Re: [Xen-devel] [PATCHv2] x86/xen: avoid m2p lookup when setting early page table entries

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 06/21/2016 12:09 PM, 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 guest (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>
> Tested-by: Keven Moraga <kmoragas@xxxxxxxxxx>
> ---
> v2:
>
> - Remove __init annotation from xen_make_pte_init() since
>   PV_CALLEE_SAVE_REGS_THUNK always puts the thunk in .text.
>
> - mask_rw_pte() -> mask_rw_pteval() for x86-64.

I don't think you actually renamed the routine.

> ---
>  arch/x86/xen/mmu.c | 28 +++++++++++++++++++++-------
>  1 file changed, 21 insertions(+), 7 deletions(-)
>
> diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
> index 478a2de..e47bc19 100644
> --- a/arch/x86/xen/mmu.c
> +++ b/arch/x86/xen/mmu.c
> @@ -1562,7 +1562,7 @@ static pte_t __init mask_rw_pte(pte_t *ptep, pte_t pte)
>       return pte;
>  }
>  #else /* CONFIG_X86_64 */
> -static pte_t __init mask_rw_pte(pte_t *ptep, pte_t pte)
> +static pteval_t __init mask_rw_pte(pteval_t pte)
>  {
>       unsigned long pfn;
>  
> @@ -1577,10 +1577,10 @@ static pte_t __init mask_rw_pte(pte_t *ptep, pte_t 
> pte)
>        * page tables for mapping the p2m list, too, and page tables MUST be
>        * mapped read-only.
>        */
> -     pfn = pte_pfn(pte);
> +     pfn = (pte & PTE_PFN_MASK) >> PAGE_SHIFT;

Is it obvious that we are holding valid PFN at this point? It wasn't
immediately obvious to me so I wonder whether a comment stating this
would be useful here (yes, you mention it in the commit messages).

-boris

>       if (pfn >= xen_start_info->first_p2m_pfn &&
>           pfn < xen_start_info->first_p2m_pfn + xen_start_info->nr_p2m_frames)
> -             pte = __pte_ma(pte_val_ma(pte) & ~_PAGE_RW);
> +             pte &= ~_PAGE_RW;
>  
>       return pte;
>  }
> @@ -1600,13 +1600,26 @@ static pte_t __init mask_rw_pte(pte_t *ptep, pte_t 
> pte)
>   * so always write the PTE directly and rely on Xen trapping and
>   * emulating any updates as necessary.
>   */
> +__visible pte_t xen_make_pte_init(pteval_t pte)
> +{
> +#ifdef CONFIG_X86_64
> +     pte = mask_rw_pte(pte);
> +#endif
> +     pte = pte_pfn_to_mfn(pte);
> +
> +     if ((pte & PTE_PFN_MASK) >> PAGE_SHIFT == INVALID_P2M_ENTRY)
> +             pte = 0;
> +
> +     return native_make_pte(pte);
> +}
> +PV_CALLEE_SAVE_REGS_THUNK(xen_make_pte_init);
> +
>  static void __init xen_set_pte_init(pte_t *ptep, pte_t pte)
>  {
> +#ifdef CONFIG_X86_32
>       if (pte_mfn(pte) != INVALID_P2M_ENTRY)
>               pte = mask_rw_pte(ptep, pte);
> -     else
> -             pte = __pte_ma(0);
> -
> +#endif
>       native_set_pte(ptep, pte);
>  }
>  
> @@ -2407,6 +2420,7 @@ static void __init xen_post_allocator_init(void)
>       pv_mmu_ops.alloc_pud = xen_alloc_pud;
>       pv_mmu_ops.release_pud = xen_release_pud;
>  #endif
> +     pv_mmu_ops.make_pte = PV_CALLEE_SAVE(xen_make_pte);
>  
>  #ifdef CONFIG_X86_64
>       pv_mmu_ops.write_cr3 = &xen_write_cr3;
> @@ -2455,7 +2469,7 @@ static const struct pv_mmu_ops xen_mmu_ops __initconst 
> = {
>       .pte_val = PV_CALLEE_SAVE(xen_pte_val),
>       .pgd_val = PV_CALLEE_SAVE(xen_pgd_val),
>  
> -     .make_pte = PV_CALLEE_SAVE(xen_make_pte),
> +     .make_pte = PV_CALLEE_SAVE(xen_make_pte_init),
>       .make_pgd = PV_CALLEE_SAVE(xen_make_pgd),
>  
>  #ifdef CONFIG_X86_PAE



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