Re: [PATCH v5 2/5] arm64/kvm: preserve host HCR_EL2/MDCR_EL2 value

James Morse
2019-01-31T16:22:12Z

Hi Amit,

On 28/01/2019 06:58, Amit Daniel Kachhap wrote:
> 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
> 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().

> While at it, host MDCR_EL2 value is fetched in a similar way and restored
> after every switch from host to guest. There should not be any change in
> functionality due to this.

Could this step be done as a separate subsequent patch? It would make review
easier! The MDCR stuff would be a simplification if done second, done in one go
like this its pretty noisy.

There ought to be some justification for moving hcr/mdcr into the cpu_context in
the commit message.

If you're keeping Mark's 'Signed-off-by' its would be normal to keep Mark as the
author in git. This shows up a an extra 'From:' when you post the patch, and
gets picked up when the maintainer runs git-am.

This patch has changed substantially from Mark's version:
https://lkml.org/lkml/2017/11/27/675

If you keep the signed-off-by, could you add a [note] in the signed-off area
with a terse summary. Something like:
> Signed-off-by: Mark Rutland <mark.rutland@arm.com>
[ Move hcr to cpu_context, added __cpu_copy_hyp_conf()]
> Signed-off-by: Amit Daniel Kachhap <amit.kachhap@arm.com>

(9c06602b1b92 is a good picked-at-random example for both of these)

> diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h
> index f5b79e9..2da6e43 100644
> --- a/arch/arm64/include/asm/kvm_asm.h
> +++ b/arch/arm64/include/asm/kvm_asm.h
> @@ -80,6 +80,8 @@ extern void __vgic_v3_init_lrs(void);
>
> extern u32 __kvm_get_mdcr_el2(void);
>
> +extern u64 __kvm_get_hcr_el2(void);

Do we need these in separate helpers? For non-vhe this means two separate trips
to EL2. Something like kvm_populate_host_context(void), and an __ version for
the bit at EL2?

We don't need to pass the host-context to EL2 as once kvm is loaded we can
access host per-cpu variables at EL2 using __hyp_this_cpu_read(). This will save
passing the vcpu around.

> @@ -458,6 +457,25 @@ int kvm_arm_vcpu_arch_has_attr(struct kvm_vcpu *vcpu,
>
> static inline void __cpu_init_stage2(void) {}
>
> +/**
> + * __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)
> +{
> + kvm_cpu_context_t *host_cxt = this_cpu_ptr(&kvm_host_cpu_state);
> +
> + host_cxt->hcr_el2 = kvm_call_hyp(__kvm_get_hcr_el2);
> +
> + /*
> + * Retrieve the initial value of mdcr_el2 so we can preserve
> + * MDCR_EL2.HPMN which has presumably been set-up by some
> + * knowledgeable bootcode.
> + */
> + host_cxt->mdcr_el2 = kvm_call_hyp(__kvm_get_mdcr_el2);
> +}

Its strange to make this an inline in a header. kvm_arm_init_debug() is a
static-inline for arch/arm, but a regular C function for arch/arm64. Can't we do
the same?

> diff --git a/arch/arm64/kvm/hyp/sysreg-sr.c b/arch/arm64/kvm/hyp/sysreg-sr.c
> index 68d6f7c..22c854a 100644
> --- a/arch/arm64/kvm/hyp/sysreg-sr.c
> +++ b/arch/arm64/kvm/hyp/sysreg-sr.c
> @@ -316,3 +316,14 @@ void __hyp_text __kvm_enable_ssbs(void)
> "msr sctlr_el2, %0"
> : "=&r" (tmp) : "L" (SCTLR_ELx_DSSBS));
> }
> +
> +/**
> + * __read_hyp_hcr_el2 - Returns hcr_el2 register value
> + *
> + * 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.
> + */
> +u64 __hyp_text __kvm_get_hcr_el2(void)
> +{
> + return read_sysreg(hcr_el2);
> +}

While I'm all in favour of kernel-doc comments for functions, it may be
over-kill in this case!

> diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
> index 9e350fd3..2d65ada 100644
> --- a/virt/kvm/arm/arm.c
> +++ b/virt/kvm/arm/arm.c
> @@ -1327,10 +1327,10 @@ static void cpu_hyp_reinit(void)
> else
> cpu_init_hyp_mode(NULL);
>
> - kvm_arm_init_debug();
> -
> if (vgic_present)
> kvm_vgic_init_cpu_hardware();
> +
> + __cpu_copy_hyp_conf();
> }

Was there a reason to make this call later than it originally was?
(kvm_vgic_init_cpu_hardware() doesn't use any of those values, so its fine, just
curious!)

Thanks,

James