a good solution,
>> but I guess we probably need to get rid of hugepd first anyway.  We may
>> somehow still need to identify this is a 8M large leaf, and I guess this is
>> again the only special case where contpte can go over >1 pmds.
> 
> Yes, we definitely need first to get rid of hugepd, which is a huge
> step, and one step closer to our goal, but at some point we will have to
> see what can we do about 8MB cont-ptes for 8xx and how to mark them,
> so ptep_get can work the same way as e.g: arm64
> (ptep_get->contpte_ptep_get).
> 
> @Christophe: Can you think of a way to flag those ptes? (e.g: a
> combination of bits etc)

We have space available in PMD if we need more flags, but in PTE I can't 
see anything possible without additional churn that would require 
additional instructions in TLB miss handlers, which is what I want to 
avoid most.

Bits mapped to HW PTE:

#define _PAGE_PRESENT	0x0001	/* V: Page is valid */
#define _PAGE_NO_CACHE	0x0002	/* CI: cache inhibit */
#define _PAGE_SH	0x0004	/* SH: No ASID (context) compare */
#define _PAGE_SPS	0x0008	/* SPS: Small Page Size (1 if 16k, 512k or 8M)*/
#define _PAGE_DIRTY	0x0100	/* C: page changed */
#define _PAGE_NA	0x0200	/* Supervisor NA, User no access */
#define _PAGE_RO	0x0600	/* Supervisor RO, User no access */

SW bits masked out in TLB miss handler:

#define _PAGE_GUARDED	0x0010	/* Copied to L1 G entry in DTLB */
#define _PAGE_ACCESSED	0x0020	/* Copied to L1 APG 1 entry in I/DTLB */
#define _PAGE_EXEC	0x0040	/* Copied to PP (bit 21) in ITLB */
#define _PAGE_SPECIAL	0x0080	/* SW entry */
#define _PAGE_HUGE	0x0800	/* Copied to L1 PS bit 29 */

All bits are used. The only thing would could do but that would have a 
performance cost is to retrieve _PAGE_SH from the PMD and use that bit 
for something else.

But I was maybe thinking another way. Lets take the exemple of 
pmd_write() helper:

#define pmd_write(pmd)		pte_write(pmd_pte(pmd))

At the time being we have

static inline pte_t pmd_pte(pmd_t pmd)
{
	return __pte(pmd_val(pmd));
}

But what about something like

static inline pte_t pmd_pte(pmd_t pmd)
{
	return *(pte_t *)pmd_page_vaddr(pmd);
}

Would it do the trick ?

Of course it would require to carefully make sure all accesses are done 
through pmd_pte().

Would that work ?



Christophe