Linux PV domU with >1 vCPU never resumes after xl save/restore

Linux PV domU with >1 vCPU never resumes after xl save/restore

[George Dunlap]

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Hello,

Saving and restoring a multi-vcpu PV guest appears to have been broken in
Linux for some time (observed 6.6.56 and 6.12.86).  Report below from
Claude Fable; I've independently verified the behavior on vanilla Linux
6.6.56.  Claude seems to think it's a bug in Linux.

Gitlab CI seems to only run with vcpus=1, which is why it didn't notice.

George Dunlap
Freelance Xen consultant
https://www.laleolanguage.com/consulting

8<----

A PV guest with more than one vCPU survives `xl save`, but after
`xl restore` it never comes back: the kernel wedges mid-resume, before
xenbus reconnect, so all frontends stay disconnected (netfront frontend
state remains XenbusStateInitialising, backend InitWait; vif shows
NO-CARRIER in dom0) and the guest is unreachable indefinitely. With
vcpus=1 the same guest/image/kernel resumes cleanly, PVH SMP
save/restore is fine, and the suspend-cancel path (a failed `xl save`
resuming the domain in place) is also fine.

Reproduced with:
 - Debian trixie kernel 6.12.86+deb13-amd64
 - the Xen-project CI test-artifacts kernel, vanilla 6.6.56
 (identical signature on both, so not a 6.12 regression; at least the
 6.6..6.12 LTS span is affected)
Host: x86-64, Xen master/staging (4.23-unstable); also reproduced on an
older commit, so the Xen version does not appear relevant. Plain
`xl save` + `xl restore` of an idle 4-vCPU, 2G PV domU, direct kernel
boot, xvda file-backed disk, one vif. 100% reproducible.

What the resume looks like (full logs available):

 - Capturing the console across a paused restore (`xl restore -p`,
   attach console, unpause) shows all secondary vCPUs immediately
   splatting:

     WARNING: CPU: 1 PID: 0 at kernel/time/timekeeping.c:747
ktime_get+0xa9/0xd0
     ...
      tick_nohz_idle_enter
      do_idle
      cpu_startup_entry
      cpu_bringup_and_idle
      asm_cpu_bringup_and_idle

   i.e. the idle task entering nohz while timekeeping is still
   suspended — with printk timestamps taken from the *uncorrected*
   clock (pre-suspend time + the save/restore wall-clock gap), while
   CPU0's own subsequent resume messages ("Grant tables using version 1
   layout", from gnttab_resume() inside xen_suspend()) carry the
   *corrected*, earlier timestamp. The secondaries therefore left the
   stop_machine corral before CPU0's post-suspend work inside
   xen_suspend() had run, which the multi_cpu_stop state machine is
   supposed to make impossible.

 - xenctx on the restored-but-still-paused domain shows the vCPU
   contexts are restored faithfully (RIP-for-RIP identical to a probe
   taken at the suspend point: vCPU0 inside the suspend hypercall stub,
   the secondaries inside the multi_cpu_stop corral loop). The
   toolstack is delivering exactly what was saved; the wedge develops
   after unpause, guest-side.

 - End state, stable forever after: vCPU0 spins at 100% inside the
   multi_cpu_stop corral code (per xenctx; `xl vcpu-list` shows r--
   accumulating time), while the secondary vCPUs sit blocked in
   SCHED_block on their idle-task stacks. Because stop_machine() never
   completes, do_suspend() never reaches xen_arch_resume() (so the
   secondaries' local ticks, suspended by xen_arch_suspend() before the
   corral, are never resumed — nothing will ever wake them) nor
   xs_resume()/dpm_resume_*() (so xenbus frontends never reconnect).

A speculative note on the trigger, from reading the 6.12 code — treat
as unverified: xen_vcpu_restore() (called from xen_pv_post_suspend()
while the secondaries are mid-corral with virtual interrupts masked)
does VCPUOP_down, re-registers vcpu_info via xen_vcpu_setup_restore(),
then VCPUOP_up on each secondary. If the re-registration ends up with
evtchn_upcall_mask clear in the newly registered vcpu_info, the vCPU
comes back up with an unexpected upcall window mid-corral; stray
exc_xen_hypervisor_callback frames in the secondaries' backtraces are
consistent with that. I stopped root-causing at this point.

Happy to provide the full console logs, xenctx dumps at
suspend/restored-paused/wedged, and the reproduction scripts, or to
test patches.

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Re: Linux PV domU with >1 vCPU never resumes after xl save/restore

[Jan Beulich]

On 04.08.2026 08:12, George Dunlap wrote:
> Saving and restoring a multi-vcpu PV guest appears to have been broken in
> Linux for some time (observed 6.6.56 and 6.12.86).  Report below from
> Claude Fable; I've independently verified the behavior on vanilla Linux
> 6.6.56.  Claude seems to think it's a bug in Linux.

Just to double check, as there was a crucial fix there recently: This is with
a Xen including bedbc17d8407 ("x86/domctl: restore all registers in
arch_{get,set}_info_guest()")?

Jan

Re: Linux PV domU with >1 vCPU never resumes after xl save/restore

[Jürgen Groß]

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On 04.08.26 10:02, Jan Beulich wrote:
> On 04.08.2026 08:12, George Dunlap wrote:
>> Saving and restoring a multi-vcpu PV guest appears to have been broken in
>> Linux for some time (observed 6.6.56 and 6.12.86).  Report below from
>> Claude Fable; I've independently verified the behavior on vanilla Linux
>> 6.6.56.  Claude seems to think it's a bug in Linux.
> 
> Just to double check, as there was a crucial fix there recently: This is with
> a Xen including bedbc17d8407 ("x86/domctl: restore all registers in
> arch_{get,set}_info_guest()")?

Thanks for bringing this up.

I just wanted to start investigation, as xl save/restore didn't work for me
either using an upstream 7.1 kernel.

Using an up-to-date Xen made the difference (I had a one month old Xen on my
test system for the initial test).


Juergen

P.S.: George, would it be possible to turn off HTML mails when sending to
       xen-devel?

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Dual content (text/plain and text/html) on xen-devel (was Re: Linux PV domU with >1 vCPU never resumes after xl save/restore)

[George Dunlap]

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[Adding back in xen-devel, since this is relevant]

On Wed, Aug 5, 2026 at 9:15 PM Jürgen Groß <jgross@suse.com> wrote:

> On 04.08.26 11:34, George Dunlap wrote:
> > On Tue, Aug 4, 2026 at 6:45 PM Jürgen Groß <jgross@suse.com
> > <mailto:jgross@suse.com>> wrote:
> >
> >
> >     P.S.: George, would it be possible to turn off HTML mails when
> sending to
> >             xen-devel?
> >
> >
> > I see both a text/plain part and a text/html part in the mail I sent;
> isn't the
> > presence of a text/plain version sufficient?
> >
> > Obviously sending patches is a different matter; but for that I'll be
> using git-
> > send-email.
>
> This is a reply to your mail using Thunderbird (which I have configured to
> use
> plain text format as the default, in order to comply with most mailing
> lists
> I'm using).
>
> I don't think Thunderbird is an exotic MUA, but please have a look how it
> rendered your HTML reply to my original mail. I can't see clearly which
> part
> was written by me originally and was cited by you in this mail.
>

Thanks, this is what I was looking for.

Attached is the message Gmail sent.  As you can see, text/plain uses normal
`>` for quotes.

That makes me think that the problem is in Thunderbird.  It should either
take the text/plan part, and reply to that as though it were the only part
it had received; or it should take the HTML part, and convert the quotes to
text properly.  Replying in HTML and then rendering it with only space
indentations seems like a bug.

Thunderbird certainly isn't exotic, but last time I used it it was
definitely under-maintained.

You're asking every person who sends an email to xen-devel to remember to
take an action before sending the mail (or to send *all* mail as
text/plain, even if it's not to xen-devel), because your MUA isn't handling
the standard properly.  Is that really reasonable?  Couldn't you tell
Thunderbird to ignore html and only render text/plain?

 -George

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MIME-Version: 1.0
Date: Tue, 4 Aug 2026 19:34:13 +1000
References: <CAFLBxZZLYxk4ZZZ9++B9qRn_J8X6ochbHr4037mbC8sDfsRqDA@mail.gmail.com>
	<61b9c9fb-3d97-4480-8171-bce986e698a8@suse.com>
	<31267d9b-022f-48b5-b583-6d9380a22740@suse.com>
In-Reply-To: <31267d9b-022f-48b5-b583-6d9380a22740@suse.com>
Message-ID: <CAFLBxZYffb3bOS4Zz8m2QX52J6V9-ZonaxPCULoqyRLpPKh-VQ@mail.gmail.com>
Subject: Re: Linux PV domU with >1 vCPU never resumes after xl save/restore
From: George Dunlap <gwd@xenproject.org>
To: =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= <jgross@suse.com>
Content-Type: multipart/alternative; boundary="0000000000009c70350658355b6f"

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On Tue, Aug 4, 2026 at 6:45=E2=80=AFPM J=C3=BCrgen Gro=C3=9F <jgross@suse.c=
om> wrote:

>
> P.S.: George, would it be possible to turn off HTML mails when sending to
>        xen-devel?
>

I see both a text/plain part and a text/html part in the mail I sent; isn't
the presence of a text/plain version sufficient?

Obviously sending patches is a different matter; but for that I'll be using
git-send-email.

 -George

--0000000000009c70350658355b6f
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<div dir=3D"ltr"><div dir=3D"ltr"><span style=3D"background-color:transpare=
nt">On Tue, Aug 4, 2026 at 6:45=E2=80=AFPM J=C3=BCrgen Gro=C3=9F &lt;<a hre=
f=3D"mailto:jgross@suse.com">jgross@suse.com</a>&gt; wrote:</span></div><di=
v class=3D"gmail_quote gmail_quote_container"><blockquote class=3D"gmail_qu=
ote" style=3D"margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,20=
4);padding-left:1ex"><br>
P.S.: George, would it be possible to turn off HTML mails when sending to<b=
r>
=C2=A0 =C2=A0 =C2=A0 =C2=A0xen-devel?<br></blockquote><div><br></div><div>I=
 see both a text/plain part and a text/html part in the mail I sent; isn&#3=
9;t the presence of a text/plain version sufficient?</div><div><br></div><d=
iv>Obviously sending patches is a different matter; but for that I&#39;ll b=
e using git-send-email.</div><div><br></div><div>=C2=A0-George</div></div><=
/div>

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Re: Dual content (text/plain and text/html) on xen-devel (was Re: Linux PV domU with >1 vCPU never resumes after xl save/restore)

[Jürgen Groß]

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On 05.08.26 14:15, George Dunlap wrote:
> [Adding back in xen-devel, since this is relevant]
> 
> On Wed, Aug 5, 2026 at 9:15 PM Jürgen Groß <jgross@suse.com 
> <mailto:jgross@suse.com>> wrote:
> 
>     On 04.08.26 11:34, George Dunlap wrote:
>      > On Tue, Aug 4, 2026 at 6:45 PM Jürgen Groß <jgross@suse.com
>     <mailto:jgross@suse.com>
>      > <mailto:jgross@suse.com <mailto:jgross@suse.com>>> wrote:
>      >
>      >
>      >     P.S.: George, would it be possible to turn off HTML mails when sending to
>      >             xen-devel?
>      >
>      >
>      > I see both a text/plain part and a text/html part in the mail I sent;
>     isn't the
>      > presence of a text/plain version sufficient?
>      >
>      > Obviously sending patches is a different matter; but for that I'll be
>     using git-
>      > send-email.
> 
>     This is a reply to your mail using Thunderbird (which I have configured to use
>     plain text format as the default, in order to comply with most mailing lists
>     I'm using).
> 
>     I don't think Thunderbird is an exotic MUA, but please have a look how it
>     rendered your HTML reply to my original mail. I can't see clearly which part
>     was written by me originally and was cited by you in this mail.
> 
> 
> Thanks, this is what I was looking for.
> 
> Attached is the message Gmail sent.  As you can see, text/plain uses normal `>` 
> for quotes.
> 
> That makes me think that the problem is in Thunderbird. It should either take 
> the text/plan part, and reply to that as though it were the only part it had 
> received; or it should take the HTML part, and convert the quotes to text 
> properly.  Replying in HTML and then rendering it with only space indentations 
> seems like a bug.
> 
> Thunderbird certainly isn't exotic, but last time I used it it was definitely 
> under-maintained.
> 
> You're asking every person who sends an email to xen-devel to remember to take 
> an action before sending the mail (or to send *all* mail as text/plain, even if 
> it's not to xen-devel), because your MUA isn't handling the standard properly.  

And you are asking every Thunderbird user to live with bad threading or to
use a different mail client.

> Is that really reasonable?  Couldn't you tell Thunderbird to ignore html and 
> only render text/plain?

I did look for a setting controlling that, but couldn't find any. I'm already
using to send plain text only. There seems to be no obvious control to prefer
text over html in alternative bodies.

On Matrix you stated you are using the same configuration for sending mails to
xen-devel since 2006. I'm not sure this is true, as most mails I've received
from you via xen-devel have been sent from your Citrix account, and those were
all text-only.


Juergen

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Re: Dual content (text/plain and text/html) on xen-devel (was Re: Linux PV domU with >1 vCPU never resumes after xl save/restore)

[George Dunlap]

On Wed, Aug 5, 2026 at 10:41 PM Jürgen Groß <jgross@suse.com> wrote:
>
> On 05.08.26 14:15, George Dunlap wrote:
> > [Adding back in xen-devel, since this is relevant]
> >
> > On Wed, Aug 5, 2026 at 9:15 PM Jürgen Groß <jgross@suse.com
> > <mailto:jgross@suse.com>> wrote:
> >
> >     On 04.08.26 11:34, George Dunlap wrote:
> >      > On Tue, Aug 4, 2026 at 6:45 PM Jürgen Groß <jgross@suse.com
> >     <mailto:jgross@suse.com>
> >      > <mailto:jgross@suse.com <mailto:jgross@suse.com>>> wrote:
> >      >
> >      >
> >      >     P.S.: George, would it be possible to turn off HTML mails when sending to
> >      >             xen-devel?
> >      >
> >      >
> >      > I see both a text/plain part and a text/html part in the mail I sent;
> >     isn't the
> >      > presence of a text/plain version sufficient?
> >      >
> >      > Obviously sending patches is a different matter; but for that I'll be
> >     using git-
> >      > send-email.
> >
> >     This is a reply to your mail using Thunderbird (which I have configured to use
> >     plain text format as the default, in order to comply with most mailing lists
> >     I'm using).
> >
> >     I don't think Thunderbird is an exotic MUA, but please have a look how it
> >     rendered your HTML reply to my original mail. I can't see clearly which part
> >     was written by me originally and was cited by you in this mail.
> >
> >
> > Thanks, this is what I was looking for.
> >
> > Attached is the message Gmail sent.  As you can see, text/plain uses normal `>`
> > for quotes.
> >
> > That makes me think that the problem is in Thunderbird. It should either take
> > the text/plan part, and reply to that as though it were the only part it had
> > received; or it should take the HTML part, and convert the quotes to text
> > properly.  Replying in HTML and then rendering it with only space indentations
> > seems like a bug.
> >
> > Thunderbird certainly isn't exotic, but last time I used it it was definitely
> > under-maintained.
> >
> > You're asking every person who sends an email to xen-devel to remember to take
> > an action before sending the mail (or to send *all* mail as text/plain, even if
> > it's not to xen-devel), because your MUA isn't handling the standard properly.
>
> And you are asking every Thunderbird user to live with bad threading or to
> use a different mail client.

Right, but Gmail is following the convention correctly, using nested >
in plain text and nested <blockquote> in html; Thunderbird (it would
appear) is not following the convention, mis-rendering HTML blockquote
into text as spaces instead of > (or instead of just using the plain
text to reply to in the first place, rather than re-rendering the HTML
into text badly).

> > Is that really reasonable?  Couldn't you tell Thunderbird to ignore html and
> > only render text/plain?
>
> I did look for a setting controlling that, but couldn't find any. I'm already
> using to send plain text only. There seems to be no obvious control to prefer
> text over html in alternative bodies.
>
> On Matrix you stated you are using the same configuration for sending mails to
> xen-devel since 2006. I'm not sure this is true, as most mails I've received
> from you via xen-devel have been sent from your Citrix account, and those were
> all text-only.

The last email I sent to the list in 2024 was dual format:

https://marc.info/?l=xen-devel&m=171405222718671

Basically, if I was writing a new email, or replying to one that I was
cc'd on, I tended to use Apple Mail (and Thunderbird before that); but
if I was responding to an email that I wasn't cc'd on, I used Google.
Apple seems to have been configured to send text-only, while Gmail
generally sends dual-format unless I ask it not to for specific
messages.  (I've done so for this message, we'll see what it looks
like.)

 -George

Re: Dual content (text/plain and text/html) on xen-devel (was Re: Linux PV domU with >1 vCPU never resumes after xl save/restore)

[Jan Beulich]

On 05.08.2026 14:15, George Dunlap wrote:
> You're asking every person who sends an email to xen-devel to remember to
> take an action before sending the mail

You're by far not the first one to be asked this; you're the first one to
have an issue with being asked, beyond some companies' IT getting in the
way. (As to the latter point, I'm already combining my observations on
xen-devel@ with those elsewhere.)

> (or to send *all* mail as
> text/plain, even if it's not to xen-devel),

By default, that is. MUAs may or may not offer options to control this in
the course of composing a message.

> because your MUA isn't handling
> the standard properly.  Is that really reasonable?  Couldn't you tell
> Thunderbird to ignore html and only render text/plain?

Both questions go together: text/plain as the only content is (not just by
us) being asked for because that's the form known to work virtually
everywhere. Wanting your mails to be properly handled at all receiving
ends is - I think - not only reasonable, but also in your own interest?

Jan

Re: Dual content (text/plain and text/html) on xen-devel (was Re: Linux PV domU with >1 vCPU never resumes after xl save/restore)

[Jürgen Groß]

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On 05.08.26 15:28, Jan Beulich wrote:
> On 05.08.2026 14:15, George Dunlap wrote:
>> You're asking every person who sends an email to xen-devel to remember to
>> take an action before sending the mail
> 
> You're by far not the first one to be asked this; you're the first one to
> have an issue with being asked, beyond some companies' IT getting in the
> way.
To put it differently: there is a statement on the Xen wiki asking to send
only text emails to xen-devel. I'm not the one to ask to relax that rule.
If you want this rule to be dropped, you probably should raise this topic
for discussion.

IMHO it is fine to question such guidelines, but just saying you don't
believe they make sense and therefor ignoring them, especially after having
been asked to obey them, is kind of rude.

This is just my opinion and I don't want to waste my time discussing this
any longer.


Juergen

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Re: Dual content (text/plain and text/html) on xen-devel (was Re: Linux PV domU with >1 vCPU never resumes after xl save/restore)

[George Dunlap]

On Wed, Aug 5, 2026 at 3:03 PM Jürgen Groß <jgross@suse.com> wrote:
>
> On 05.08.26 15:28, Jan Beulich wrote:
> > On 05.08.2026 14:15, George Dunlap wrote:
> >> You're asking every person who sends an email to xen-devel to remember to
> >> take an action before sending the mail
> >
> > You're by far not the first one to be asked this; you're the first one to
> > have an issue with being asked, beyond some companies' IT getting in the
> > way.
> To put it differently: there is a statement on the Xen wiki asking to send
> only text emails to xen-devel. I'm not the one to ask to relax that rule.
> If you want this rule to be dropped, you probably should raise this topic
> for discussion.
>
> IMHO it is fine to question such guidelines, but just saying you don't
> believe they make sense and therefor ignoring them, especially after having
> been asked to obey them, is kind of rude.

The rule seems ambiguous to me.  Here it is for those following along
at home [1]:

> Please post in plain text (i.e. not HTML), word-wrapped to somewehere around 72 characters.

It never says "only".   Recall that frequently, mailers are configured
to send *only* HTML emails; or, they send HTML emails with plain-text
attachments that do not reflect everything in the HTML version of the
email (perhaps because people actually use the mark-up to convey
information).  Given that, the rule could mean two things:

1. Please post at least plain-text, and expect that the plain-text
version will be the only one read.  Do not post HTML-only, and do not
post an email where the plain text is difficult to read (mis-formatted
or garbled) or is missing information present in the HTML version.

2. Please post in *only* plain text; mail to xen-devel should contain
no HTML attachment whatsoever.

Either way, it seems to me like the expectations could be clarified.

It looks like I can send plain-text only by selecting it for each
individual message.  I may try to do that, but knowing the way my
brain works, I wouldn't be surprised if I frequently forget.

 -George

[1] https://wiki.xenproject.org/wiki/Asking_Developer_Questions

Re: Dual content (text/plain and text/html) on xen-devel (was Re: Linux PV domU with >1 vCPU never resumes after xl save/restore)

[George Dunlap]

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On Wed, Aug 5, 2026 at 11:28 PM Jan Beulich <jbeulich@suse.com> wrote:

> On 05.08.2026 14:15, George Dunlap wrote:
> > You're asking every person who sends an email to xen-devel to remember to
> > take an action before sending the mail
>
> You're by far not the first one to be asked this; you're the first one to
> have an issue with being asked, beyond some companies' IT getting in the
> way. (As to the latter point, I'm already combining my observations on
> xen-devel@ with those elsewhere.)
>

And indeed, if I were sending to a community I was new to, I would just
suck it up.  I'm pushing back for two reasons: first, it wasn't an issue
for the previous 18 years, so I'm trying to figure out why it's an issue
now; secondly, whether it's a big barrier or not, it adds a barrier, which
seems to me unnecessary.

I do feel a bit weird starting an argument about this on my second or third
public post coming back.  But, the arguments put forth to support the
"text/plain *only*" position don't seem to me (at the moment) to be
justified; and in this community I'm in a position to push back in a way a
newbie would not be.


> > because your MUA isn't handling
> > the standard properly.  Is that really reasonable?  Couldn't you tell
> > Thunderbird to ignore html and only render text/plain?
>
> Both questions go together: text/plain as the only content is (not just by
> us) being asked for because that's the form known to work virtually
> everywhere. Wanting your mails to be properly handled at all receiving
> ends is - I think - not only reasonable, but also in your own interest?
>

As I said in a sibling thread, the fact stands that (as far as I can tell)
Gmail is following long-established mail conventions, and Thunderbird is
not.  Juergen can read my mail just fine.  *His* MUA generates somewhat
mangled responses, specifically because *it* is using HTML rather than
text/plain; I don't see why that should be my issue.

 -George

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Re: Linux PV domU with >1 vCPU never resumes after xl save/restore

[George Dunlap]

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On Tue, Aug 4, 2026 at 6:02 PM Jan Beulich <jbeulich@suse.com> wrote:

> On 04.08.2026 08:12, George Dunlap wrote:
> > Saving and restoring a multi-vcpu PV guest appears to have been broken in
> > Linux for some time (observed 6.6.56 and 6.12.86).  Report below from
> > Claude Fable; I've independently verified the behavior on vanilla Linux
> > 6.6.56.  Claude seems to think it's a bug in Linux.
>
> Just to double check, as there was a crucial fix there recently: This is
> with
> a Xen including bedbc17d8407 ("x86/domctl: restore all registers in
> arch_{get,set}_info_guest()")?
>

Ah, yes, that seems to have done the trick.  Sorry for he noise.

 -George

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