RE: [Xen-ia64-devel] RFC: ptc.ga implementation for SMP-g

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

>From: Tristan Gingold [mailto:Tristan.Gingold@xxxxxxxx]
>Sent: 2006年4月4日 16:39
>Le Mardi 04 Avril 2006 10:15, Tian, Kevin a écrit :
>[...]
>> >diff -r ddc279c91502 xen/arch/ia64/xen/vcpu.c
>> >--- a/xen/arch/ia64/xen/vcpu.c      Fri Mar 31 21:04:16 2006
>> >+++ b/xen/arch/ia64/xen/vcpu.c      Mon Apr  3 11:20:57 2006
>> >+                   do {
>> >+                           /* Wait until the tlb is not used.  */
>> >+                           while ((count = PSCBX(vcpu, tlb_inuse)) & 1)
>> >+                                   cpu_relax ();
>> >+
>> >+                           /* Purge tc entries.  */
>> >+                           vcpu_purge_tr_entry(&PSCBX(vcpu,dtlb));
>> >+                           vcpu_purge_tr_entry(&PSCBX(vcpu,itlb));
>>
>> It's possible tlb_inuse is set again here, however for a new guest tlb
>> entry which means content of dtlb/itlb may change within this window.
>Then
>> the new tlb entry will be incorrectly purged in next loop.
>The original code unconditionnaly purge tc.  This code does too.  For
>sure, it
>is inefficient.
>Is it incorrect too ?
>
>Tristan.

It's an overkill to unconditionally purge dtlb/itlb without check against 
destination range. Guest may still work, since it's only TC. However we'd 
better not do that since it's not syntax of ptc.g.

Your code only based on assumption that domain has single entry 
itlb/dtlb, which doesn't apply to hash vtlb approach. Once Anthony clears 
up the patch which get checked-in, your code doesn't work since you 
need to walk hash vtlb table to compare the destination range that time.

That's why I still prefer to merging vtlb purge together with vhpt purge 
within IPI handler, which saves you from complex thought to handle race 
condition. Anyway, you already have an IPI sent out following vTLB purge. 
Then you can define a vtlb_flush_address within ptc_ga_remote_func, 
with the former defined differently for one entry dtlb/itlb and hash vtlb 
approach. 

How do you think?

Thanks,
Kevin

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