Re: [PATCH v3 08/10] arm/cpu: more customization for the kvm host cpu model

[Daniel P . Berrangé]

On Wed, May 14, 2025 at 05:36:58PM +0200, Cornelia Huck wrote:
> On Tue, May 13 2025, Daniel P. Berrangé <berrange@redhat.com> wrote:
> 
> > On Mon, Apr 14, 2025 at 06:38:47PM +0200, Cornelia Huck wrote:
> >> From: Eric Auger <eric.auger@redhat.com>
> >> 
> >> If the interface for writable ID registers is available, expose 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 host,SYSREG_ID_AA64ISAR0_EL1_DP=0x0
> >> which sets DP field of ID_AA64ISAR0_EL1 to 0.
> >
> > For the value you are illustrating 0x0 - is this implying that
> > all the flags take an arbitrary integer hex value ?
> >
> > This would be different from x86, where CPU feature flags are
> > a boolean on/off state.
> 
> Most of the fields are 4 bits, the allowed values vary (there are also
> some fields that are single bits, or wider.) The FEAT_xxx values (which
> can be expressed via ID register fields, or a combination thereof) are
> mostly boolean, but there are also some of them that can take values.
> 
> We could cook up pseudo-features that are always on/off, but I don't
> like that approach: they would be QEMU only, whereas the ID register
> fields and FEAT_xxx features are all defined in the Arm documentation.

Fortunately from a libvirt POV we can likely expand our config
to cope with hex values for arm features without too much
trouble.

> 
> An additional difference from x86 would be that FEAT_xxx featues are not
> neccessarily configurable (only if the host kernel supports changing the
> ID register field(s) backing the feature.)

Is the kernel able to tell us which ones are configurable and which
are not ? If so, it'd be helpful to expose this info in QAPI some
place.

> 

With regards,
Daniel
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