RE: [Xen-ia64-devel] Re: [PATCH]: ptc.ga for SMP-g

["Xu, Anthony" <anthony.xu@xxxxxxxxx>]

>From: Tristan Gingold
>Sent: 2006?3?30? 16:31
>To: Isaku Yamahata
>Cc: xen-ia64-devel@xxxxxxxxxxxxxxxxxxx
>Subject: Re: [Xen-ia64-devel] Re: [PATCH]: ptc.ga for SMP-g
>
>Le Mardi 28 Mars 2006 04:04, Isaku Yamahata a écrit :
>> Hi Tristan.
>>
>>
>> +                    /* Purge tc entry.
>> +                       Can we do this directly ?  Well, this is just a
>> +                       single atomic write.  */
>> +                    vcpu_purge_tr_entry(&PSCBX(v,dtlb));
>> +                    vcpu_purge_tr_entry(&PSCBX(v,itlb));
>>
>> These aren't SMP-safe because dtlb isn't read atomically.
>> From vcpu_translate()
>>
>> #define vcpu_match_tr_entry(_trp,_ifa,_rid)             \
>>     ((_trp->p && (_trp->rid==_rid) && (_ifa >= _trp->vadr) &&   \
>>     (_ifa < (_trp->vadr + (1L<< _trp->ps)) - 1)))
>>
>>         trp = &vcpu->arch.dtlb;
>>         if (/* is_data && */ vcpu_match_tr_entry(trp,address,rid)) {
>>                 <<<<<<<<<<<<If other processor executes
>>                             vcpu_purge_tr_entry(&PSCBX(v,dtlb)) here,
>>                             then a disaster will happen >>>>>>>>>>>>>>>>
>Can you elaborate ? Do you have a disaster scenario ?
>vcpu_purge_tr_entry only clears the p bit.
>
>> In fact it's more subtle because of memory ordering.
>I don't think it is the case, because vcpu_purge_tr_entry modify only one bit
>in one word.

There seems no issue, because vcpu_purge_tr_entry() only sets ->p=0.
In some cases, this patch may cause a not-present tlb inserted into machine 
TLB, this is harmless, when guest accesses this page, page not present fault
 happens, and guest kernel will purge this tlb and insert a new one, there are 
a little performance penalty.
Sine you will send IPI to flush vhpt later, why not purge this tlb as well as 
flush vhpt.


Thanks,
Anthony

_______________________________________________
Xen-ia64-devel mailing list
Xen-ia64-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-ia64-devel