On Wed, Nov 23, 2022 at 12:51:39PM +0100, Jiri Denemark wrote:

On Wed, Nov 23, 2022 at 11:41:03 +0000, Daniel P. Berrangé wrote:

On Wed, Nov 23, 2022 at 12:29:24PM +0100, Jiri Denemark wrote:

On Wed, Nov 23, 2022 at 11:18:01 +0000, Daniel P. Berrangé wrote:

On Tue, Nov 22, 2022 at 01:48:39PM +0100, Jiri Denemark wrote:

On Mon, Nov 21, 2022 at 12:47:58 +0530, manish.mishra wrote:

On 18/11/22 5:00 pm, Jiri Denemark wrote:

On Fri, Nov 18, 2022 at 10:18:59 +0000, John Levon wrote:

On Fri, Nov 18, 2022 at 10:52:32AM +0100, Jiri Denemark wrote:

   * Qemu already provide an option 'enforce' to validate if features
   with which vm is started is exactly same as one provided and nothing
   is silently dropped.

Right, but it's not enough. In addition to removed features libvirt also
checks for unexpectedly added features. And you really need to do both.
Because if you ask for -cpu Model,feat1=on,feat2=on,enforce and QEMU
says everything is fine, the guest might see more than what you asked.
For example, if a feature is enabled only if a host supports it you may
or may not get it without any complains from QEMU. But if you get it you
really need to explicitly ask for it during migration, otherwise the
feature can just silently disappear. Of course, this would be a really
bad behavior from QEMU, but that does not mean it can't happen (I think
SVM is a bit problematic in this way) and the whole point of libvirt's
checks is to prevent this kind of issues.

Hi Jiri, I'm not following this very well. I think you're saying that qemu has
had bugs previously where features get silently enabled,

Personally, I haven't actually witnessed any bug in this area (as far as
I can remember, which is not that far :-)), but got some reports of at
least one, even though without any proof.

Specifically, SVM is quite strange as it is included in all AMD CPU
models in QEMU and yet if you try to start it on a host without nesting
enabled "enforce" does not complain. I saw the feature is enabled with
older machine types, but I was told the magic behind this feature looks
like not only machine type but even host configuration itself is
involved. Anyway, although I saw reports of that the feature could be
enabled without an explicit request even with new machine types I still
haven't seen any proof of this happening. So I hope it just does not
happen and users are only afraid of this possibility.

and it's libvirt's job/role to paper over those issues? Do you have
some specific cases of this?

This is not about papering. It's actually the opposite, that is about
detecting if something like this happens and reporting it as a failure
rather than papering it and hoping everything goes well. So I think
doing this is a good idea even though I don't think we actually saw any
issue of this kind.

Hi Jiri, I see now with libvirt master, with check=='full' we verify
both silently dropped as well as added features. But as you already
stated Qemu silently adding feature is a Qemu bug and libvirt just
reports that bug, so it should be very unlikely, i agree that is not a
good reasoning :). Our requirement is that we want to use CPU Models
and features which are defined in Qemu but not in libvirt for e.g if
we want to use Icelake-Server-V4 directly or newly added vmx feature,
libvirt does not allow. Currently we take help of qemu to do
validations but for cpu feature verfication and model definations we
still use libvirt defined definations which prevent us to use anything
which is not defined in libvirt. I see there are already efforts going
on to get all model and feature defination from qemu itself, but not
sure how much time it will take. Till that happens we thought safest
option is to have an option to remove all validations from libvirt and
rely on qemu 'enforce' for migration safetly. I understand
qemu-enforce does not check for silently added features, but that case
we can assume is very unlikely and Qemu should fix otherwise VMs will
not poweron anyway with check=='full'. Basically we want it as an
modification of check='none' but also skipping things like
virCPUValidateFeatures and passing option 'enforce' to Qemu. Or if
silently adding features is that big concern we can have a check in
Qemu itself? I understand current qemu-enforce is not as migration
safe as check=='full' but probably suitable for our use case for time
being?

I understand why you want this, but on the other hand adding just a
plain pass-through for CPU model and features is quite dangerous as it
can be used to set any -cpu option even if it's not a feature. And
especially the parts that are configured in other areas of domain XML
(such as hypervisor features). So I think instead of adding a new mode
for <cpu> we should go another way. For things that are not yet
supported by libvirt we support various elements in qemu namespace,
e.g., setting QEMU command line options, environment variables, or even
overriding options libvirt would normally set on the command line. So I
guess we could have a special <qemu:cpu> element which would be used
when a user wants full control over the -cpu option without any libvirt
intervention.

I really don't think we should allow this, especially not for something
which is clearly intended to be used for production deployment. Our
hypervisor CLI passthrough is there to allow for short term workarounds
for bugs, or to experiment with a feature before it is mapped into
libvirt in a supported manner.

If there are aspects related to QEMU configuration thiat are not in
libvirt yet, we need to address those gaps, not provide yet another
way to bypass libvirt mapping.

Indeed, this was definitely meant as a short term workaround for stuff
we don't have support for yet, for testing or experimental purposes. The
supported approach is for implement the missing parts in libvirt (and
QEMU) as soon as possible. I don't see why would a properly documented
support for experiments with -cpu would be an issue.

Why can't it just use the exsting QEMU passthrough syntax we have.
I don't think we should be adding specifial support just for CPUs

That would be nice, but the QEMU passthrough syntax cannot be used for
changing options that libvirt already passes to QEMU. So using it would
likely result in two separate -cpu options on QEMU command line. And it
would not rule out the CPU verification code in libvirt. Remember, we
add a default -cpu model in case there's none configured in the XML.

Two -cpu options isn't a problem, the latter will override the former,
which is fine from the level of support intended for QEMU passthrough
usage.

For the libvirt checking, isn't the 'check=none' attr sufficient
to skip checks libvirt does.


With regards,
Daniel

Actually even within check='none' libvirt does few checks e.g.  virCPUValidateFeatures if feature string passed is proper and defined in libvirt. We could extend check='none' to remove even these kind of things. So Jiri suggested that making such kind of changes in general path is not good idea so discussion went to using separate cpu class <qemu:cpu>.

Thanks

Manish Mishra