linux-kernel - Re: [PATCH] xen: adjust early dom0 p2m handling to xen hypervisor behavior

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Date:   Wed, 10 May 2017 09:45:33 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Juergen Gross <jgross@...e.com>, linux-kernel@...r.kernel.org,
        xen-devel@...ts.xenproject.org
Subject: Re: [PATCH] xen: adjust early dom0 p2m handling to xen hypervisor
 behavior

On 05/10/2017 12:08 AM, Juergen Gross wrote:
> When booted as pv-guest the p2m list presented by the Xen is already
> mapped to virtual addresses. In dom0 case the hypervisor might make use
> of 2M- or 1G-pages for this mapping. Unfortunately while being properly
> aligned in virtual and machine address space, those pages might not be
> aligned properly in guest physical address space.

Is this the only place where we shouldn't assume that large page is
properly aligned (in pfn space)?

-boris

>
> So when trying to obtain the guest physical address of such a page
> pud_pfn() and pmd_pfn() must be avoided as those will mask away guest
> physical address bits not being zero in this special case.
>
> Signed-off-by: Juergen Gross <jgross@...e.com>
> ---
>  arch/x86/xen/mmu_pv.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/xen/mmu_pv.c b/arch/x86/xen/mmu_pv.c
> index 9d9ae6650aa1..7397d8b8459d 100644
> --- a/arch/x86/xen/mmu_pv.c
> +++ b/arch/x86/xen/mmu_pv.c
> @@ -2025,7 +2025,8 @@ static unsigned long __init xen_read_phys_ulong(phys_addr_t addr)
>  
>  /*
>   * Translate a virtual address to a physical one without relying on mapped
> - * page tables.
> + * page tables. Don't rely on big pages being aligned in (guest) physical
> + * space!
>   */
>  static phys_addr_t __init xen_early_virt_to_phys(unsigned long vaddr)
>  {
> @@ -2046,7 +2047,7 @@ static phys_addr_t __init xen_early_virt_to_phys(unsigned long vaddr)
>  						       sizeof(pud)));
>  	if (!pud_present(pud))
>  		return 0;
> -	pa = pud_pfn(pud) << PAGE_SHIFT;
> +	pa = pud_val(pud) & PTE_PFN_MASK;
>  	if (pud_large(pud))
>  		return pa + (vaddr & ~PUD_MASK);
>  
> @@ -2054,7 +2055,7 @@ static phys_addr_t __init xen_early_virt_to_phys(unsigned long vaddr)
>  						       sizeof(pmd)));
>  	if (!pmd_present(pmd))
>  		return 0;
> -	pa = pmd_pfn(pmd) << PAGE_SHIFT;
> +	pa = pmd_val(pmd) & PTE_PFN_MASK;
>  	if (pmd_large(pmd))
>  		return pa + (vaddr & ~PMD_MASK);
>