Re: [RFC PATCH v4 02/26] KVM: arm64: Save ID registers' sanitized value per guest

[Reiji Watanabe <reijiw@google.com>]

Hi Ricardo,

> > > > +
> > > > +/*
> > > > + * Set the guest's ID registers that are defined in sys_reg_descs[]
> > > > + * with ID_SANITISED() to the host's sanitized value.
> > > > + */
> > > > +void set_default_id_regs(struct kvm *kvm)
> > > > +{
> > > > +     int i;
> > > > +     u32 id;
> > > > +     const struct sys_reg_desc *rd;
> > > > +     u64 val;
> > > > +
> > > > +     for (i = 0; i < ARRAY_SIZE(sys_reg_descs); i++) {
> > > > +             rd = &sys_reg_descs[i];
> > > > +             if (rd->access != access_id_reg)
> > > > +                     /* Not ID register, or hidden/reserved ID register */
> > > > +                     continue;
> > > > +
> > > > +             id = reg_to_encoding(rd);
> > > > +             if (WARN_ON_ONCE(!is_id_reg(id)))
> > > > +                     /* Shouldn't happen */
> > > > +                     continue;
> > > > +
> > > > +             val = read_sanitised_ftr_reg(id);
> > >
> > > I'm a bit confused. Shouldn't the default+sanitized values already use
> > > arm64_ftr_bits_kvm (instead of arm64_ftr_regs)?
> >
> > I'm not sure if I understand your question.
> > arm64_ftr_bits_kvm is used for feature support checkings when
> > userspace tries to modify a value of ID registers.
> > With this patch, KVM just saves the sanitized values in the kvm's
> > buffer, but userspace is still not allowed to modify values of ID
> > registers yet.
> > I hope it answers your question.
>
> Based on the previous commit I was assuming that some registers, like
> id_aa64dfr0,
> would default to the overwritten values as the sanitized values. More
> specifically: if
> userspace doesn't modify any ID reg, shouldn't the defaults have the
> KVM overwritten
> values (arm64_ftr_bits_kvm)?

arm64_ftr_bits_kvm doesn't have arm64_ftr_reg but arm64_ftr_bits,
and arm64_ftr_bits_kvm doesn't have the sanitized values.

Thanks,
Reiji