Re: arm: "max" CPU class hierarchy changes possible?

From: Peter Maydell <peter.maydell@linaro.org>
To: Claudio Fontana <cfontana@suse.de>
Cc: "Andrew Jones" <drjones@redhat.com>,
	"Eduardo Habkost" <ehabkost@redhat.com>,
	"Richard Henderson" <richard.henderson@linaro.org>,
	"Philippe Mathieu-Daudé" <f4bug@amsat.org>,
	qemu-devel <qemu-devel@nongnu.org>,
	qemu-arm <qemu-arm@nongnu.org>,
	"Paolo Bonzini" <pbonzini@redhat.com>
Subject: Re: arm: "max" CPU class hierarchy changes possible?
Date: Thu, 11 Mar 2021 18:33:15 +0000	[thread overview]
Message-ID: <CAFEAcA_j-0+vmNFtPjcxEXC9r4bFrebDfGjq-x1SfguzUG4qcw@mail.gmail.com> (raw)
In-Reply-To: <2277fdf5-ec92-476a-8fe5-0d4eee23dfef@suse.de>

On Thu, 11 Mar 2021 at 17:16, Claudio Fontana <cfontana@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.
>
> 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?

Maybe; that's a question for Richard or Drew...

> Are "max" and "host" for KVM supposed to be the same like with x86?

Yes.

thanks
-- PMM