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
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