Re: [Xen-devel] [PATCH v6] x86: introduce a new set of APIs to manage Xen page tables

["Xia, Hongyan" <hongyxia@xxxxxxxxxx>]

Ping.

On Tue, 2020-01-28 at 13:50 +0000, Hongyan Xia wrote:
> From: Wei Liu <wei.liu2@xxxxxxxxxx>
> 
> We are going to switch to using domheap page for page tables.
> A new set of APIs is introduced to allocate and free pages of page
> tables based on mfn instead of the xenheap direct map address. The
> allocation and deallocation work on mfn_t but not page_info, because
> they are required to work even before frame table is set up.
> 
> Implement the old functions with the new ones. We will rewrite, site
> by site, other mm functions that manipulate page tables to use the
> new
> APIs.
> 
> After the allocation, one needs to map and unmap via map_domain_page
> to
> access the PTEs. This does not break xen half way, since the new APIs
> still use xenheap pages underneath, and map_domain_page will just use
> the directmap for mappings. They will be switched to use domheap and
> dynamic mappings when usage of old APIs is eliminated.
> 
> No functional change intended in this patch.
> 
> Signed-off-by: Wei Liu <wei.liu2@xxxxxxxxxx>
> Signed-off-by: Hongyan Xia <hongyxia@xxxxxxxxxx>
> Reviewed-by: Julien Grall <jgrall@xxxxxxxxxx>
> 
> ---
> Changed since v5:
> - sounds like we are happy to use map_domain_page for Xen PTEs.
> Remove
>   map/unmap_xen_pagetable, just use map/unmap_domain_page instead.
> - remove redundant logic in free_xen_pagetable.
> 
> Changed since v4:
> - properly handle INVALID_MFN.
> - remove the _new suffix for map/unmap_xen_pagetable because they do
> not
>   have old alternatives.
> 
> Changed since v3:
> - const qualify unmap_xen_pagetable_new().
> - remove redundant parentheses.
> ---
>  xen/arch/x86/mm.c        | 32 +++++++++++++++++++++++++++-----
>  xen/include/asm-x86/mm.h |  3 +++
>  2 files changed, 30 insertions(+), 5 deletions(-)
> 
> diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
> index f50c065af3..fa824d5252 100644
> --- a/xen/arch/x86/mm.c
> +++ b/xen/arch/x86/mm.c
> @@ -119,6 +119,7 @@
>  #include <xen/efi.h>
>  #include <xen/grant_table.h>
>  #include <xen/hypercall.h>
> +#include <xen/mm.h>
>  #include <asm/paging.h>
>  #include <asm/shadow.h>
>  #include <asm/page.h>
> @@ -4947,22 +4948,43 @@ int mmcfg_intercept_write(
>  }
>  
>  void *alloc_xen_pagetable(void)
> +{
> +    mfn_t mfn = alloc_xen_pagetable_new();
> +
> +    return mfn_eq(mfn, INVALID_MFN) ? NULL :
> mfn_to_virt(mfn_x(mfn));
> +}
> +
> +void free_xen_pagetable(void *v)
> +{
> +    mfn_t mfn = v ? virt_to_mfn(v) : INVALID_MFN;
> +
> +    free_xen_pagetable_new(mfn);
> +}
> +
> +/*
> + * For these PTE APIs, the caller must follow the alloc-map-unmap-
> free
> + * lifecycle, which means explicitly mapping the PTE pages before
> accessing
> + * them. The caller must check whether the allocation has succeeded,
> and only
> + * pass valid MFNs to map_domain_page().
> + */
> +mfn_t alloc_xen_pagetable_new(void)
>  {
>      if ( system_state != SYS_STATE_early_boot )
>      {
>          void *ptr = alloc_xenheap_page();
>  
>          BUG_ON(!hardware_domain && !ptr);
> -        return ptr;
> +        return ptr ? virt_to_mfn(ptr) : INVALID_MFN;
>      }
>  
> -    return mfn_to_virt(mfn_x(alloc_boot_pages(1, 1)));
> +    return alloc_boot_pages(1, 1);
>  }
>  
> -void free_xen_pagetable(void *v)
> +/* mfn can be INVALID_MFN */
> +void free_xen_pagetable_new(mfn_t mfn)
>  {
> -    if ( system_state != SYS_STATE_early_boot )
> -        free_xenheap_page(v);
> +    if ( system_state != SYS_STATE_early_boot && !mfn_eq(mfn,
> INVALID_MFN) )
> +        free_xenheap_page(mfn_to_virt(mfn_x(mfn)));
>  }
>  
>  static DEFINE_SPINLOCK(map_pgdir_lock);
> diff --git a/xen/include/asm-x86/mm.h b/xen/include/asm-x86/mm.h
> index 2ca8882ad0..ac81991e62 100644
> --- a/xen/include/asm-x86/mm.h
> +++ b/xen/include/asm-x86/mm.h
> @@ -582,6 +582,9 @@ void *do_page_walk(struct vcpu *v, unsigned long
> addr);
>  /* Allocator functions for Xen pagetables. */
>  void *alloc_xen_pagetable(void);
>  void free_xen_pagetable(void *v);
> +mfn_t alloc_xen_pagetable_new(void);
> +void free_xen_pagetable_new(mfn_t mfn);
> +
>  l1_pgentry_t *virt_to_xen_l1e(unsigned long v);
>  
>  int __sync_local_execstate(void);
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