Re: [Libguestfs] [PATCH] always 'max' for the appliance CPU model on all targes except ppc

On Wed, 25 May 2022 at 16:07, Laszlo Ersek <lersek(a)redhat.com&gt; 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.) 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.