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Subject: Re: pstore does not work under xen
To:     James Dingwall <james@dingwall.me.uk>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Cc:     Kees Cook <keescook@chromium.org>,
        Anton Vorontsov <anton@enomsg.org>,
        Colin Cross <ccross@android.com>,
        Juergen Gross <jgross@suse.com>,
        "Luck, Tony" <tony.luck@intel.com>
References: <20190919102643.GA9400@dingwall.me.uk>
 <3908561D78D1C84285E8C5FCA982C28F7F472015@ORSMSX115.amr.corp.intel.com>
 <20190919161430.GA28042@dingwall.me.uk>
From:   Boris Ostrovsky <boris.ostrovsky@oracle.com>
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Message-ID: <ae56e2c0-b2d3-835d-04cb-e4404d979859@oracle.com>
Date:   Thu, 19 Sep 2019 12:37:40 -0400
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Sender: linux-kernel-owner@vger.kernel.org
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On 9/19/19 12:14 PM, James Dingwall wrote:
> On Thu, Sep 19, 2019 at 03:51:33PM +0000, Luck, Tony wrote:
>>> I have been investigating a regression in our environment where pstore 
>>> (efi-pstore specifically but I suspect this would affect all 
>>> implementations) no longer works after upgrading from a 4.4 to 5.0 
>>> kernel when running under xen.  (This is an Ubuntu kernel but I don't 
>>> think there are patches which affect this area.)
>> I don't have any answer for this ... but want to throw out the idea that
>> VMM systems could provide some hypercalls to guests to save/return
>> some blob of memory (perhaps the "save" triggers automagically if the
>> guest crashes?).
>>
>> That would provide a much better pstore back end than relying on emulation
>> of EFI persistent variables (which have severe contraints on size, and don't
>> support some pstore modes because you can't dynamically update EFI variables
>> hundreds of times per second).
>>
> For clarification this is a dom0 crash rather than an HVM guest with EFI.  I
> should probably have also mentioned the xen verion has changed from 4.8.4 to
> 4.11.2 in case its behaviour on detection of crashed domain has changed.
>
> (For capturing guest crashes we have enabled xenconsole logging so the
> hvc0 log is available in dom0.)


Do you only see this difference between 4.4 and 5.0 when you crash via
sysrq?

Because that's where things changed. On 4.4 we seem to be forcing an
oops, which eventually calls kmsg_dump() and then panic. On 5.0 we call
panic() directly from sysrq handler. And because Xen's panic notifier
doesn't return we never get a chance to call kmsg_dump().

-boris