Re: [PATCH v5 06/15] KVM: arm64: Add support for FEAT_HDBSS

[Leonardo Bras <leo.bras@arm.com>]

On Fri, Oct 30, 2026 at 10:53:57AM +0800, Tian Zheng wrote:
> 
> 
> On 10/27/2026 2:12 AM, Leonardo Bras wrote:
> > On Tue, Sep 29, 2026 at 06:36:46PM +0800, Tian Zheng wrote:
> > > diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> > > index 32102c3912fa..12fa37328dd2 100644
> > > --- a/arch/arm64/kernel/cpufeature.c
> > > +++ b/arch/arm64/kernel/cpufeature.c
> > > @@ -2158,6 +2158,11 @@ static bool hvhe_possible(const struct arm64_cpu_capabilities *entry,
> > >   	return arm64_test_sw_feature_override(ARM64_SW_FEATURE_OVERRIDE_HVHE);
> > >   }
> > > 
> > > +static bool has_vhe_hdbss(const struct arm64_cpu_capabilities *entry, int scope)
> > > +{
> > > +	return is_kernel_in_hyp_mode() && has_cpuid_feature(entry, scope);
> > > +}
> > > +
> > 
> 
> Hi leo,
> 
> > IIUC you run this on every cpu in the system, to make sure it's matching
> > the feature. Is there any chance the kernel is in EL2 only on some of the
> > cpus here?
> > 
> 
> It's impossible for the system to have only some CPUs in EL2 and others
>  not.
> 
> > If not, maybe we can run it on system_supports_hdbss() instead, which would
> > be easier for the user to understand it only runs in EL2, on top of not
> > needing the above.
> > 
> 
> However, The question is whether a cap should mean "this system can use
> the feature" or "this hardware supports the feature". If the cap only
> tracks hardware support, users may be confused when the hardware
> supports HDBSS but the system runs in nVHE mode — they would mistakenly
> believe they can use the feature.
> 
> So I think we should keep is_kernel_in_hyp_mode() in has_vhe_hdbss(),
> and add a comment in system_supports_hdbss() explaining this.
> 
> Does that work for you? Looking forward to your reply.

Well, 
That's just my personal view of this:

If we have a function called "has_hdbss" it should only report if the 
system has the feature available.

On the "system_supports_*" we have to figure out if the system itself 
supports the feature, which checks the feature bits itself and could mean 
checking for other features being there, such as hyp_mode.  

What do you think?

Thx,
Leo