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RE: [Xen-ia64-devel] [PATCH] Fix severe vtlb BUG which made domU crashra

To: "Magenheimer, Dan \(HP Labs Fort Collins\)" <dan.magenheimer@xxxxxx>, <xen-ia64-devel@xxxxxxxxxxxxxxxxxxx>
Subject: RE: [Xen-ia64-devel] [PATCH] Fix severe vtlb BUG which made domU crashrandomly
From: "Tian, Kevin" <kevin.tian@xxxxxxxxx>
Date: Fri, 16 Sep 2005 08:50:00 +0800
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Thread-topic: [Xen-ia64-devel] [PATCH] Fix severe vtlb BUG which made domU crashrandomly
>From: Magenheimer, Dan (HP Labs Fort Collins)
>Related topic: In another message you said that you were
>having some hg problems... was that on xenlinux or linux?

Oh, the hg problems I encountered is on the build machine, instead of
target machine running xen/xenlinux. Finally I just confirmed it's a
local env issue, and a fresh clone can work again.

>> -----Original Message-----
>> From: xen-ia64-devel-bounces@xxxxxxxxxxxxxxxxxxx
>> [mailto:xen-ia64-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf
>> Of Tian, Kevin
>> Sent: Thursday, September 15, 2005 4:21 AM
>> To: xen-ia64-devel@xxxxxxxxxxxxxxxxxxx
>> Subject: [Xen-ia64-devel] [PATCH] Fix severe vtlb BUG which
>> made domU crashrandomly
>> When I was debugging domU, machine crash often occurred when domU is
>> detecting IDE disk (need disable later with different config file for
>> xen0/xenU) which preventing further progress. After some
>> investigation,
>> the root cause is that page fault handler invokes
>> vcpu_itc_no_srlz with
>> 5th param (guest pte) to be (-1UL). However there's no detection upon
>> such deliberate value in vcpu_itc_no_srlz, which thus updates (-1UL)
>> into copy of guest pte (vcpu->arch.dtlb_pte).
>> However match_dtlb only checks machine pte (vcpu->arch.dtlb) which is
>> still valid. Once this entry is hit in next page fault handler,
>> will be returned as the guest pte value and later translation for
>> machine pfn failed.
>> I think this can explain why Dan saw dom0 unstable after my previous
>> vtlb fix. Then after Dan pushed out a temporary solution for dom0,
>> issue was left only to domU. I'm not sure why the phenomenon
>> related to
>> this reason didn't occur to me previously, but big merges in
>> the middle
>> may be the cause. John, maybe you can make a try since you saw domU
>> crash for a long time (But has to be upon your previous Rev). ;-)
>> A side effect of this change is that vtlb can be hit more frequently
>> than before. I didn't remove the print line after match_dtlb, and you
>> can see it occurring more. The possible reason is that guest
>> svhpt entry
>> will live longer now.
>> (BTW, this fix doesn't bring domU up, and blkfront still failed to
>> connect. But it can provide a pretty stable/reproducible env now.)
>> Thanks,
>> Kevin

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