Re: [Xen-devel] [PATCH v2 4/6] xen/arm: implement get/put_page_type

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Fri, 2012-07-20 at 16:05 +0100, Stefano Stabellini wrote:
> Add a basic get_page_type and put_page_type implementation: the
> implementation is similar to the x86 one, without all the code to handle
> shadow pagetables and other unneeded features.
> 
> Also remove PGT_shared_page, that is unused.

I suppose it'll come back some day?

Is the existing page sharing stuff completely contained within the x86
arch code and not in generic code at all?
> +int get_page_type(struct page_info *page, unsigned long type)
> +{
> +    unsigned long nx, x, y = page->u.inuse.type_info;
> +
> +    for ( ; ; )
> +    {
> +        x  = y;
> +        nx = x + 1;
> +        if ( unlikely((nx & PGT_count_mask) == 0) )
> +        {
> +            printk("Type count overflow on pfn %lx", page_to_mfn(page));
> +            return -EINVAL;
> +        }
> +        else if ( (x & PGT_count_mask) == 0 )
> +        {
> +            struct domain *d = page_get_owner(page);
> +
> +            if ( (x & PGT_type_mask) != type )
> +            {
> +                /*
> +                 * On type change we check to flush stale TLB entries. This 
> +                 * may be unnecessary (e.g., page was GDT/LDT) but those 
> +                 * circumstances should be very rare.
> +                 */
> +                cpumask_t mask;
> +
> +                cpumask_copy(&mask, d->domain_dirty_cpumask);
> +
> +                /* Don't flush if the timestamp is old enough */
> +                tlbflush_filter(mask, page->tlbflush_timestamp);
> +
> +                if ( unlikely(!cpumask_empty(&mask)) )
> +                {
> +                    perfc_incr(need_flush_tlb_flush);
> +                    flush_tlb_mask(&mask);
> +                }
> +
> +                /* We lose existing type and validity. */
> +                nx &= ~(PGT_type_mask | PGT_validated);
> +                nx |= type;
> +
> +                /* No special validation needed for writable pages. */
> +                /* Page tables and GDT/LDT need to be scanned for validity. 
> */

There is no GDT/LDT on ARM.

And do we really need PGT_validated when there is no direct paging
support? (Hrm, maybe I'm confused about what PGT_validated means).

In fact, do the types make any sense at all for ARM? Is PGT_writable is
really only meaningful when you have direct paging?

Ian.



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