Re: [PATCH v4 3/6] KVM: arm64: add emulation for CTR_EL0 register

[Sebastian Ott]

Hi Oliver,

On Thu, 13 Jun 2024, Oliver Upton wrote:
> On Thu, Jun 13, 2024 at 10:19:56PM +0000, Oliver Upton wrote:
> > On Mon, Jun 03, 2024 at 03:05:04PM +0200, Sebastian Ott wrote:
> > > +static int validate_cache_topology(struct kvm_vcpu *vcpu, u64 ctr_el0)
> > > +{
> > > +	const struct sys_reg_desc *clidr_el1;
> > > +	unsigned int i;
> > > +	int ret;
> > > +
> > > +	clidr_el1 = get_sys_reg_desc(SYS_CLIDR_EL1);
> > > +	if (!clidr_el1)
> > > +		return -ENOENT;
> >
> > This doesn't actually matter if we agree on dropping the cross-checking,
> > but if this lookup fails it is 100% a KVM bug. Returning ENOENT isn't
> > exactly right here, since it gives userspace the impression that the
> > sysreg index it tried to access does not exist.
> >
> > So in the future it'd be good to return EINVAL in places where the
> > kernel did something stupid, probably with a warning for good measure.

OK.

> > > +static int set_ctr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
> > > +		   u64 val)
> > > +{
> > > +	u64 ctr, writable_mask = rd->val;
> > > +	int ret = 0;
> > > +
> > > +	mutex_lock(&vcpu->kvm->arch.config_lock);
> > > +	ctr  = vcpu->kvm->arch.ctr_el0;
> > > +	if (val == ctr)
> > > +		goto out_unlock;
> > > +
> > > +	ret = -EBUSY;
> > > +	if (kvm_vm_has_ran_once(vcpu->kvm))
> > > +		goto out_unlock;
> > > +
> > > +	ret = -EINVAL;
> > > +	if ((ctr & ~writable_mask) != (val & ~writable_mask))
> > > +		goto out_unlock;
> > > +
> > > +	if (((ctr & CTR_EL0_DIC_MASK) < (val & CTR_EL0_DIC_MASK)) ||
> > > +	    ((ctr & CTR_EL0_IDC_MASK) < (val & CTR_EL0_IDC_MASK)) ||
> > > +	    ((ctr & CTR_EL0_DminLine_MASK) < (val & CTR_EL0_DminLine_MASK)) ||
> > > +	    ((ctr & CTR_EL0_IminLine_MASK) < (val & CTR_EL0_IminLine_MASK))) {
> > > +		goto out_unlock;
> >
> > I'd prefer if we addressed the issue w/ arm64_check_features() by making
> > CTR_EL0 behave like the other registers in the ID space instead of
> > open-coding these sorts of checks.
> >
> > I believe that can be accomplished by using kvm_read_sanitised_id_reg()
> > as the ::reset() function in the descriptor and initializing
> > kvm->arch.ctr_el0 in kvm_reset_id_regs().
>
> Durr, I got rid of kvm_reset_id_regs() in commit 44cbe80b7616 ("KVM: arm64:
> Reset VM feature ID regs from kvm_reset_sys_regs()"), I should engage
> brain before responding.
>
> Adding a check for encoding == CTR_EL0 to is_vm_ftr_id_reg() seems to be
> the best way out.

Hm, but in that case we'd use reset_vm_ftr_id_reg() meaning we would have
to make IDREG() work for this reg. Either by adding special handling to
that macro or by increasing kvm->arch.id_regs[] a lot - both options don't
sound very appealing.

I'll think of smth to make arm64_check_features() work for this.

Thanks,
Sebastian