Re: [PATCH V3 16/23] xen/mm: Handle properly reference in set_foreign_p2m_entry() on Arm

[Stefano Stabellini <sstabellini@kernel.org>]

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On Mon, 30 Nov 2020, Oleksandr Tyshchenko wrote:
> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
> 
> This patch implements reference counting of foreign entries in
> in set_foreign_p2m_entry() on Arm. This is a mandatory action if
> we want to run emulator (IOREQ server) in other than dom0 domain,
> as we can't trust it to do the right thing if it is not running
> in dom0. So we need to grab a reference on the page to avoid it
> disappearing.
> 
> It is valid to always pass "p2m_map_foreign_rw" type to
> guest_physmap_add_entry() since the current and foreign domains
> would be always different. A case when they are equal would be
> rejected by rcu_lock_remote_domain_by_id(). Besides the similar
> comment in the code put a respective ASSERT() to catch incorrect
> usage in future.
> 
> It was tested with IOREQ feature to confirm that all the pages given
> to this function belong to a domain, so we can use the same approach
> as for XENMAPSPACE_gmfn_foreign handling in xenmem_add_to_physmap_one().
> 
> This involves adding an extra parameter for the foreign domain to
> set_foreign_p2m_entry() and a helper to indicate whether the arch
> supports the reference counting of foreign entries and the restriction
> for the hardware domain in the common code can be skipped for it.
> 
> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
> CC: Julien Grall <julien.grall@arm.com>

The arm side looks OK to me


> ---
> Please note, this is a split/cleanup/hardening of Julien's PoC:
> "Add support for Guest IO forwarding to a device emulator"
> 
> Changes RFC -> V1:
>    - new patch, was split from:
>      "[RFC PATCH V1 04/12] xen/arm: Introduce arch specific bits for IOREQ/DM features"
>    - rewrite a logic to handle properly reference in set_foreign_p2m_entry()
>      instead of treating foreign entries as p2m_ram_rw
> 
> Changes V1 -> V2:
>    - rebase according to the recent changes to acquire_resource()
>    - update patch description
>    - introduce arch_refcounts_p2m()
>    - add an explanation why p2m_map_foreign_rw is valid
>    - move set_foreign_p2m_entry() to p2m-common.h
>    - add const to new parameter
> 
> Changes V2 -> V3:
>    - update patch description
>    - rename arch_refcounts_p2m() to arch_acquire_resource_check()
>    - move comment to x86’s arch_acquire_resource_check()
>    - return rc in Arm's set_foreign_p2m_entry()
>    - put a respective ASSERT() into Arm's set_foreign_p2m_entry()
> ---
> ---
>  xen/arch/arm/p2m.c           | 24 ++++++++++++++++++++++++
>  xen/arch/x86/mm/p2m.c        |  5 +++--
>  xen/common/memory.c          | 10 +++-------
>  xen/include/asm-arm/p2m.h    | 19 +++++++++----------
>  xen/include/asm-x86/p2m.h    | 16 +++++++++++++---
>  xen/include/xen/p2m-common.h |  4 ++++
>  6 files changed, 56 insertions(+), 22 deletions(-)
> 
> diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
> index 4eeb867..5b8d494 100644
> --- a/xen/arch/arm/p2m.c
> +++ b/xen/arch/arm/p2m.c
> @@ -1380,6 +1380,30 @@ int guest_physmap_remove_page(struct domain *d, gfn_t gfn, mfn_t mfn,
>      return p2m_remove_mapping(d, gfn, (1 << page_order), mfn);
>  }
>  
> +int set_foreign_p2m_entry(struct domain *d, const struct domain *fd,
> +                          unsigned long gfn, mfn_t mfn)
> +{
> +    struct page_info *page = mfn_to_page(mfn);
> +    int rc;
> +
> +    if ( !get_page(page, fd) )
> +        return -EINVAL;
> +
> +    /*
> +     * It is valid to always use p2m_map_foreign_rw here as if this gets
> +     * called then d != fd. A case when d == fd would be rejected by
> +     * rcu_lock_remote_domain_by_id() earlier. Put a respective ASSERT()
> +     * to catch incorrect usage in future.
> +     */
> +    ASSERT(d != fd);
> +
> +    rc = guest_physmap_add_entry(d, _gfn(gfn), mfn, 0, p2m_map_foreign_rw);
> +    if ( rc )
> +        put_page(page);
> +
> +    return rc;
> +}
> +
>  static struct page_info *p2m_allocate_root(void)
>  {
>      struct page_info *page;
> diff --git a/xen/arch/x86/mm/p2m.c b/xen/arch/x86/mm/p2m.c
> index 7a2ba82..4772c86 100644
> --- a/xen/arch/x86/mm/p2m.c
> +++ b/xen/arch/x86/mm/p2m.c
> @@ -1321,7 +1321,8 @@ static int set_typed_p2m_entry(struct domain *d, unsigned long gfn_l,
>  }
>  
>  /* Set foreign mfn in the given guest's p2m table. */
> -int set_foreign_p2m_entry(struct domain *d, unsigned long gfn, mfn_t mfn)
> +int set_foreign_p2m_entry(struct domain *d, const struct domain *fd,
> +                          unsigned long gfn, mfn_t mfn)
>  {
>      return set_typed_p2m_entry(d, gfn, mfn, PAGE_ORDER_4K, p2m_map_foreign,
>                                 p2m_get_hostp2m(d)->default_access);
> @@ -2621,7 +2622,7 @@ int p2m_add_foreign(struct domain *tdom, unsigned long fgfn,
>       * will update the m2p table which will result in  mfn -> gpfn of dom0
>       * and not fgfn of domU.
>       */
> -    rc = set_foreign_p2m_entry(tdom, gpfn, mfn);
> +    rc = set_foreign_p2m_entry(tdom, fdom, gpfn, mfn);
>      if ( rc )
>          gdprintk(XENLOG_WARNING, "set_foreign_p2m_entry failed. "
>                   "gpfn:%lx mfn:%lx fgfn:%lx td:%d fd:%d\n",
> diff --git a/xen/common/memory.c b/xen/common/memory.c
> index 3363c06..49e3001 100644
> --- a/xen/common/memory.c
> +++ b/xen/common/memory.c
> @@ -1134,12 +1134,8 @@ static int acquire_resource(
>      xen_pfn_t mfn_list[32];
>      int rc;
>  
> -    /*
> -     * FIXME: Until foreign pages inserted into the P2M are properly
> -     *        reference counted, it is unsafe to allow mapping of
> -     *        resource pages unless the caller is the hardware domain.
> -     */
> -    if ( paging_mode_translate(currd) && !is_hardware_domain(currd) )
> +    if ( paging_mode_translate(currd) && !is_hardware_domain(currd) &&
> +         !arch_acquire_resource_check() )
>          return -EACCES;
>  
>      if ( copy_from_guest(&xmar, arg, 1) )
> @@ -1207,7 +1203,7 @@ static int acquire_resource(
>  
>          for ( i = 0; !rc && i < xmar.nr_frames; i++ )
>          {
> -            rc = set_foreign_p2m_entry(currd, gfn_list[i],
> +            rc = set_foreign_p2m_entry(currd, d, gfn_list[i],
>                                         _mfn(mfn_list[i]));
>              /* rc should be -EIO for any iteration other than the first */
>              if ( rc && i )
> diff --git a/xen/include/asm-arm/p2m.h b/xen/include/asm-arm/p2m.h
> index 28ca9a8..4f8056e 100644
> --- a/xen/include/asm-arm/p2m.h
> +++ b/xen/include/asm-arm/p2m.h
> @@ -161,6 +161,15 @@ typedef enum {
>  #endif
>  #include <xen/p2m-common.h>
>  
> +static inline bool arch_acquire_resource_check(void)
> +{
> +    /*
> +     * The reference counting of foreign entries in set_foreign_p2m_entry()
> +     * is supported on Arm.
> +     */
> +    return true;
> +}
> +
>  static inline
>  void p2m_altp2m_check(struct vcpu *v, uint16_t idx)
>  {
> @@ -392,16 +401,6 @@ static inline gfn_t gfn_next_boundary(gfn_t gfn, unsigned int order)
>      return gfn_add(gfn, 1UL << order);
>  }
>  
> -static inline int set_foreign_p2m_entry(struct domain *d, unsigned long gfn,
> -                                        mfn_t mfn)
> -{
> -    /*
> -     * NOTE: If this is implemented then proper reference counting of
> -     *       foreign entries will need to be implemented.
> -     */
> -    return -EOPNOTSUPP;
> -}
> -
>  /*
>   * A vCPU has cache enabled only when the MMU is enabled and data cache
>   * is enabled.
> diff --git a/xen/include/asm-x86/p2m.h b/xen/include/asm-x86/p2m.h
> index 4603560..8d2dc22 100644
> --- a/xen/include/asm-x86/p2m.h
> +++ b/xen/include/asm-x86/p2m.h
> @@ -382,6 +382,19 @@ struct p2m_domain {
>  #endif
>  #include <xen/p2m-common.h>
>  
> +static inline bool arch_acquire_resource_check(void)
> +{
> +    /*
> +     * The reference counting of foreign entries in set_foreign_p2m_entry()
> +     * is not supported on x86.
> +     *
> +     * FIXME: Until foreign pages inserted into the P2M are properly
> +     * reference counted, it is unsafe to allow mapping of
> +     * resource pages unless the caller is the hardware domain.
> +     */
> +    return false;
> +}
> +
>  /*
>   * Updates vCPU's n2pm to match its np2m_base in VMCx12 and returns that np2m.
>   */
> @@ -647,9 +660,6 @@ int p2m_finish_type_change(struct domain *d,
>  int p2m_is_logdirty_range(struct p2m_domain *, unsigned long start,
>                            unsigned long end);
>  
> -/* Set foreign entry in the p2m table (for priv-mapping) */
> -int set_foreign_p2m_entry(struct domain *d, unsigned long gfn, mfn_t mfn);
> -
>  /* Set mmio addresses in the p2m table (for pass-through) */
>  int set_mmio_p2m_entry(struct domain *d, gfn_t gfn, mfn_t mfn,
>                         unsigned int order);
> diff --git a/xen/include/xen/p2m-common.h b/xen/include/xen/p2m-common.h
> index 58031a6..b4bc709 100644
> --- a/xen/include/xen/p2m-common.h
> +++ b/xen/include/xen/p2m-common.h
> @@ -3,6 +3,10 @@
>  
>  #include <xen/mm.h>
>  
> +/* Set foreign entry in the p2m table */
> +int set_foreign_p2m_entry(struct domain *d, const struct domain *fd,
> +                          unsigned long gfn, mfn_t mfn);
> +
>  /* Remove a page from a domain's p2m table */
>  int __must_check
>  guest_physmap_remove_page(struct domain *d, gfn_t gfn, mfn_t mfn,
> -- 
> 2.7.4
>