Re: [Xen-devel] [PATCH v5] x86/mm: Clean up p2m_finish_type_change return value

[Alexandru Stefan ISAILA <aisaila@xxxxxxxxxxxxxxx>]

Roger/Paul are you ok with the latest changes? Can this go in?

Regards,
Alex

On 29.03.2019 14:50, Alexandru Stefan ISAILA wrote:
> In the case of any errors, finish_type_change() passes values returned
> from p2m->recalc() up the stack (with some exceptions in the case where
> an error is expected); this eventually ends up being returned to the
> XEN_DOMOP_map_mem_type_to_ioreq_server hypercall.
> 
> However, on Intel processors (but not on AMD processor), p2m->recalc()
> can also return '1' as well as '0'.  This case is handled very
> inconsistently: finish_type_change() will return the value of the final
> entry it attempts, discarding results for other entries;
> p2m_finish_type_change() will attempt to accumulate '1's, so that it
> returns '1' if any of the calls to finish_type_change() returns '1'; and
> dm_op() will again return '1' only if the very last call to
> p2m_finish_type_change() returns '1'.  The result is that the
> XEN_DMOP_map_mem_type_to_ioreq_server() hypercall will sometimes return
> 0 and sometimes return 1 on success, in an unpredictable manner.
> 
> The hypercall documentation doesn't mention return values; but it's not
> clear what the caller could do with the information about whether
> entries had been changed or not.  At the moment it's always 0 on AMD
> boxes, and *usually* 1 on Intel boxes; so nothing can be relying on a
> '1' return value for correctness (or if it is, it's broken).
> 
> Make the return value on success consistently '0' by only returning
> 0/-ERROR from finish_type_change().  Also remove the accumulation code
> from p2m_finish_type_change().
> 
> Suggested-by: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
> Signed-off-by: Alexandru Isaila <aisaila@xxxxxxxxxxxxxxx>
> 
> ---
> Changes since V4:
>       - Undo changes in p2m->recalc() and have 1 turn into 0 in
>       finish_type_change().
> ---
>   xen/arch/x86/mm/p2m.c | 16 +++++-----------
>   1 file changed, 5 insertions(+), 11 deletions(-)
> 
> diff --git a/xen/arch/x86/mm/p2m.c b/xen/arch/x86/mm/p2m.c
> index b9bbb8f485..5508cedc55 100644
> --- a/xen/arch/x86/mm/p2m.c
> +++ b/xen/arch/x86/mm/p2m.c
> @@ -1158,7 +1158,7 @@ void p2m_change_type_range(struct domain *d,
>    * Finish p2m type change for gfns which are marked as need_recalc in a 
> range.
>    * Uses the current p2m's max_mapped_pfn to further clip the invalidation
>    * range for alternate p2ms.
> - * Returns: 0/1 for success, negative for failure
> + * Returns: 0 for success, negative for failure
>    */
>   static int finish_type_change(struct p2m_domain *p2m,
>                                 gfn_t first_gfn, unsigned long max_nr)
> @@ -1174,9 +1174,9 @@ static int finish_type_change(struct p2m_domain *p2m,
>           /*
>            * ept->recalc could return 0/1/-ENOMEM. pt->recalc could return
>            * 0/-ENOMEM/-ENOENT, -ENOENT isn't an error as we are looping
> -         * gfn here.
> +         * gfn here. If rc is 1 we need to have it 0 for success.
>            */
> -        if ( rc == -ENOENT )
> +        if ( rc == -ENOENT || rc == 1 )
>               rc = 0;
>           else if ( rc < 0 )
>           {
> @@ -1213,19 +1213,13 @@ int p2m_finish_type_change(struct domain *d,
>               if ( d->arch.altp2m_eptp[i] != mfn_x(INVALID_MFN) )
>               {
>                   struct p2m_domain *altp2m = d->arch.altp2m_p2m[i];
> -                int rc1;
>   
>                   p2m_lock(altp2m);
> -                rc1 = finish_type_change(altp2m, first_gfn, max_nr);
> +                rc = finish_type_change(altp2m, first_gfn, max_nr);
>                   p2m_unlock(altp2m);
>   
> -                if ( rc1 < 0 )
> -                {
> -                    rc = rc1;
> +                if ( rc < 0 )
>                       goto out;
> -                }
> -
> -                rc |= rc1;
>               }
>       }
>   #endif
> 
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