Re: [Xen-devel] [PATCHv4 04/14] xen: remove scratch frames for ballooned pages and m2p override

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Mon, 26 Jan 2015, David Vrabel wrote:
> The scratch frame mappings for ballooned pages and the m2p override
> are broken.  Remove them in preparation for replacing them with
> simpler mechanisms that works.
> 
> The scratch pages did not ensure that the page was not in use.  In
> particular, the foreign page could still be in use by hardware.  If
> the guest reused the frame the hardware could read or write that
> frame.
> 
> The m2p override did not handle the same frame being granted by two
> different grant references.  Trying an M2P override lookup in this
> case is impossible.
> 
> Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>

[...]

>  int set_foreign_p2m_mapping(struct gnttab_map_grant_ref *map_ops,
>                           struct gnttab_map_grant_ref *kmap_ops,
>                           struct page **pages, unsigned int count)
>  {
>       int i, ret = 0;
> -     bool lazy = false;
>       pte_t *pte;
>  
>       if (xen_feature(XENFEAT_auto_translated_physmap))
>               return 0;
>  
> -     if (kmap_ops &&
> -         !in_interrupt() &&
> -         paravirt_get_lazy_mode() == PARAVIRT_LAZY_NONE) {
> -             arch_enter_lazy_mmu_mode();
> -             lazy = true;
> +     if (kmap_ops) {
> +             ret = HYPERVISOR_grant_table_op(GNTTABOP_map_grant_ref,
> +                                             kmap_ops, count);
> +             if (ret)
> +                     goto out;
>       }

I think that you should explain why you are adding one more grant table
mapping to a function meant to work on the p2m/m2p.  Everything else is
clear.

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