Re: [PATCH] xen: x86/32: perform initial startup on initial_page_table

[Jeremy Fitzhardinge <jeremy@goop.org>]

On 11/24/2010 04:09 AM, Ian Campbell wrote:
> Only make swapper_pg_dir readonly and pinned when generic x86 architecture code
> (which also starts on initial_page_table) switches to it.  This helps ensure
> that the generic setup paths work on Xen unmodified. In particular
> clone_pgd_range writes directly to the destination pgd entries and is used to
> initialise swapper_pg_dir so we need to ensure that it remains writeable until
> the last possible moment during bring up.
>
> This is complicated slightly by the need to avoid sharing kernel PMD entries
> when running under Xen, therefore the Xen implementation must make a copy of
> the kernel PMD (which is otherwise referred to by both intial_page_table and
> swapper_pg_dir) before switching to swapper_pg_dir.

The one thing I'd add is the commit ID of the change which made this
necessary.

    J

> Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
> Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> Tested-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: H. Peter Anvin <hpa@linux.intel.com>
> Cc: Jeremy Fitzhardinge <jeremy@goop.org>
> ---
>  arch/x86/xen/enlighten.c |    2 -
>  arch/x86/xen/mmu.c       |   69 +++++++++++++++++++++++++++++++++++++--------
>  2 files changed, 56 insertions(+), 15 deletions(-)
>
> diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
> index 7250bef..02c710b 100644
> --- a/arch/x86/xen/enlighten.c
> +++ b/arch/x86/xen/enlighten.c
> @@ -1200,8 +1200,6 @@ asmlinkage void __init xen_start_kernel(void)
>  	/* Allocate and initialize top and mid mfn levels for p2m structure */
>  	xen_build_mfn_list_list();
>  
> -	init_mm.pgd = pgd;
> -
>  	/* keep using Xen gdt for now; no urgent need to change it */
>  
>  #ifdef CONFIG_X86_32
> diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
> index 790af90..a1feff9 100644
> --- a/arch/x86/xen/mmu.c
> +++ b/arch/x86/xen/mmu.c
> @@ -2133,44 +2133,83 @@ __init pgd_t *xen_setup_kernel_pagetable(pgd_t *pgd,
>  	return pgd;
>  }
>  #else	/* !CONFIG_X86_64 */
> -static RESERVE_BRK_ARRAY(pmd_t, level2_kernel_pgt, PTRS_PER_PMD);
> +static RESERVE_BRK_ARRAY(pmd_t, initial_kernel_pmd, PTRS_PER_PMD);
> +static RESERVE_BRK_ARRAY(pmd_t, swapper_kernel_pmd, PTRS_PER_PMD);
> +
> +static __init void xen_write_cr3_init(unsigned long cr3)
> +{
> +	unsigned long pfn = PFN_DOWN(__pa(swapper_pg_dir));
> +
> +	BUG_ON(read_cr3() != __pa(initial_page_table));
> +	BUG_ON(cr3 != __pa(swapper_pg_dir));
> +
> +	/*
> +	 * We are switching to swapper_pg_dir for the first time (from
> +	 * initial_page_table) and therefore need to mark that page
> +	 * read-only and then pin it.
> +	 *
> +	 * Xen disallows sharing of kernel PMDs for PAE
> +	 * guests. Therefore we must copy the kernel PMD from
> +	 * initial_page_table into a new kernel PMD to be used in
> +	 * swapper_pg_dir.
> +	 */
> +	swapper_kernel_pmd =
> +		extend_brk(sizeof(pmd_t) * PTRS_PER_PMD, PAGE_SIZE);
> +	memcpy(swapper_kernel_pmd, initial_kernel_pmd,
> +	       sizeof(pmd_t) * PTRS_PER_PMD);
> +	swapper_pg_dir[KERNEL_PGD_BOUNDARY] =
> +		__pgd(__pa(swapper_kernel_pmd) | _PAGE_PRESENT);
> +	set_page_prot(swapper_kernel_pmd, PAGE_KERNEL_RO);
> +
> +	set_page_prot(swapper_pg_dir, PAGE_KERNEL_RO);
> +	xen_write_cr3(cr3);
> +	pin_pagetable_pfn(MMUEXT_PIN_L3_TABLE, pfn);
> +
> +	pin_pagetable_pfn(MMUEXT_UNPIN_TABLE,
> +			  PFN_DOWN(__pa(initial_page_table)));
> +	set_page_prot(initial_page_table, PAGE_KERNEL);
> +	set_page_prot(initial_kernel_pmd, PAGE_KERNEL);
> +
> +	pv_mmu_ops.write_cr3 = &xen_write_cr3;
> +}
>  
>  __init pgd_t *xen_setup_kernel_pagetable(pgd_t *pgd,
>  					 unsigned long max_pfn)
>  {
>  	pmd_t *kernel_pmd;
>  
> -	level2_kernel_pgt = extend_brk(sizeof(pmd_t) * PTRS_PER_PMD, PAGE_SIZE);
> +	initial_kernel_pmd =
> +		extend_brk(sizeof(pmd_t) * PTRS_PER_PMD, PAGE_SIZE);
>  
>  	max_pfn_mapped = PFN_DOWN(__pa(xen_start_info->pt_base) +
>  				  xen_start_info->nr_pt_frames * PAGE_SIZE +
>  				  512*1024);
>  
>  	kernel_pmd = m2v(pgd[KERNEL_PGD_BOUNDARY].pgd);
> -	memcpy(level2_kernel_pgt, kernel_pmd, sizeof(pmd_t) * PTRS_PER_PMD);
> +	memcpy(initial_kernel_pmd, kernel_pmd, sizeof(pmd_t) * PTRS_PER_PMD);
>  
> -	xen_map_identity_early(level2_kernel_pgt, max_pfn);
> +	xen_map_identity_early(initial_kernel_pmd, max_pfn);
>  
> -	memcpy(swapper_pg_dir, pgd, sizeof(pgd_t) * PTRS_PER_PGD);
> -	set_pgd(&swapper_pg_dir[KERNEL_PGD_BOUNDARY],
> -			__pgd(__pa(level2_kernel_pgt) | _PAGE_PRESENT));
> +	memcpy(initial_page_table, pgd, sizeof(pgd_t) * PTRS_PER_PGD);
> +	initial_page_table[KERNEL_PGD_BOUNDARY] =
> +		__pgd(__pa(initial_kernel_pmd) | _PAGE_PRESENT);
>  
> -	set_page_prot(level2_kernel_pgt, PAGE_KERNEL_RO);
> -	set_page_prot(swapper_pg_dir, PAGE_KERNEL_RO);
> +	set_page_prot(initial_kernel_pmd, PAGE_KERNEL_RO);
> +	set_page_prot(initial_page_table, PAGE_KERNEL_RO);
>  	set_page_prot(empty_zero_page, PAGE_KERNEL_RO);
>  
>  	pin_pagetable_pfn(MMUEXT_UNPIN_TABLE, PFN_DOWN(__pa(pgd)));
>  
> -	xen_write_cr3(__pa(swapper_pg_dir));
> -
> -	pin_pagetable_pfn(MMUEXT_PIN_L3_TABLE, PFN_DOWN(__pa(swapper_pg_dir)));
> +	pin_pagetable_pfn(MMUEXT_PIN_L3_TABLE,
> +			  PFN_DOWN(__pa(initial_page_table)));
> +	xen_write_cr3(__pa(initial_page_table));
>  
>  	memblock_x86_reserve_range(__pa(xen_start_info->pt_base),
>  		      __pa(xen_start_info->pt_base +
>  			   xen_start_info->nr_pt_frames * PAGE_SIZE),
>  		      "XEN PAGETABLES");
>  
> -	return swapper_pg_dir;
> +	return initial_page_table;
>  }
>  #endif	/* CONFIG_X86_64 */
>  
> @@ -2304,7 +2343,11 @@ static const struct pv_mmu_ops xen_mmu_ops __initdata = {
>  	.write_cr2 = xen_write_cr2,
>  
>  	.read_cr3 = xen_read_cr3,
> +#ifdef CONFIG_X86_32
> +	.write_cr3 = xen_write_cr3_init,
> +#else
>  	.write_cr3 = xen_write_cr3,
> +#endif
>  
>  	.flush_tlb_user = xen_flush_tlb,
>  	.flush_tlb_kernel = xen_flush_tlb,