On 06/11/2015 10:07 AM, Ingo Molnar wrote:
diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c index fb0a9dd1d6e4..e0bf90470d70 100644 --- a/arch/x86/mm/pgtable.c +++ b/arch/x86/mm/pgtable.c @@ -391,6 +391,63 @@ pgd_t *pgd_alloc(struct mm_struct *mm) return NULL; }+/* + * Initialize the kernel portion of the PGD. + * + * This is done separately, because pgd_alloc() happens when + * the task is not on the task list yet - and PGD updates + * happen by walking the task list. + * + * No locking is needed here, as we just copy over the reference + * PGD. The reference PGD (pgtable_init) is only ever expanded + * at the highest, PGD level. Thus any other task extending it + * will first update the reference PGD, then modify the task PGDs. + */ +void arch_pgd_init_late(struct mm_struct *mm, pgd_t *pgd) +{ + /* + * This is called after a new MM has been made visible + * in fork() or exec(). + * + * This barrier makes sure the MM is visible to new RCU + * walkers before we initialize it, so that we don't miss + * updates: + */ + smp_wmb(); + + /* + * If the pgd points to a shared pagetable level (either the + * ptes in non-PAE, or shared PMD in PAE), then just copy the + * references from swapper_pg_dir: + */ + if (CONFIG_PGTABLE_LEVELS == 2 || + (CONFIG_PGTABLE_LEVELS == 3 && SHARED_KERNEL_PMD) || + CONFIG_PGTABLE_LEVELS == 4) { + + pgd_t *pgd_src = swapper_pg_dir + KERNEL_PGD_BOUNDARY; + pgd_t *pgd_dst = pgd + KERNEL_PGD_BOUNDARY; + int i; + + for (i = 0; i < KERNEL_PGD_PTRS; i++, pgd_src++, pgd_dst++) { + /* + * This is lock-less, so it can race with PGD updates + * coming from vmalloc() or CPA methods, but it's safe, + * because: + * + * 1) this PGD is not in use yet, we have still not + * scheduled this task. + * 2) we only ever extend PGD entries + * + * So if we observe a non-zero PGD entry we can copy it, + * it won't change from under us. Parallel updates (new + * allocations) will modify our (already visible) PGD: + */ + if (pgd_val(*pgd_src)) + WRITE_ONCE(*pgd_dst, *pgd_src);
This should be set_pgd(pgd_dst, *pgd_src) in order for it to work as a Xen PV guest.
I don't know whether anything would need to be done wrt WRITE_ONCE. Perhaps put it into native_set_pgd()?
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