Re: [Xen-devel] [PATCH V11 4/5] p2m: Always use hostp2m when clipping rangesets

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 12/5/18 9:18 AM, Razvan Cojocaru wrote:
> The logdirty rangesets of the altp2ms need to be kept in sync with the
> hostp2m. This means when iterating through the altp2ms, we need to
> use the host p2m to clip the rangeset, not the indiviual altp2m's
> value.
> 
> This change also:
> 
> - Documents that the end is non-inclusive
> 
> - Calculates an "inclusive" value for the end once, rather than
>   open-coding the modification, and (worse) back-modifying updates so
>   that the calculation ends up correct
> 
> - Clarifies the logic deciding whether to call
>   change_entry_type_global() or change_entry_type_range()
> 
> - Handles the case where start >= hostp2m->max_mapped_pfn
> 
> Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxx>
> Signed-off-by: Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>
> Tested-by: Tamas K Lengyel <tamas@xxxxxxxxxxxxx>
> 
> ---
> CC: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
> CC: Jan Beulich <jbeulich@xxxxxxxx>
> CC: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> CC: Wei Liu <wei.liu2@xxxxxxxxxx>
> CC: "Roger Pau Monné" <roger.pau@xxxxxxxxxx>
> 
> ---
> Changes since V10:
>  - Fixed a double-space in the patch description.
>  - Fixed a coding style issue for
>    "if ( !start && end >= p2m->max_mapped_pfn)" (no space before
>    closing ')').
>  - Switched the early return comment back to "/* If the requested
>    range is out of scope, return doing nothing. */.
>  - Added Tamas' Tested-by.
> ---
>  xen/arch/x86/mm/p2m.c | 47 ++++++++++++++++++++++++++++++-----------------
>  1 file changed, 30 insertions(+), 17 deletions(-)
> 
> diff --git a/xen/arch/x86/mm/p2m.c b/xen/arch/x86/mm/p2m.c
> index d145850..539ea16 100644
> --- a/xen/arch/x86/mm/p2m.c
> +++ b/xen/arch/x86/mm/p2m.c
> @@ -1002,30 +1002,43 @@ int p2m_change_type_one(struct domain *d, unsigned 
> long gfn_l,
>      return rc;
>  }
>  
> -/* Modify the p2m type of a range of gfns from ot to nt. */
> +/* Modify the p2m type of [start, end) from ot to nt. */
>  static void change_type_range(struct p2m_domain *p2m,
>                                unsigned long start, unsigned long end,
>                                p2m_type_t ot, p2m_type_t nt)
>  {
> -    unsigned long gfn = start;
>      struct domain *d = p2m->domain;
> +    const unsigned long host_max_pfn = p2m_get_hostp2m(d)->max_mapped_pfn;
>      int rc = 0;
>  
> -    if ( unlikely(end > p2m->max_mapped_pfn) )
> -    {
> -        if ( !gfn )
> -        {
> -            p2m->change_entry_type_global(p2m, ot, nt);
> -            gfn = end;
> -        }
> -        end = p2m->max_mapped_pfn + 1;
> -    }
> -    if ( gfn < end )
> -        rc = p2m->change_entry_type_range(p2m, ot, nt, gfn, end - 1);
> +    --end;

I don't like the idea of modifying arguments, which is why I duplicated
them in the first place.

What about calling the argument end_exclusive, and having a local
variable, 'end', which we set to be end_exclusive-1?

> +    if ( start >= host_max_pfn )
> +        printk(XENLOG_G_WARNING "Dom%d logdirty rangeset clipped to 
> max_mapped_pfn\n",
> +               d->domain_id);

This doesn't seem to be the right condition for this warning..  Surely
this warning would go better under the next conditional, where we
actually do the clipping?

> +    /* Always clip the rangeset down to the host p2m. */
> +    if ( unlikely(end > host_max_pfn) )
> +        end = host_max_pfn;

So what about modifying this comment thus:

"Always clip the rangeset down to the host p2m.  This is probably not
the right behavior.  This should be revisited later, but for now post a
warning."

Thanks,
 -George

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