Re: [Xen-devel] [PATCH v14 02/17] libxc: Move temporary grant table mapping to end of memory

[Roger Pau Monnà <roger.pau@xxxxxxxxxx>]

Ccing tools maintainers.

On 04/11/13 13:14, George Dunlap wrote:
> From: Roger Pau Monnà <roger.pau@xxxxxxxxxx>
> 
> In order to set up the grant table for HVM guests, libxc needs to map
> the grant table temporarily.  At the moment, it does this by adding the
> grant page to the HVM guest's p2m table in the MMIO hole (at gfn 0xFFFFE),
> then mapping that gfn, setting up the table, then unmapping the gfn and
> removing it from the p2m table.
> 
> This breaks with PVH guests with 4G or more of ram, because there is
> no MMIO hole; so it ends up clobbering a valid RAM p2m entry, then
> leaving a "hole" when it removes the grant map from the p2m table.
> Since the guest thinks this is normal ram, when it maps it and tries
> to access the page, it crashes.
> 
> This patch maps the page at max_gfn+1 instead.
> 
> Signed-off-by: Roger Pau Monnà <roger.pau@xxxxxxxxxx>
> ---
>  tools/libxc/xc_dom.h      |    3 ---
>  tools/libxc/xc_dom_boot.c |   14 ++++++++++++--
>  2 files changed, 12 insertions(+), 5 deletions(-)
> 
> diff --git a/tools/libxc/xc_dom.h b/tools/libxc/xc_dom.h
> index 86e23ee..935b49e 100644
> --- a/tools/libxc/xc_dom.h
> +++ b/tools/libxc/xc_dom.h
> @@ -18,9 +18,6 @@
>  
>  #define INVALID_P2M_ENTRY   ((xen_pfn_t)-1)
>  
> -/* Scrach PFN for temporary mappings in HVM */
> -#define SCRATCH_PFN_GNTTAB 0xFFFFE
> -
>  /* --- typedefs and structs ---------------------------------------- */
>  
>  typedef uint64_t xen_vaddr_t;
> diff --git a/tools/libxc/xc_dom_boot.c b/tools/libxc/xc_dom_boot.c
> index 71e1897..fdfeaf8 100644
> --- a/tools/libxc/xc_dom_boot.c
> +++ b/tools/libxc/xc_dom_boot.c
> @@ -361,17 +361,27 @@ int xc_dom_gnttab_hvm_seed(xc_interface *xch, domid_t 
> domid,
>                             domid_t xenstore_domid)
>  {
>      int rc;
> +    xen_pfn_t max_gfn;
>      struct xen_add_to_physmap xatp = {
>          .domid = domid,
>          .space = XENMAPSPACE_grant_table,
>          .idx   = 0,
> -        .gpfn  = SCRATCH_PFN_GNTTAB
>      };
>      struct xen_remove_from_physmap xrfp = {
>          .domid = domid,
> -        .gpfn  = SCRATCH_PFN_GNTTAB
>      };
>  
> +    max_gfn = xc_domain_maximum_gpfn(xch, domid);
> +    if ( max_gfn <= 0 ) {
> +        xc_dom_panic(xch, XC_INTERNAL_ERROR,
> +                     "%s: failed to get max gfn "
> +                     "[errno=%d]\n",
> +                     __FUNCTION__, errno);
> +        return -1;
> +    }
> +    xatp.gpfn = max_gfn + 1;
> +    xrfp.gpfn = max_gfn + 1;
> +
>      rc = do_memory_op(xch, XENMEM_add_to_physmap, &xatp, sizeof(xatp));
>      if ( rc != 0 )
>      {
> 


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