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

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

>From: Tristan Gingold
>Sent: 2006年4月3日 21:38
>
>The other conflict is use.  This is within ia64_page_fault, between
>vcpu_translate and vcpu_itc_no_srlz.  This part of code is protected by
>a
>flag + counter: At entry the flag is set and the counter increment, at exit
>the flag is reset.  Ptc.ga waits if the flag is set and retries if the
>counter has changes.

Agree with Alex to use seqlock here.

>
>+      /* Bit 0 must not be set.  */
>+      BUG_ON (PSCBX(current, tlb_inuse) & 1);
>+      /* Set bit 0 and increment counter (from bit 1).  */
>+      current->arch.tlb_inuse += 3;
>+
>       fault = vcpu_translate(current,address,is_data,0,&pteval,&itir,&iha);
>       if (fault == IA64_NO_FAULT) {
>               pteval = translate_domain_pte(pteval,address,itir);
>
>       vcpu_itc_no_srlz(current,is_data?2:1,address,pteval,-1UL,(itir>>2)
>&0x3f);
>+              /* Clear in_use flag.  */
>+              current->arch.tlb_inuse--;
>               return;
>       }
>+      /* Clear in_use flag.  */
>+      current->arch.tlb_inuse--;
>+

Seems you need current->arch.tlb_inuse -= 3 here.

>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.

>+
>+                              /* Loop until the entry is not used.  */
>+                      } while (count != PSCBX(vcpu, tlb_inuse));

Thanks,
Kevin

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