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[Xen-devel] Re: [PATCH] xen: modify kernel mappings corresponding to gra

On Tue, 2011-07-26 at 17:55 +0100, stefano.stabellini@xxxxxxxxxxxxx
wrote:
> From: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
> 
> If we want to use granted pages for AIO, changing the mappings of a user
> vma and the corresponding p2m is not enough, we also need to update the
> kernel mappings accordingly.
> On x86_64 it is easy, we can issue another HYPERVISOR_grant_table_op
> right away in m2p_add_override. We can remove the mappings using another
> HYPERVISOR_grant_table_op in m2p_remove_override.
> On x86_32 it is more difficult because the pages are highmem pages and
> therefore we need to catch the set_pte that tries to map a granted page
> and issue an HYPERVISOR_grant_table_op instead.
> Same thing for unmapping them: we need to catch the pte clear or the
> set_pte that try to unmap a granted page and issue an
> HYPERVISOR_grant_table_op.
> 
> Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
> ---
>  arch/x86/include/asm/xen/page.h |    5 ++-
>  arch/x86/xen/mmu.c              |   68 
> +++++++++++++++++++++++++++++++++++++++
>  arch/x86/xen/p2m.c              |   47 ++++++++++++++++++++------
>  drivers/xen/gntdev.c            |   27 +++++++++++++++-
>  drivers/xen/grant-table.c       |    4 +-
>  include/xen/grant_table.h       |    1 +
>  6 files changed, 137 insertions(+), 15 deletions(-)
> 
> diff --git a/arch/x86/include/asm/xen/page.h b/arch/x86/include/asm/xen/page.h
> index 64a619d..ec7bbfb 100644
> --- a/arch/x86/include/asm/xen/page.h
> +++ b/arch/x86/include/asm/xen/page.h
> @@ -12,6 +12,7 @@
>  #include <asm/pgtable.h>
>  
>  #include <xen/interface/xen.h>
> +#include <xen/grant_table.h>
>  #include <xen/features.h>
>  
>  /* Xen machine address */
> @@ -31,8 +32,10 @@ typedef struct xpaddr {
>  #define INVALID_P2M_ENTRY    (~0UL)
>  #define FOREIGN_FRAME_BIT    (1UL<<(BITS_PER_LONG-1))
>  #define IDENTITY_FRAME_BIT   (1UL<<(BITS_PER_LONG-2))
> +#define GRANT_FRAME_BIT      (1UL<<(BITS_PER_LONG-3))

We need to be a bit careful about stealing more bits from the top of the
MFN space, especially on 32 bit. Stealing the top three bits of a 32 bit
MFN leaves a total (host) address space of 2^(29+12) == 2TB, which seems
large today but... To be honest even the current stealing the top 2 bits
(==8TB) seems a bit shaky for the not too distant future...

I'm not sure what the alternatives are though. Perhaps we can switch
FOREIGN_FRAME_BIT to SOMETHING_SPECIAL_BIT (with a better name ;-)) and
do a secondary lookup based on that? That might be too heavy weight for
FOREIGN and IDENTITY lookups? But for the grant frame bits we are
already manipulating a hashtable too aren't we?

The secondary lookup could be PFN indexed so wouldn't necessarily need
to scale to TB sizes, probably a bitmap would do?

Alternatively perhaps we can reuse the other 31 bits when the
SOMETHING_SPECIAL_BIT is set to indicate what kind of special? e.g.
FOREIGN == 0x80000000, IDENTITY == 0x80000001, GRANT == 0x80000002 etc?
Might interact strangely with the save/restore protocol (ISTR Konrad
investigated that when he added IDENTITY_FRAME_BIT but don't recall the
result).

Ian.

>  #define FOREIGN_FRAME(m)     ((m) | FOREIGN_FRAME_BIT)
>  #define IDENTITY_FRAME(m)    ((m) | IDENTITY_FRAME_BIT)
> +#define GRANT_FRAME(m)       ((m) | GRANT_FRAME_BIT)
>  
>  /* Maximum amount of memory we can handle in a domain in pages */
>  #define MAX_DOMAIN_PAGES                                             \
> @@ -48,7 +51,7 @@ extern unsigned long set_phys_range_identity(unsigned long 
> pfn_s,
>                                            unsigned long pfn_e);
>  
>  extern int m2p_add_override(unsigned long mfn, struct page *page,
> -                         bool clear_pte);
> +                         struct gnttab_map_grant_ref *kmap_op);
>  extern int m2p_remove_override(struct page *page, bool clear_pte);
>  extern struct page *m2p_find_override(unsigned long mfn);
>  extern unsigned long m2p_find_override_pfn(unsigned long mfn, unsigned long 
> pfn);
> diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
> index 4e37a7c0..13f20d8 100644
> --- a/arch/x86/xen/mmu.c
> +++ b/arch/x86/xen/mmu.c
> @@ -282,8 +282,70 @@ static bool xen_batched_set_pte(pte_t *ptep, pte_t 
> pteval)
>       return true;
>  }
>  
> +#ifdef CONFIG_HIGHMEM
> +static int xen_unmap_granted_page(pte_t *ptep)
> +{
> +     unsigned long mfn;
> +     struct page *page;
> +
> +     if (pte_flags(*ptep) & _PAGE_USER)
> +             return 1;
> +     mfn = (ptep->pte & PTE_PFN_MASK) >> PAGE_SHIFT;
> +     page = m2p_find_override(mfn);
> +     if (page != NULL && (page->private & GRANT_FRAME_BIT)) {
> +             int ret;
> +             struct gnttab_unmap_grant_ref kunmap_op;
> +             struct gnttab_map_grant_ref *kmap_op =
> +                     (struct gnttab_map_grant_ref *) page->index;
> +             kunmap_op.host_addr = kmap_op->host_addr;
> +             kunmap_op.handle = kmap_op->handle;
> +             kunmap_op.dev_bus_addr = 0;
> +             ret = HYPERVISOR_grant_table_op(
> +                             GNTTABOP_unmap_grant_ref, &kunmap_op, 1);
> +             WARN(ret, "m2p_remove_override: pfn %lx mfn %lx, failed to "
> +                     "modify kernel mappings", page_to_pfn(page), mfn);
> +             return ret;
> +     }
> +     return 1;
> +}
> +#endif
> +
>  static void xen_set_pte(pte_t *ptep, pte_t pteval)
>  {
> +#ifdef CONFIG_HIGHMEM
> +     if (!(pte_flags(pteval) & _PAGE_USER)) {
> +             int ret;
> +             struct page *page;
> +             unsigned long mfn = (pteval.pte & PTE_PFN_MASK) >> PAGE_SHIFT;
> +             page = m2p_find_override(mfn);
> +             /*
> +              * if this is a granted page we need to use a grant table
> +              * hypercall to map it instead
> +              */
> +             if (page != NULL && (page->private & GRANT_FRAME_BIT)) {
> +                     struct gnttab_map_grant_ref *kmap_op =
> +                             (struct gnttab_map_grant_ref *) page->index;
> +                     unsigned long old_mfn = kmap_op->dev_bus_addr;
> +                     kmap_op->host_addr =
> +                             arbitrary_virt_to_machine(ptep).maddr;
> +                     kmap_op->dev_bus_addr = 0;
> +                     ret = HYPERVISOR_grant_table_op(
> +                                     GNTTABOP_map_grant_ref, kmap_op, 1);
> +                     WARN(ret, "xen_set_pte: pfn %lx mfn %lx, failed to "
> +                             "modify kernel mappings",
> +                             page_to_pfn(page), mfn);
> +                     kmap_op->dev_bus_addr = old_mfn;
> +                     return;
> +             }
> +             /*
> +              * even if it is not a granted page, the old page we are about
> +              * to overwrite could be a granted page and in that case we need
> +              * to unmap it using a grant table hypercall
> +              */
> +             xen_unmap_granted_page(ptep);
> +     }
> +#endif
> +
>       if (!xen_batched_set_pte(ptep, pteval))
>               native_set_pte(ptep, pteval);
>  }
> @@ -548,6 +610,12 @@ static void xen_set_pte_atomic(pte_t *ptep, pte_t pte)
>  
>  static void xen_pte_clear(struct mm_struct *mm, unsigned long addr, pte_t 
> *ptep)
>  {
> +     /*
> +      * check if this is a granted page and unmap it using a grant table
> +      * hypercall in that case
> +      */
> +     if (!xen_unmap_granted_page(ptep))
> +             return;
>       if (!xen_batched_set_pte(ptep, native_make_pte(0)))
>               native_pte_clear(mm, addr, ptep);
>  }
> diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
> index 58efeb9..1c4d2b5 100644
> --- a/arch/x86/xen/p2m.c
> +++ b/arch/x86/xen/p2m.c
> @@ -161,6 +161,7 @@
>  #include <asm/xen/page.h>
>  #include <asm/xen/hypercall.h>
>  #include <asm/xen/hypervisor.h>
> +#include <xen/grant_table.h>
>  
>  #include "xen-ops.h"
>  
> @@ -676,7 +677,8 @@ static unsigned long mfn_hash(unsigned long mfn)
>  }
>  
>  /* Add an MFN override for a particular page */
> -int m2p_add_override(unsigned long mfn, struct page *page, bool clear_pte)
> +int m2p_add_override(unsigned long mfn, struct page *page,
> +             struct gnttab_map_grant_ref *kmap_op)
>  {
>       unsigned long flags;
>       unsigned long pfn;
> @@ -699,9 +701,18 @@ int m2p_add_override(unsigned long mfn, struct page 
> *page, bool clear_pte)
>       if (unlikely(!set_phys_to_machine(pfn, FOREIGN_FRAME(mfn))))
>               return -ENOMEM;
>  
> -     if (clear_pte && !PageHighMem(page))
> -             /* Just zap old mapping for now */
> -             pte_clear(&init_mm, address, ptep);
> +     if (kmap_op != NULL) {
> +             if (!PageHighMem(page)) {
> +                     int ret = HYPERVISOR_grant_table_op(
> +                                     GNTTABOP_map_grant_ref, kmap_op, 1);
> +                     WARN(ret, "m2p_add_override: pfn %lx mfn %lx, "
> +                             "failed to modify kernel mappings", pfn, mfn);
> +             }
> +             page->private |= GRANT_FRAME_BIT;
> +             /* let's use dev_bus_addr to record the old mfn instead */
> +             kmap_op->dev_bus_addr = page->index;
> +             page->index = (unsigned long) kmap_op;
> +     }
>       spin_lock_irqsave(&m2p_override_lock, flags);
>       list_add(&page->lru,  &m2p_overrides[mfn_hash(mfn)]);
>       spin_unlock_irqrestore(&m2p_override_lock, flags);
> @@ -735,13 +746,27 @@ int m2p_remove_override(struct page *page, bool 
> clear_pte)
>       spin_lock_irqsave(&m2p_override_lock, flags);
>       list_del(&page->lru);
>       spin_unlock_irqrestore(&m2p_override_lock, flags);
> -     set_phys_to_machine(pfn, page->index);
>  
> -     if (clear_pte && !PageHighMem(page))
> -             set_pte_at(&init_mm, address, ptep,
> -                             pfn_pte(pfn, PAGE_KERNEL));
> -             /* No tlb flush necessary because the caller already
> -              * left the pte unmapped. */
> +     if (clear_pte) {
> +             struct gnttab_map_grant_ref *map_op =
> +                     (struct gnttab_map_grant_ref *) page->index;
> +             set_phys_to_machine(pfn, map_op->dev_bus_addr);
> +             if (!PageHighMem(page)) {
> +                     int ret;
> +                     struct gnttab_unmap_grant_ref unmap_op;
> +                     unmap_op.host_addr = map_op->host_addr;
> +                     unmap_op.handle = map_op->handle;
> +                     unmap_op.dev_bus_addr = 0;
> +                     ret = HYPERVISOR_grant_table_op(
> +                                     GNTTABOP_unmap_grant_ref, &unmap_op, 1);
> +                     WARN(ret, "m2p_remove_override: pfn %lx mfn %lx, "
> +                             "failed to modify kernel mappings", pfn, mfn);
> +                     set_pte_at(&init_mm, address, ptep,
> +                                     pfn_pte(pfn, PAGE_KERNEL));
> +                     __flush_tlb_single(address);
> +             }
> +     } else
> +             set_phys_to_machine(pfn, page->index);
>  
>       return 0;
>  }
> @@ -758,7 +783,7 @@ struct page *m2p_find_override(unsigned long mfn)
>       spin_lock_irqsave(&m2p_override_lock, flags);
>  
>       list_for_each_entry(p, bucket, lru) {
> -             if (p->private == mfn) {
> +             if ((p->private & (~GRANT_FRAME_BIT)) == mfn) {
>                       ret = p;
>                       break;
>               }
> diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
> index f914b26..ca41772 100644
> --- a/drivers/xen/gntdev.c
> +++ b/drivers/xen/gntdev.c
> @@ -83,6 +83,7 @@ struct grant_map {
>       struct ioctl_gntdev_grant_ref *grants;
>       struct gnttab_map_grant_ref   *map_ops;
>       struct gnttab_unmap_grant_ref *unmap_ops;
> +     struct gnttab_map_grant_ref   *kmap_ops;
>       struct page **pages;
>  };
>  
> @@ -116,10 +117,12 @@ static struct grant_map *gntdev_alloc_map(struct 
> gntdev_priv *priv, int count)
>       add->grants    = kzalloc(sizeof(add->grants[0])    * count, GFP_KERNEL);
>       add->map_ops   = kzalloc(sizeof(add->map_ops[0])   * count, GFP_KERNEL);
>       add->unmap_ops = kzalloc(sizeof(add->unmap_ops[0]) * count, GFP_KERNEL);
> +     add->kmap_ops  = kzalloc(sizeof(add->kmap_ops[0])  * count, GFP_KERNEL);
>       add->pages     = kzalloc(sizeof(add->pages[0])     * count, GFP_KERNEL);
>       if (NULL == add->grants    ||
>           NULL == add->map_ops   ||
>           NULL == add->unmap_ops ||
> +         NULL == add->kmap_ops  ||
>           NULL == add->pages)
>               goto err;
>  
> @@ -129,6 +132,7 @@ static struct grant_map *gntdev_alloc_map(struct 
> gntdev_priv *priv, int count)
>       for (i = 0; i < count; i++) {
>               add->map_ops[i].handle = -1;
>               add->unmap_ops[i].handle = -1;
> +             add->kmap_ops[i].handle = -1;
>       }
>  
>       add->index = 0;
> @@ -142,6 +146,7 @@ err:
>       kfree(add->grants);
>       kfree(add->map_ops);
>       kfree(add->unmap_ops);
> +     kfree(add->kmap_ops);
>       kfree(add);
>       return NULL;
>  }
> @@ -243,10 +248,30 @@ static int map_grant_pages(struct grant_map *map)
>                       gnttab_set_unmap_op(&map->unmap_ops[i], addr,
>                               map->flags, -1 /* handle */);
>               }
> +     } else {
> +             for (i = 0; i < map->count; i++) {
> +                     unsigned level;
> +                     unsigned long address = (unsigned long)
> +                             pfn_to_kaddr(page_to_pfn(map->pages[i]));
> +                     pte_t *ptep;
> +                     u64 pte_maddr = 0;
> +                     if (!PageHighMem(map->pages[i])) {
> +                             ptep = lookup_address(address, &level);
> +                             pte_maddr =
> +                                     arbitrary_virt_to_machine(ptep).maddr;
> +                     }
> +                     gnttab_set_map_op(&map->kmap_ops[i], pte_maddr,
> +                             map->flags |
> +                             GNTMAP_host_map |
> +                             GNTMAP_contains_pte,
> +                             map->grants[i].ref,
> +                             map->grants[i].domid);
> +             }
>       }
>  
>       pr_debug("map %d+%d\n", map->index, map->count);
> -     err = gnttab_map_refs(map->map_ops, map->pages, map->count);
> +     err = gnttab_map_refs(map->map_ops, use_ptemod ? map->kmap_ops : NULL,
> +                     map->pages, map->count);
>       if (err)
>               return err;
>  
> diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c
> index fd725cd..1e8669a 100644
> --- a/drivers/xen/grant-table.c
> +++ b/drivers/xen/grant-table.c
> @@ -448,6 +448,7 @@ unsigned int gnttab_max_grant_frames(void)
>  EXPORT_SYMBOL_GPL(gnttab_max_grant_frames);
>  
>  int gnttab_map_refs(struct gnttab_map_grant_ref *map_ops,
> +                     struct gnttab_map_grant_ref *kmap_ops,
>                   struct page **pages, unsigned int count)
>  {
>       int i, ret;
> @@ -488,8 +489,7 @@ int gnttab_map_refs(struct gnttab_map_grant_ref *map_ops,
>                        */
>                       return -EOPNOTSUPP;
>               }
> -             ret = m2p_add_override(mfn, pages[i],
> -                                    map_ops[i].flags & GNTMAP_contains_pte);
> +             ret = m2p_add_override(mfn, pages[i], &kmap_ops[i]);
>               if (ret)
>                       return ret;
>       }
> diff --git a/include/xen/grant_table.h b/include/xen/grant_table.h
> index b1fab6b..6b99bfb 100644
> --- a/include/xen/grant_table.h
> +++ b/include/xen/grant_table.h
> @@ -156,6 +156,7 @@ unsigned int gnttab_max_grant_frames(void);
>  #define gnttab_map_vaddr(map) ((void *)(map.host_virt_addr))
>  
>  int gnttab_map_refs(struct gnttab_map_grant_ref *map_ops,
> +                     struct gnttab_map_grant_ref *kmap_ops,
>                   struct page **pages, unsigned int count);
>  int gnttab_unmap_refs(struct gnttab_unmap_grant_ref *unmap_ops,
>                     struct page **pages, unsigned int count);



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