Re: [Xen-devel] [PATCH 1 of 3] p2m: Allow l2 pages to be replaced by superpages

[Tim Deegan <Tim.Deegan@xxxxxxxxxx>]

Hi, 

At 14:36 +0000 on 17 Jan (1295274993), George Dunlap wrote:
> # HG changeset patch
> # User George Dunlap <george.dunlap@xxxxxxxxxxxxx>
> # Date 1295274250 0
> # Node ID 366d675630fd6ecbd6228426b3f7723d8a9dd944
> # Parent  75b6287626ee0b852d725543568001e99b13be5b
> p2m: Allow l2 pages to be replaced by superpages
> 
> Allow second-level p2m pages to be replaced with superpage entries,
> freeing the p2m table page properly.  This allows, for example, a
> sequence of 512 singleton zero pages to be replaced with a superpage
> populate-on-demand entry.

This problem became more general under your feet - xen 4.1 supports 1GiB
superpages as well, so the same thing needs to be fixed for them.  
(I understand that PoD only uses 2MiB superpages but to half-fix it
invites future bugs.)

Also, although in the EPT code you're careful to do all the flushing &c
before freeing the old page, in the vanilla p2m code you free the page
even before writing the new l2e!

Cheers,

Tim.

> Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
> 
> diff -r 75b6287626ee -r 366d675630fd xen/arch/x86/mm/hap/hap.c
> --- a/xen/arch/x86/mm/hap/hap.c       Fri Jan 14 16:38:51 2011 +0000
> +++ b/xen/arch/x86/mm/hap/hap.c       Mon Jan 17 14:24:10 2011 +0000
> @@ -333,9 +333,11 @@
>  
>      ASSERT(page_get_owner(pg) == d);
>      /* Should have just the one ref we gave it in alloc_p2m_page() */
> -    if ( (pg->count_info & PGC_count_mask) != 1 )
> -        HAP_ERROR("Odd p2m page count c=%#lx t=%"PRtype_info"\n",
> -                  pg->count_info, pg->u.inuse.type_info);
> +    if ( (pg->count_info & PGC_count_mask) != 1 ) {
> +        HAP_ERROR("Odd p2m page %p count c=%#lx t=%"PRtype_info"\n",
> +                     pg, pg->count_info, pg->u.inuse.type_info);
> +        WARN();
> +    }
>      pg->count_info &= ~PGC_count_mask;
>      /* Free should not decrement domain's total allocation, since
>       * these pages were allocated without an owner. */
> diff -r 75b6287626ee -r 366d675630fd xen/arch/x86/mm/hap/p2m-ept.c
> --- a/xen/arch/x86/mm/hap/p2m-ept.c   Fri Jan 14 16:38:51 2011 +0000
> +++ b/xen/arch/x86/mm/hap/p2m-ept.c   Mon Jan 17 14:24:10 2011 +0000
> @@ -317,6 +317,7 @@
>      int vtd_pte_present = 0;
>      int needs_sync = 1;
>      struct domain *d = p2m->domain;
> +    struct page_info *intermediate_table = NULL;
>  
>      /*
>       * the caller must make sure:
> @@ -369,6 +370,12 @@
>          /* No need to flush if the old entry wasn't valid */
>          if ( !is_epte_present(ept_entry) )
>              needs_sync = 0;
> +        else if ( order == 9 && ! ept_entry->sp )
> +        {
> +            /* We're replacing a non-SP page with a superpage.  Make sure to
> +             * handle freeing the table properly. */
> +            intermediate_table = mfn_to_page(ept_entry->mfn);
> +        }
>  
>          if ( mfn_valid(mfn_x(mfn)) || direct_mmio || p2m_is_paged(p2mt) ||
>               (p2mt == p2m_ram_paging_in_start) )
> @@ -487,6 +494,17 @@
>          }
>      }
>  
> +    if ( intermediate_table )
> +    {
> +        /* Release the old intermediate table.  This has to be the
> +           last thing we do, after the ept_sync_domain() and removal
> +           from the iommu tables, so as to avoid a potential
> +           use-after-free. */
> +
> +        page_list_del(intermediate_table, &d->arch.p2m->pages);
> +        d->arch.paging.free_page(d, intermediate_table);
> +    }
> +
>      return rv;
>  }
>  
> diff -r 75b6287626ee -r 366d675630fd xen/arch/x86/mm/p2m.c
> --- a/xen/arch/x86/mm/p2m.c   Fri Jan 14 16:38:51 2011 +0000
> +++ b/xen/arch/x86/mm/p2m.c   Mon Jan 17 14:24:10 2011 +0000
> @@ -1361,9 +1361,15 @@
>          if ( (l1e_get_flags(*p2m_entry) & _PAGE_PRESENT) &&
>               !(l1e_get_flags(*p2m_entry) & _PAGE_PSE) )
>          {
> -            P2M_ERROR("configure P2M table 4KB L2 entry with large page\n");
> -            domain_crash(p2m->domain);
> -            goto out;
> +            struct page_info *pg;
> +
> +            /* We're replacing a non-SP page with a superpage.  Make sure to
> +             * handle freeing the table properly. */
> +            pg = mfn_to_page(_mfn(l1e_get_pfn(*p2m_entry)));
> +            paging_write_p2m_entry(p2m->domain, gfn, p2m_entry, table_mfn,
> +                                   l1e_empty(), 2);
> +            page_list_del(pg, &p2m->pages);
> +            p2m->domain->arch.paging.free_page(p2m->domain, pg);
>          }
>          
>          ASSERT(!mfn_valid(mfn) || p2mt != p2m_mmio_direct);
> 
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-- 
Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Principal Software Engineer, Xen Platform Team
Citrix Systems UK Ltd.  (Company #02937203, SL9 0BG)

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