Re: [Xen-devel] [PATCHv3] x86/xen: avoid m2p lookup when setting early page table entries

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 06/23/2016 05:51 AM, David Vrabel wrote:
> When page tables entries are set using xen_set_pte_init() during early
> boot there is no page fault handler that could handle a fault when
> performing an M2P lookup.
>
> In 64 bit guests (usually dom0) early_ioremap() would fault in
> xen_set_pte_init() because an M2P lookup faults because the MFN is in
> MMIO space and not mapped in the M2P.  This lookup is done to see if
> the PFN in in the range used for the initial page table pages, so that
> the PTE may be set as read-only.
>
> The M2P lookup can be avoided by moving the check (and clear of RW)
> earlier when the PFN is still available.
>
> Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
> ---
> Cc: Kevin Moraga <kmoragas@xxxxxxxxxx>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>



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