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

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

On 12/20/18 6:09 PM, George Dunlap wrote:
> 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?

Of course, I'll do that.

>> +    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."

You're right, the warning is a bit out of place above. I'll move it and
update the comment as indicated.


Thank you,
Razvan

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