On 3/11/21 5:18 PM, Paolo Bonzini wrote: > On 11/03/21 16:02, Peter Maydell wrote: >> On Thu, 11 Mar 2021 at 14:27, Claudio Fontana <cfont...@suse.de> wrote: >>> the "max" cpu in x86 and s390 is a class, >>> >>> and then "host" has "max" as parent. >>> >>> This would be a convenient setup for ARM too, as it would allow to put >>> common code between kvm and tcg in the "max" class, >>> and allow "host" to specialize the behavior for KVM (and in the future HVF >>> probably). >>> >>> Would changing the class hierarchy this way be acceptable, cause any >>> problems? >> >> It's not clear to me why 'host' would be a subtype of 'max': >> that doesn't seem like an obvious relationship. > > On x86, "-cpu host" is essentially the same as "-cpu max" with the only > difference that it errors out on TCG. So: > > - with TCG: "-cpu max" enables all that can be emulated, "-cpu host" fails > > - with KVM: "-cpu max" enables all that can be virtualized, "-cpu host" > does the same > > Paolo >
Maybe Peter you could clarify similarly what the intended meaning of "max" is on ARM? TCG: for aarch64, the starting point seems to be Cortex-A57, and then lots of other features are added on top of it, and for non-aarch64, the starting point seems to be Cortex-A15, plus "old-style VFP short-vector support". Is the intention to enable all that can be emulated here too, like for X86? 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. 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? Are "max" and "host" for KVM supposed to be the same like with x86? Thanks, Claudio
