On 3/19/21 9:23 AM, Claudio Fontana wrote: > Hi Markus, > > could you help me untangle the arm_cpu_post_init question?
Nevermind, I think I figured it out. The arm_cpu_post_init are indeed called only for the "leaf" class, via the "instance_init" functions. I think I can use it to do things reliably "post init" for all classes in there. Thanks, Claudio > > I am trying to cleanup a bit the initialization path for ARM, > and it seems that arm_cpu_post_init is called numerous times for AArch64 in > particular, > > while for "tcg cpus", 32bit it is called only once. > > Any reason for the multiple calls in the hierarchy? > Was the intention to actually call this just once from the final leaf classes? > > The ability to execute code after the initialization would come in handy in > an ARM CPU class refactoring I am doing, > but I stopped short of adding anything to arm_cpu_post_init since I noticed > the inconsistencies. > > Thanks, > > Claudio > > > On 3/18/21 12:06 PM, Claudio Fontana wrote: >> 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 >>> >> >> > >
