On 05/25/22 18:13, Peter Maydell wrote: > On Wed, 25 May 2022 at 16:07, Laszlo Ersek <ler...@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.) > 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.
Ah, so that's my big misunderstanding -- I didn't realize it was *just* a QOM property. I thought it caused a guest-visible VCPU feature to appear, only on "max" (with KVM). If that's not the case, then my concern is gone. Thanks! Laszlo _______________________________________________ Libguestfs mailing list Libguestfs@redhat.com https://listman.redhat.com/mailman/listinfo/libguestfs