Re: [RFC 18/21] arm/cpu: Introduce a customizable kvm host cpu model

[Eric Auger <eric.auger@redhat.com>]

Hi Daniel,

On 10/25/24 15:06, Daniel P. Berrangé wrote:
> On Fri, Oct 25, 2024 at 12:17:37PM +0200, Eric Auger wrote:
>> This new cpu model takes by default the host cpu values.
>> However it exposes uint64 SYSREG properties for writable ID reg
>> fields exposed by the host kernel. Properties are named
>> SYSREG_<REG>_<FIELD> with REG and FIELD being those used
>> in linux arch/arm64/tools/sysreg. This done by matching the
>> writable fields retrieved from the host kernel against the
>> generated description of sysregs.
>>
>> An example of invocation is:
>> -cpu custom,SYSREG_ID_AA64ISAR0_EL1_DP=0x0
>> which sets DP field of ID_AA64ISAR0_EL1 to 0.
> "SYSREG_" feels kinda redundant to repeat on every single
> feature. 

I do agree. To be honest this was mostly driven my implementation need
for cpu model expansion. Given the high number of props which are
getting exposed, I iterate on all props and having a prefix let me
return only those SYSREG props. Most probably we can get rid of the
prefix by using some generated code as well.
>
> Also, is this naming convention really the same one that users
> will see when they look at /proc/cpuinfo to view features ? It
No it is not. I do agree that the custom cpu model is very low level. It
is very well suited to test all series turning ID regs as writable but
this would require an extra layer that adapts /proc/cpuinfo feature
level to this regid/field abstraction.

In /cpu/proc you will see somethink like:
 Features    : fp asimd evtstrm aes pmull sha1 sha2 crc32 atomics fphp
asimdhp cpuid asimdrdm lrcpc dcpop asimddp
> feels pretty low level to me ?  Naming after the registers &
> fields, would be like configuring x86 CPU features by asking
> for "SYSREG_EAX_1_ECX_20" instead of saying "vmx" which is the
> human friendly name.
agreed.
>
>
>> Signed-off-by: Eric Auger <eric.auger@redhat.com>
>> Signed-off-by: Cornelia Huck <cohuck@redhat.com>
>>
>> ---
>>
>> At the moment, the custom model does not support legacy options
>> of the host cpu model. We need to understand what we do with those
>> latter (SVE, ...). This means that related KVM ioctl are
>> not called yet.
> It will be pretty painful to have to use different feature
> terminology for different CPU models. Everything in libvirt
> assuming feature terminology varies per-arch, not per-CPU
> model.
Actually as far as I understand those regids/fields would fit all kind
of aarch64 Cortex-A CPUs. So they wouldn't vary per-CPU (I mean their
terminology. Their availability will).

Thanks

Eric
>
>
> With regards,
> Daniel