[Libguestfs] [PATCH] always 'max' for the appliance CPU model on all targes except ppc
Richard W.M. Jones
rjones at redhat.com
Wed May 25 17:34:44 UTC 2022
On Wed, May 25, 2022 at 05:13:53PM +0100, Peter Maydell wrote:
> On Wed, 25 May 2022 at 16:07, Laszlo Ersek <lersek at redhat.com> wrote:
> >
> > + Drew & Peter
> >
> > On 05/25/22 15:30, Daniel P. Berrangé wrote:
> > - The patch seems to do what it says in the commit message.
> >
> > - QEMU commit bab52d4bba3f ("target/arm: Add "-cpu max" support",
> > 2018-03-09) confirms what the commit message says, about both TCG and
> > KVM.
> >
> > - To smoke-test the TCG-related change, I've edited a long-term TCG
> > aarch64 libvirt domain of mine, replacing "cortex-a57" with "max".
> > Both edk2 and the Linux guest continued working. So I guess the TCG
> > change is OK.
>
> One thing to note here is that if you are using:
> * TCG -cpu max
> * 'virt' with no named version or with 'virt-7.0' or later
> * a Linux kernel version prior to v5.12
> then a bug in Linux means it won't boot. (This is because of
> the LPA2 CPU feature which TCG -cpu max now emulates; older
> kernels were buggy and won't boot on an LPA2 CPU, including
> a real hardware one.)
Is this related at all to the 5-level page tables (la57) failure with
TCG and -cpu max?
https://gitlab.com/qemu-project/qemu/-/issues/1023
Rich.
> You might or might not feel this is something worth noting in
> release notes or equivalent.
>
>
> > - In additional support of the above, QEMU commit ddaebdda53fc
> > ("target/arm: Unindent unnecessary else-clause", 2022-02-21) added a
> > comment saying
> >
> > /* '-cpu max' for TCG: we currently do this as "A57 with extra things" */
> >
> > - Although I was more surprised by the TCG-related statement initially
> > (i.e. that "max" was a superset of "cortex-a57" when using TCG), now
> > I'm actually more concerned about the KVM case.
>
> The TCG part is an implementation convenience (mostly it just
> means that 'max' has the A57's IMPDEF registers).
>
> > Specifically QEMU commit 0baa21be497d ("target/arm: Make KVM -cpu max
> > exactly like -cpu host", 2022-02-21) eliminated a difference where
> > "-cpu max" had been a superset of "-cpu host", featuring the
> > "sve-max-vq" extra property.
> >
> > The fix is part of release v7.0.0.
> >
> > The difference was introduced in commits
> >
> > [1] 6fa8a37949d9 ("target/arm/cpu64: max cpu: Support sve properties
> > with KVM", 2019-11-01)
> >
> > [2] 87014c6b3660 ("target/arm/kvm: host cpu: Add support for sve<N>
> > properties", 2019-11-01)
> >
> > and apparently *deliberately*.
>
> No, this was unintentional. See the discussion in this thread:
> https://lore.kernel.org/qemu-devel/20220203173640.shxkmatdcsfzzvtj@gator/
>
> In particular, although KVM '-cpu max' had the sve-max-vq
> property, Drew notes "sve-max-vq won't work for any of the machines that
> support SVE that I know of". Which is why we removed it, rather
> than adding it to '-cpu host'.
>
> > Therefore it seems that starting with qemu-4.2, but strictly preceding
> > qemu-7.0, "-cpu max" and "-cpu host" are not "identical" when KVM is
> > enabled; "-cpu max" has more features. Because of that, I think there
> > are two options:
> >
> > (a) This extra feature is actually harmless, so we should only update
> > the commit message (i.e., generally speaking, "-cpu max" has been
> > a superset of "-cpu host" on KVM and of "-cpu cortex-a57" on TCG).
> >
> > (b) The feature actually presents a problem, and qemu in [v4.2.0,
> > v7.0.0) will not start when KVM accel and "-cpu max" are requested
> > simultaneously. In this case, I think the appliance needs to stick
> > with "-cpu host" on KVM.
>
> I don't understand why you think these are the only two options. The
> actual situation is:
>
> (c) -cpu max and -cpu host have always been identical on KVM,
> and this commit does not change that.
> There happens to have been a QOM property 'sve-max-vq' on 'max'
> that should not have existed there and that nobody can actually have
> been usefully setting, but now there isn't.
>
> thanks
> -- PMM
>
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--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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