Re: [PATCH] gnttab: miscellaneous fixes

[Keir Fraser <keir.xen@gmail.com>]

On 09/02/2012 03:15, "Jan Beulich" <JBeulich@suse.com> wrote:

> - _GTF_* constants name bit positions, so binary arithmetic on them is
>   wrong
> - gnttab_clear_flag() cannot (on x86 and ia64 at least) simply use
>   clear_bit(), as that may access more than the two bytes that are
>   intended to be accessed
> 
> Signed-off-by: Jan Beulich <jbeulich@suse.com>

Acked-by: Keir Fraser <keir@xen.org>

> --- a/xen/common/grant_table.c
> +++ b/xen/common/grant_table.c
> @@ -397,7 +397,8 @@ static int _set_status_v2(domid_t  domid
>               (id != domid) ||
>               (!readonly && (flags & GTF_readonly)) )
>          {
> -            gnttab_clear_flag(_GTF_reading | _GTF_writing, status);
> +            gnttab_clear_flag(_GTF_writing, status);
> +            gnttab_clear_flag(_GTF_reading, status);
>              PIN_FAIL(done, GNTST_general_error,
>                       "Unstable flags (%x) or dom (%d). (expected dom %d) "
>                       "(r/w: %d)\n",
> @@ -1716,14 +1717,14 @@ __release_grant_for_copy(
>     under the domain's grant table lock. */
>  /* Only safe on transitive grants.  Even then, note that we don't
>     attempt to drop any pin on the referent grant. */
> -static void __fixup_status_for_pin(struct active_grant_entry *act,
> +static void __fixup_status_for_pin(const struct active_grant_entry *act,
>                                     uint16_t *status)
>  {
>      if ( !(act->pin & GNTPIN_hstw_mask) )
> -        *status &= ~_GTF_writing;
> +        *status &= ~GTF_writing;
>  
>      if ( !(act->pin & GNTPIN_hstr_mask) )
> -        *status &= ~_GTF_reading;
> +        *status &= ~GTF_reading;
>  }
>  
>  /* Grab a frame number from a grant entry and update the flags and pin
> --- a/xen/include/asm-ia64/grant_table.h
> +++ b/xen/include/asm-ia64/grant_table.h
> @@ -5,6 +5,8 @@
>  #ifndef __ASM_GRANT_TABLE_H__
>  #define __ASM_GRANT_TABLE_H__
>  
> +#include <asm/intrinsics.h>
> +
>  #define INITIAL_NR_GRANT_FRAMES 1
>  
>  // for grant map/unmap
> @@ -82,9 +84,19 @@ int guest_physmap_add_page(struct domain
>  
>  #define gnttab_mark_dirty(d, f) ((void)f)
>  
> -static inline void gnttab_clear_flag(unsigned long nr, uint16_t *addr)
> +static inline void gnttab_clear_flag(unsigned int nr, volatile uint16_t *st)
>  {
> - clear_bit(nr, addr);
> + /*
> +  * Note that this cannot be clear_bit(), as the access must be
> +  * confined to the specified 2 bytes.
> +  */
> + uint16_t mask = ~(1 << nr), old;
> + CMPXCHG_BUGCHECK_DECL
> +
> + do {
> +  CMPXCHG_BUGCHECK(st);
> +  old = *st;
> + } while (cmpxchg_rel(st, old, old & mask) != old);
>  }
>  
>  #define gnttab_host_mapping_get_page_type(op, ld, rd)   \
> --- a/xen/include/asm-x86/grant_table.h
> +++ b/xen/include/asm-x86/grant_table.h
> @@ -48,9 +48,13 @@ int replace_grant_host_mapping(
>  
>  #define gnttab_mark_dirty(d, f) paging_mark_dirty((d), (f))
>  
> -static inline void gnttab_clear_flag(unsigned long nr, uint16_t *addr)
> +static inline void gnttab_clear_flag(unsigned int nr, uint16_t *st)
>  {
> -    clear_bit(nr, (unsigned long *)addr);
> +    /*
> +     * Note that this cannot be clear_bit(), as the access must be
> +     * confined to the specified 2 bytes.
> +     */
> +    asm volatile ("lock btrw %1,%0" : "=m" (*st) : "Ir" (nr), "m" (*st));
>  }
>  
>  /* Foreign mappings of HHVM-guest pages do not modify the type count. */
> 
> 
> 
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