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Re: [Xen-devel] [PATCH 2/6] xen/hvm kexec: unregister shutdown+sysrq wat

To: Olaf Hering <olaf@xxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH 2/6] xen/hvm kexec: unregister shutdown+sysrq watches during reboot
From: Keir Fraser <keir.xen@xxxxxxxxx>
Date: Thu, 28 Jul 2011 21:30:24 +0100
Cc: "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Ian Campbell <Ian.Campbell@xxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxxxx>
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Thread-topic: [Xen-devel] [PATCH 2/6] xen/hvm kexec: unregister shutdown+sysrq watches during reboot
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On 28/07/2011 20:50, "Olaf Hering" <olaf@xxxxxxxxx> wrote:

> On Thu, Jul 28, Keir Fraser wrote:
>> On 28/07/2011 15:07, "Olaf Hering" <olaf@xxxxxxxxx> wrote:
>>> On Thu, Jul 28, Ian Campbell wrote:
>>>>> Are you suggesting that there have to be ways for a domU to query the
>>>>> state of its registered watches and shut them all down during very early
>>>>> boot?
>>>> Perhaps the xenstore protocol could be enhanced with a mechanism to
>>>> clear all existing watches? A kernel could call that at start of day.
>>> I poked around in the xenstore sources, there is appearently nothing
>>> that can be used to shutdown all. Or would calling XS_RELEASE do the trick?
> Unfortunately do_introduce() is not preprared for that.
> On enter, conn->id contains the domain_id, so it errors out early.
> Later it compares domain->conn != conn and does not enter the correct path.

Oh of course you want to do it from *inside* the guest...

> If I remove both checks the kexec appears to work ok.
> Is it save to remove both checks?

Hmm, no. Attached patch would be safer, give it a try.

And note that it is *dangerous* to reset the domain xenstore connection if
there could be any other concurrent activity on the connection that could
confuse the guest kernel. So doing the reset during kernel bringup might be
safest, there you can do it in the kernel before the whole xenbus subsystem
is fully up.

 -- Keir

> The only issue I havent figured out:
> How to get the domid from within the kernel?
> Olaf
> diff -r 42edf1481c57 tools/xenstore/xenstored_domain.c
> --- a/tools/xenstore/xenstored_domain.c
> +++ b/tools/xenstore/xenstored_domain.c
> @@ -326,7 +326,7 @@ void do_introduce(struct connection *con
> return;
> }
> - if (conn->id != 0 || !conn->can_write) {
> + if (!conn->can_write) {
> send_error(conn, EACCES);
> return;
> }
> @@ -365,7 +365,7 @@ void do_introduce(struct connection *con
> talloc_steal(domain->conn, domain);
> fire_watches(NULL, "@introduceDomain", false);
> - } else if ((domain->mfn == mfn) && (domain->conn != conn)) {
> + } else if (domain->mfn == mfn) {
> /* Use XS_INTRODUCE for recreating the xenbus event-channel. */
> if (domain->port)
> xc_evtchn_unbind(xce_handle, domain->port);

Attachment: 00-xenstore-introduce
Description: Binary data

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