Re: [PATCH v6 1/6] arm64/kvm: preserve host HCR_EL2 value

[Amit Daniel Kachhap]

Hi,

On 2/21/19 5:20 PM, Mark Rutland wrote:
> Hi,
>
> On Tue, Feb 19, 2019 at 02:54:26PM +0530, Amit Daniel Kachhap wrote:
> > From: Mark Rutland <mark.rutland@xxxxxxx>
> >
> > When restoring HCR_EL2 for the host, KVM uses HCR_HOST_VHE_FLAGS, which
> > is a constant value. This works today, as the host HCR_EL2 value is
> > always the same, but this will get in the way of supporting extensions
> > that require HCR_EL2 bits to be set conditionally for the host.
> >
> > To allow such features to work without KVM having to explicitly handle
> > every possible host feature combination, this patch has KVM save/restore
> > for the host HCR when switching to/from a guest HCR. The saving of the
> > register is done once during cpu hypervisor initialization state and is
> > just restored after switch from guest.
> >
> > For fetching HCR_EL2 during kvm initialisation, a hyp call is made using
> > kvm_call_hyp and is helpful in NHVE case.
> >
> > For the hyp TLB maintenance code, __tlb_switch_to_host_vhe() is updated
> > to toggle the TGE bit with a RMW sequence, as we already do in
> > __tlb_switch_to_guest_vhe().
> >
> > The value of hcr_el2 is now stored in struct kvm_cpu_context as both host
> > and guest can now use this field in a common way.
> >
> > Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
> > [Added __cpu_copy_hyp_conf, hcr_el2 field in struct kvm_cpu_context]
> > Signed-off-by: Amit Daniel Kachhap <amit.kachhap@xxxxxxx>
> > Cc: Marc Zyngier <marc.zyngier@xxxxxxx>
> > Cc: Christoffer Dall <christoffer.dall@xxxxxxx>
> > Cc: kvmarm@xxxxxxxxxxxxxxxxxxxxx
>
> [...]
>
> > +/**
> > + * __cpu_copy_hyp_conf - copy the boot hyp configuration registers
> > + *
> > + * It is called once per-cpu during CPU hyp initialisation.
> > + */
> > +static inline void __cpu_copy_hyp_conf(void)
>
> I think this would be better named as something like:
>
>    cpu_init_host_ctxt()
>
> ... as that makes it a bit clearer as to what is being initialized.
ok, Agreed with the name.
> [...]
>
> > +/**
> > + * __kvm_populate_host_regs - Stores host register values
> > + *
> > + * This function acts as a function handler parameter for kvm_call_hyp and
> > + * may be called from EL1 exception level to fetch the register value.
> > + */
> > +void __hyp_text __kvm_populate_host_regs(void)
> > +{
> > +	struct kvm_cpu_context *host_ctxt;
> > +
> > +	if (has_vhe())
> > +		host_ctxt = this_cpu_ptr(&kvm_host_cpu_state);
> > +	else
> > +		host_ctxt = __hyp_this_cpu_ptr(kvm_host_cpu_state);
>
> Do we need the has_vhe() check here?
>
> Can't we always do:
>
> 	host_ctxt = __hyp_this_cpu_ptr(kvm_host_cpu_state);
>
> ... regardless of VHE? Or is that broken for VHE somehow?
Yes it works fine for VHE. I missed this.

Thanks,
Amit
> Thanks,
> Mark.