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

From: Claudio Fontana <cfontana@suse.de>
To: Paolo Bonzini <pbonzini@redhat.com>,
	Peter Maydell <peter.maydell@linaro.org>
Cc: "Philippe Mathieu-Daudé" <f4bug@amsat.org>,
	qemu-arm <qemu-arm@nongnu.org>,
	qemu-devel <qemu-devel@nongnu.org>,
	"Eduardo Habkost" <ehabkost@redhat.com>
Subject: Re: arm: "max" CPU class hierarchy changes possible?
Date: Thu, 11 Mar 2021 18:16:52 +0100	[thread overview]
Message-ID: <2277fdf5-ec92-476a-8fe5-0d4eee23dfef@suse.de> (raw)
In-Reply-To: <2e6a5d98-e022-0b39-5f30-92eb74491d3b@redhat.com>

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 <cfontana@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