Question re live migrate on Xen 4.2 re different cpu capabilities

Question re live migrate on Xen 4.2 re different cpu capabilities

[Alex Bligh]

We've run into an issue on live migrate on Xen 4.2. We've mainly tested HVM 
on qemu-upstream DM with my live migrate patches in, but it seems to apply 
in any HVM live migrate.

What we see is under certain circumstances a linux guest booted on machine 
A will live-migrate to machine B and back again, but a linux guest booted 
on machine B will not live migrate to machine A - it hangs after the 
migration.

Tracking this down to different pairs of machines A and B, the difference 
seems to be down to different CPU capabilities. In particular, if B has 
identical CPU flags to A except that B supports xsave, the above seems to 
happen reliably. I presume what may be happening is that the guest notes 
and uses the presence of xsave on boot, and becomes unhappy when the 
recipient machine does not have it.

KVM with default settings does not suffer from this issue. I presume it is 
presenting by default a masked set of CPU capabilities.

Is there any way to do similarly in Xen? Or is Xen live migrate effectively 
restricted to live migrate between identical CPUs?

-- 
Alex Bligh

Re: Question re live migrate on Xen 4.2 re different cpu capabilities

[Olaf Hering]

On Thu, Feb 07, Alex Bligh wrote:

> Is there any way to do similarly in Xen? Or is Xen live migrate effectively
> restricted to live migrate between identical CPUs?

Its mentionend somewhere in the docs that both cpus must be sufficient
compatible. Use the cpuid= setting in the domU config file to enforce
this.

Olaf

Re: Question re live migrate on Xen 4.2 re different cpu capabilities

[Ian Campbell]

On Thu, 2013-02-07 at 09:19 +0000, Olaf Hering wrote:
> On Thu, Feb 07, Alex Bligh wrote:
> 
> > Is there any way to do similarly in Xen? Or is Xen live migrate effectively
> > restricted to live migrate between identical CPUs?
> 
> Its mentionend somewhere in the docs that both cpus must be sufficient
> compatible. Use the cpuid= setting in the domU config file to enforce
> this.

You can also level down an entire host using the cpuid_mask_* command
line options described in
http://xenbits.xen.org/docs/unstable/misc/xen-command-line.html 

Ian.

Re: Question re live migrate on Xen 4.2 re different cpu capabilities

[Alex Bligh]

Ian, Olaf,

--On 7 February 2013 09:47:01 +0000 Ian Campbell <Ian.Campbell@citrix.com> 
wrote:

> On Thu, 2013-02-07 at 09:19 +0000, Olaf Hering wrote:
>> On Thu, Feb 07, Alex Bligh wrote:
>>
>> > Is there any way to do similarly in Xen? Or is Xen live migrate
>> > effectively restricted to live migrate between identical CPUs?
>>
>> Its mentionend somewhere in the docs that both cpus must be sufficient
>> compatible. Use the cpuid= setting in the domU config file to enforce
>> this.
>
> You can also level down an entire host using the cpuid_mask_* command
> line options described in
> http://xenbits.xen.org/docs/unstable/misc/xen-command-line.html

Thanks - that's really helpful. Looking at the documentation for cpuid=
I can't see an obvious way of saying 'mask everything except the
least common denominator of features' using the libxl method, without
having prior knowledge of what each version of xen supports. I'm not
even clear how to do this on the xend method (obviously I want to
pass long bitstrings of zeros, but how many?). Am I missing something?

-- 
Alex Bligh

Re: Question re live migrate on Xen 4.2 re different cpu capabilities

[Olaf Hering]

On Thu, Feb 07, Alex Bligh wrote:

> >You can also level down an entire host using the cpuid_mask_* command
> >line options described in
> >http://xenbits.xen.org/docs/unstable/misc/xen-command-line.html
> 
> Thanks - that's really helpful. Looking at the documentation for cpuid=
> I can't see an obvious way of saying 'mask everything except the
> least common denominator of features' using the libxl method, without
> having prior knowledge of what each version of xen supports. I'm not
> even clear how to do this on the xend method (obviously I want to
> pass long bitstrings of zeros, but how many?). Am I missing something?

cpuid= is what the guest sees, so its not so much a feature of Xen
itself but what the host cpu provides. I'm not sure if Xen can emulate
certain important bits. The wikipedia CPUID entry has a list what each
bit means.

Olaf

Re: Question re live migrate on Xen 4.2 re different cpu capabilities

[Alex Bligh]

Olaf,

--On 7 February 2013 16:11:26 +0100 Olaf Hering <olaf@aepfle.de> wrote:

> cpuid= is what the guest sees, so its not so much a feature of Xen
> itself but what the host cpu provides. I'm not sure if Xen can emulate
> certain important bits. The wikipedia CPUID entry has a list what each
> bit means.

I know what how to find out what the bits mean. But in some sense
I don't care.

What I want to do is take a particular set of cpu features (which for the
sake of argument I will do by booting a kvm domain), and say "mask off
any additional cpuid flags beyond these", so if a new feature gets
introduced in the Xen codebase, it won't show in the guest. I think the
only thing I can do at the moment is check the Xen code for every cpu
feature Xen knows about, remove those I still want, and mask off the rest.
What I actually want to do is to say "please don't expose any features
other than these ones, and only expose these if the host supports them",
so that if we change versions of Xen to one supporting another feature,
we won't need to poke around in every domain config.

-- 
Alex Bligh

Re: Question re live migrate on Xen 4.2 re different cpu capabilities

[Ian Campbell]

On Thu, 2013-02-07 at 15:59 +0000, Alex Bligh wrote:
> Olaf,
> 
> --On 7 February 2013 16:11:26 +0100 Olaf Hering <olaf@aepfle.de> wrote:
> 
> > cpuid= is what the guest sees, so its not so much a feature of Xen
> > itself but what the host cpu provides. I'm not sure if Xen can emulate
> > certain important bits. The wikipedia CPUID entry has a list what each
> > bit means.
> 
> I know what how to find out what the bits mean. But in some sense
> I don't care.
> 
> What I want to do is take a particular set of cpu features (which for the
> sake of argument I will do by booting a kvm domain), and say "mask off
> any additional cpuid flags beyond these", so if a new feature gets
> introduced in the Xen codebase, it won't show in the guest. I think the
> only thing I can do at the moment is check the Xen code for every cpu
> feature Xen knows about, remove those I still want, and mask off the rest.
> What I actually want to do is to say "please don't expose any features
> other than these ones, and only expose these if the host supports them",
> so that if we change versions of Xen to one supporting another feature,
> we won't need to poke around in every domain config.

I think you can do this using what xl.cfg(5) describes as the "xend"
syntax, by setting all the ones you aren't explicitly exposing to 0.

The "libxl syntax" is currently "=host,flag=value,..." which starts from
the host and modifies the flags. I can't offhand thing of a reason why
we wouldn't also want to support a different keyword ("explicit"?) which
means "starting from an empty slate add these". Patches accepted...

Ian.

Re: Question re live migrate on Xen 4.2 re different cpu capabilities

[Alex Bligh]

Ian,

--On 7 February 2013 16:16:55 +0000 Ian Campbell <Ian.Campbell@citrix.com> 
wrote:

> I think you can do this using what xl.cfg(5) describes as the "xend"
> syntax, by setting all the ones you aren't explicitly exposing to 0.

OK thanks.

> The "libxl syntax" is currently "=host,flag=value,..." which starts from
> the host and modifies the flags. I can't offhand thing of a reason why
> we wouldn't also want to support a different keyword ("explicit"?) which
> means "starting from an empty slate add these". Patches accepted...

I may just take you up on that.

-- 
Alex Bligh

Re: Question re live migrate on Xen 4.2 re different cpu capabilities

[Jan Beulich]

>>> On 07.02.13 at 16:59, Alex Bligh <alex@alex.org.uk> wrote:
> What I want to do is take a particular set of cpu features (which for the
> sake of argument I will do by booting a kvm domain), and say "mask off
> any additional cpuid flags beyond these", so if a new feature gets
> introduced in the Xen codebase, it won't show in the guest. I think the
> only thing I can do at the moment is check the Xen code for every cpu
> feature Xen knows about, remove those I still want, and mask off the rest.
> What I actually want to do is to say "please don't expose any features
> other than these ones, and only expose these if the host supports them",
> so that if we change versions of Xen to one supporting another feature,
> we won't need to poke around in every domain config.

You're heading in a slightly wrong direction here: You don't really
care what features Xen know about or supports. What you do care
about is what features your DomU-s get to see. And that's where
the masking comes into play (and why this can be done on a per
guest basis as well as at the host level).

Jan

Re: Question re live migrate on Xen 4.2 re different cpu capabilities

[Alex Bligh]

--On 7 February 2013 16:19:29 +0000 Jan Beulich <JBeulich@suse.com> wrote:

> You're heading in a slightly wrong direction here: You don't really
> care what features Xen know about or supports. What you do care
> about is what features your DomU-s get to see. And that's where
> the masking comes into play (and why this can be done on a per
> guest basis as well as at the host level).

I want my domUs to see a no more than a fixed set of CPU flags
(obviously if those CPU flags aren't present, I don't want to
lie that they are). I have no visibility of what hardware my
software may be installed on in the future. EG if Intel introduces
a megawidget CPU flag and Xen adds support for it, I want to be
guaranteed this is masked out as it will break live migrate
between megawidget and non-megawidget compatible machines.

-- 
Alex Bligh

Re: Question re live migrate on Xen 4.2 re different cpu capabilities

[Olaf Hering]

On Thu, Feb 07, Alex Bligh wrote:

> 
> 
> --On 7 February 2013 16:19:29 +0000 Jan Beulich <JBeulich@suse.com> wrote:
> 
> >You're heading in a slightly wrong direction here: You don't really
> >care what features Xen know about or supports. What you do care
> >about is what features your DomU-s get to see. And that's where
> >the masking comes into play (and why this can be done on a per
> >guest basis as well as at the host level).
> 
> I want my domUs to see a no more than a fixed set of CPU flags
> (obviously if those CPU flags aren't present, I don't want to
> lie that they are). I have no visibility of what hardware my
> software may be installed on in the future. EG if Intel introduces
> a megawidget CPU flag and Xen adds support for it, I want to be
> guaranteed this is masked out as it will break live migrate
> between megawidget and non-megawidget compatible machines.

I'm not sure what the question is, in a recent bug I had to disable
popcnt because one of the hosts did not have it. So I came up with this
config entry, which disables popcnt and sse4* bits:

cpuid=[ '1:ecx=xxxxxxxx0xx00xxxxxxxxxxxxxxxxxxx' ]

I think you have to read through the CPUID entry and set a few "required" bits
and a few "common across your hardware zoo" bits to 1 and set everything else
to 0 to handle the upcoming megawidget bit:
http://en.wikipedia.org/wiki/CPUID


If you use xend, you may need this patch to handle cpuid properly:

Only add cpuid and cpuid_check to sexpr once

When converting a XendConfig object to sexpr, cpuid and cpuid_check
were being emitted twice in the resulting sexpr.  The first conversion
writes incorrect sexpr, causing parsing of the sexpr to fail when xend
is restarted and domain sexpr files in /var/lib/xend/domains/<dom-uuid>
are read and parsed.

This patch skips the first conversion, and uses only the custom
cpuid{_check} conversion methods called later.  It is not pretty, but
is the least invasive fix in this complex code.

Index: xen-4.2.0-testing/tools/python/xen/xend/XendConfig.py
===================================================================
--- xen-4.2.0-testing.orig/tools/python/xen/xend/XendConfig.py
+++ xen-4.2.0-testing/tools/python/xen/xend/XendConfig.py
@@ -1126,6 +1126,10 @@ class XendConfig(dict):
         else:
             for name, typ in XENAPI_CFG_TYPES.items():
                 if name in self and self[name] not in (None, []):
+                    # Skip cpuid and cpuid_check.  Custom conversion
+                    # methods for these are called below.
+                    if name in ("cpuid", "cpuid_check"):
+                        continue
                     if typ == dict:
                         s = self[name].items()
                     elif typ == list:


Olaf

Re: Question re live migrate on Xen 4.2 re different cpu capabilities

[Konrad Rzeszutek Wilk]

On Fri, Feb 8, 2013 at 8:36 AM, Olaf Hering <olaf@aepfle.de> wrote:
> On Thu, Feb 07, Alex Bligh wrote:
>
>>
>>
>> --On 7 February 2013 16:19:29 +0000 Jan Beulich <JBeulich@suse.com> wrote:
>>
>> >You're heading in a slightly wrong direction here: You don't really
>> >care what features Xen know about or supports. What you do care
>> >about is what features your DomU-s get to see. And that's where
>> >the masking comes into play (and why this can be done on a per
>> >guest basis as well as at the host level).
>>
>> I want my domUs to see a no more than a fixed set of CPU flags
>> (obviously if those CPU flags aren't present, I don't want to
>> lie that they are). I have no visibility of what hardware my
>> software may be installed on in the future. EG if Intel introduces
>> a megawidget CPU flag and Xen adds support for it, I want to be
>> guaranteed this is masked out as it will break live migrate
>> between megawidget and non-megawidget compatible machines.
>
> I'm not sure what the question is, in a recent bug I had to disable
> popcnt because one of the hosts did not have it. So I came up with this
> config entry, which disables popcnt and sse4* bits:
>
> cpuid=[ '1:ecx=xxxxxxxx0xx00xxxxxxxxxxxxxxxxxxx' ]
>
> I think you have to read through the CPUID entry and set a few "required" bits
> and a few "common across your hardware zoo" bits to 1 and set everything else
> to 0 to handle the upcoming megawidget bit:
> http://en.wikipedia.org/wiki/CPUID
>
>
> If you use xend, you may need this patch to handle cpuid properly:
>
> Only add cpuid and cpuid_check to sexpr once
>
> When converting a XendConfig object to sexpr, cpuid and cpuid_check
> were being emitted twice in the resulting sexpr.  The first conversion
> writes incorrect sexpr, causing parsing of the sexpr to fail when xend
> is restarted and domain sexpr files in /var/lib/xend/domains/<dom-uuid>
> are read and parsed.
>
> This patch skips the first conversion, and uses only the custom
> cpuid{_check} conversion methods called later.  It is not pretty, but
> is the least invasive fix in this complex code.


I recall seeing that libvirt had some of this figured out. It would
know which CPUID flags each CPU family had - and you could actually
set ('I am a Westmere CPU') or it would use the lowest common CPU
family support for all the guest.

Granted that means you need to know _which_ of the machines has the
lowest common CPU family first. Or you set the guest to say 'Core' .

Anyhow, perhaps looking at libvirt and implementing something similar
in 'xl' would be beneficial for these issues?

Re: Question re live migrate on Xen 4.2 re different cpu capabilities

[Olaf Hering]

On Fri, Feb 08, Konrad Rzeszutek Wilk wrote:

> I recall seeing that libvirt had some of this figured out. It would
> know which CPUID flags each CPU family had - and you could actually
> set ('I am a Westmere CPU') or it would use the lowest common CPU
> family support for all the guest.
> 
> Granted that means you need to know _which_ of the machines has the
> lowest common CPU family first. Or you set the guest to say 'Core' .
> 
> Anyhow, perhaps looking at libvirt and implementing something similar
> in 'xl' would be beneficial for these issues?

I havent looked at the code, but I can imagine it does that via qemu.
But it would be nice thing to have proper cpuid= handling were libvirt
can force a certain cpu type.

Olaf

Question re live migrate on Xen 4.2 re different cpu capabilities

[Alex Bligh]

We've run into an issue on live migrate on Xen 4.2. We've mainly tested HVM 
on qemu-upstream DM with my live migrate patches in, but it seems to apply 
in any HVM live migrate.

What we see is under certain circumstances a linux guest booted on machine 
A will live-migrate to machine B and back again, but a linux guest booted 
on machine B will not live migrate to machine A - it hangs after the 
migration.

Tracking this down to different pairs of machines A and B, the difference 
seems to be down to different CPU capabilities. In particular, if B has 
identical CPU flags to A except that B supports xsave, the above seems to 
happen reliably. I presume what may be happening is that the guest notes 
and uses the presence of xsave on boot, and becomes unhappy when the 
recipient machine does not have it.

KVM with default settings does not suffer from this issue. I presume it is 
presenting by default a masked set of CPU capabilities.

Is there any way to do similarly in Xen? Or is Xen live migrate effectively 
restricted to live migrate between identical CPUs?

-- 
Alex Bligh