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SubjectRe: [PATCH v6 1/6] arm64/kvm: preserve host HCR_EL2 value
On Tue, Feb 19, 2019 at 02:54:26PM +0530, Amit Daniel Kachhap wrote:
> From: Mark Rutland <mark.rutland@arm.com>
>
> 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.

Minor nit: NVHE misspelled. This looks a bit like it's naming an arch
feature rather than a kernel implementation detail though. Maybe write
"non-VHE".

> 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@arm.com>
> [Added __cpu_copy_hyp_conf, hcr_el2 field in struct kvm_cpu_context]
> Signed-off-by: Amit Daniel Kachhap <amit.kachhap@arm.com>
> Cc: Marc Zyngier <marc.zyngier@arm.com>
> Cc: Christoffer Dall <christoffer.dall@arm.com>
> Cc: kvmarm@lists.cs.columbia.edu
> ---
> arch/arm/include/asm/kvm_host.h | 2 ++
> arch/arm64/include/asm/kvm_asm.h | 2 ++
> arch/arm64/include/asm/kvm_emulate.h | 22 +++++++++++-----------
> arch/arm64/include/asm/kvm_host.h | 13 ++++++++++++-
> arch/arm64/include/asm/kvm_hyp.h | 2 +-
> arch/arm64/kvm/guest.c | 2 +-
> arch/arm64/kvm/hyp/switch.c | 23 +++++++++++++----------
> arch/arm64/kvm/hyp/sysreg-sr.c | 21 ++++++++++++++++++++-
> arch/arm64/kvm/hyp/tlb.c | 6 +++++-
> virt/kvm/arm/arm.c | 1 +
> 10 files changed, 68 insertions(+), 26 deletions(-)
>
> diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h
> index ca56537..05706b4 100644
> --- a/arch/arm/include/asm/kvm_host.h
> +++ b/arch/arm/include/asm/kvm_host.h
> @@ -273,6 +273,8 @@ static inline void __cpu_init_stage2(void)
> kvm_call_hyp(__init_stage2_translation);
> }
>
> +static inline void __cpu_copy_hyp_conf(void) {}
> +
> static inline int kvm_arch_vm_ioctl_check_extension(struct kvm *kvm, long ext)
> {
> return 0;
> diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h
> index f5b79e9..8acd73f 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 void __kvm_populate_host_regs(void);
> +
> /* Home-grown __this_cpu_{ptr,read} variants that always work at HYP */
> #define __hyp_this_cpu_ptr(sym) \
> ({ \
> diff --git a/arch/arm64/include/asm/kvm_emulate.h b/arch/arm64/include/asm/kvm_emulate.h
> index 506386a..0dbe795 100644
> --- a/arch/arm64/include/asm/kvm_emulate.h
> +++ b/arch/arm64/include/asm/kvm_emulate.h
> @@ -50,25 +50,25 @@ void kvm_inject_pabt32(struct kvm_vcpu *vcpu, unsigned long addr);
>
> static inline bool vcpu_el1_is_32bit(struct kvm_vcpu *vcpu)
> {
> - return !(vcpu->arch.hcr_el2 & HCR_RW);
> + return !(vcpu->arch.ctxt.hcr_el2 & HCR_RW);

Putting hcr_el2 into struct kvm_cpu_context creates a lot of splatter
here, and I'm wondering whether it's really necessary. Otherwise,
we could just put the per-vcpu guest HCR_EL2 value in struct
kvm_vcpu_arch.

Is the *host* hcr_el2 value really different per-vcpu? That looks
odd. I would have thought this is fixed across the system at KVM
startup time.

Having a single global host hcr_el2 would also avoid the need for
__kvm_populate_host_regs(): instead, we just decide what HCR_EL2 is to
be ahead of time and set a global variable that we map into Hyp.


Or does the host HCR_EL2 need to vary at runtime for some reason I've
missed?

[...]

+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);

According to the comment by the definition of __hyp_this_cpu_ptr(), this
always works at Hyp. I also see other calls with no fallback
this_cpu_ptr() call like we have here.

So, can we simply always call __hyp_this_cpu_ptr() here?

(I'm not familiar with this, myself.)

Cheers
---Dave

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