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?
And why do we have a separate arm_cpu_finalize_features()? Nothing in the ARM cpu classes initializations ever seems to be "final" to me. > > 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 >> >
