On 3/11/21 8:10 PM, Andrew Jones wrote: > On Thu, Mar 11, 2021 at 06:33:15PM +0000, Peter Maydell wrote: >> On Thu, 11 Mar 2021 at 17:16, Claudio Fontana <cfont...@suse.de> wrote: >>> Maybe Peter you could clarify similarly what the intended meaning of "max" >>> is on ARM? >> >> "max" is "best we can do, whatever that is". (On KVM this is "same as >> the host".) >> "host" is "whatever the host is (KVM only)". >> >>> KVM: (aarch64-only): aarch64_max_initfn(): >>> >>> The following comment in the code seems wrong to me: >>> >>> /* -cpu max: if KVM is enabled, like -cpu host (best possible with this >>> host); */ >>> >>> This is not exactly true: >>> >>> "-cpu max" calls kvm_arm_set_cpu_features_from_host(), (which checks >>> "dtb_compatible", and if not set gets the features from the host, if set >>> ...?) >>> After that, calls aarch64_add_sve_properties() and then adds also >>> "svw-max-vq". This code is common with TCG.
As part of this research I noticed that arm_cpu_post_init() is quite confusing, seems really inconsistent to me. Apparently the intention was to call it from the leaf classes: commit 51e5ef459eca045d7e8afe880ee60190f0b75b26 Author: Marc-André Lureau <marcandre.lur...@redhat.com> Date: Tue Nov 27 12:55:59 2018 +0400 arm: replace instance_post_init() Replace arm_cpu_post_init() instance callback by calling it from leaf classes, to avoid potential ordering issue with other post_init callbacks. Signed-off-by: Marc-André Lureau <marcandre.lur...@redhat.com> Suggested-by: Igor Mammedov <imamm...@redhat.com> Reviewed-by: Igor Mammedov <imamm...@redhat.com> Acked-by: Eduardo Habkost <ehabk...@redhat.com> but then we end up calling it multiple times in the class hierarch, which is a recipe for bugs, and makes it difficult to understand what arm_cpu_post_init() even means, what calling this function is supposed to do. For a "max" or "host" cpu on AArch64, this function is called: for the ARM CPU base class, TYPE_ARM_CPU, in cpu.c::arm_cpu_instance_init, then later again for the TYPE_AARCH64_CPU class, child of TYPE_ARM_CPU, in cpu64.c::aarch64_cpu_instance_init, then later again for the TYPE_ARM_HOST_CPU class, child of TYPE_AARCH64_CPU, in cpu.c::arm_host_initfn. Same for "max". When looking at 32bit CPUs instead, only the ARM CPU base class ends up calling arm_cpu_post_init. "Leaf" classes do not do it (see cpu_tcg.c). What is then arm_cpu_post_init even supposed to mean? Thanks, Claudio >>> >>> In the case of cpu host instead, >>> >>> "-cpu host" calls kvm_arm_set_cpu_features_from_host(), same as max, then >>> calls aarch64_add_sve_properties() but does NOT add "svw-max-vq". >>> >>> Is this a bug? > > It was left out intentionally. More below. > >> >> Maybe; that's a question for Richard or Drew... >> >>> Are "max" and "host" for KVM supposed to be the same like with x86? > > Yes, but my understanding of "max" == "host" for KVM is that that only > applies to the perspective of the guest. What CPU and what CPU features > the guest can see should be exactly the same with either "max" or "host", > depending on the enabling/disabling of any optional CPU properties. > > The question here seems to be that, if one has a CPU property, does that > imply the other should have the same? Which would effectively allow the > two to be aliases (when KVM is enabled). I don't know, does x86 ensure > 100% property compatibility? > > I opted not to support sve-max-vq for "host" because I consider it a > legacy CPU property, one I didn't want to propagate. Indeed it may > make more sense to depreciate sve-max-vq than to "fix" this issue > by adding it to "host". Note, we can already create equivalent SVE > CPUs. The following are the same from the perspective of the guest > > -accel kvm -cpu host,sve512=on > -accel kvm -cpu max,sve512=on > > And, for TCG, these are the same from the perspective of the guest > > -accel tcg -cpu max,sve512=on > -accel tcg -cpu max,sve-max-vq=4 > > So we already don't need sve-max-vq. > > Thanks, > drew >