Re: [PATCH v1 13/16] kvm: arm64: Configure VTCR per VM

[Suzuki K Poulose]

Hi Christoffer,

On 08/02/18 18:04, Christoffer Dall wrote:
> On Tue, Jan 09, 2018 at 07:04:08PM +0000, Suzuki K Poulose wrote:
> > We set VTCR_EL2 very early during the stage2 init and don't
> > touch it ever. This is fine as we had a fixed IPA size. This
> > patch changes the behavior to set the VTCR for a given VM,
> > depending on its stage2 table. The common configuration for
> > VTCR is still performed during the early init. But the SL0
> > and T0SZ are programmed for each VM and is cleared once we
> > exit the VM.
> >
> > Cc: Marc Zyngier <marc.zyngier@xxxxxxx>
> > Cc: Christoffer Dall <cdall@xxxxxxxxxx>
> > Signed-off-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>

> > diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
> > index f7c651f3a8c0..523471f0af7b 100644
> > --- a/arch/arm64/kvm/hyp/switch.c
> > +++ b/arch/arm64/kvm/hyp/switch.c
> > @@ -157,11 +157,20 @@ static void __hyp_text __deactivate_traps(struct kvm_vcpu *vcpu)
> >   static void __hyp_text __activate_vm(struct kvm_vcpu *vcpu)
> >   {
> >    struct kvm *kvm = kern_hyp_va(vcpu->kvm);
> > + u64 vtcr = read_sysreg(vtcr_el2);
> > +
> > + vtcr &= ~VTCR_EL2_PRIVATE_MASK;
> > + vtcr |= VTCR_EL2_SL0(stage2_pt_levels(kvm)) |
> > + VTCR_EL2_T0SZ(kvm_phys_shift(kvm));
> > + write_sysreg(vtcr, vtcr_el2);
>
> If we're writing VTCR_EL2 on each entry, do we really need to read the
> value back first and calculate things on every entry to the VM?  It
> seems to me we should be able to compute the vtcr_el2 and store it on
> struct kvm, and simply restore that per-VM value upon entering the VM?

I took a look at this and we need to do this to make sure we retain the
Hardware update of Access flags for stage2 (VTCR_EL2_HA) bits on the CPUs
that has it and it is safe to run a mix of CPUs with and without the feature.

> >    write_sysreg(kvm->arch.vttbr, vttbr_el2);
> >   }
> >
> >   static void __hyp_text __deactivate_vm(struct kvm_vcpu *vcpu)
> >   {
> > + u64 vtcr = read_sysreg(vtcr_el2) & ~VTCR_EL2_PRIVATE_MASK;
> > +
> > + write_sysreg(vtcr, vtcr_el2);
>
> Why do we need to care about restoring VTCR when returning to the host?

Yes, this can be skipped.

Cheers
Suzuki