linux-kernel - Re: [PATCH v6 1/6] arm64/kvm: preserve host HCR

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Date:   Thu, 21 Feb 2019 15:49:35 +0000
From:   Dave Martin <Dave.Martin@....com>
To:     Amit Daniel Kachhap <amit.kachhap@....com>
Cc:     linux-arm-kernel@...ts.infradead.org,
        Marc Zyngier <marc.zyngier@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Kristina Martsenko <kristina.martsenko@....com>,
        kvmarm@...ts.cs.columbia.edu,
        Ramana Radhakrishnan <ramana.radhakrishnan@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [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@....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@....com>
> [Added __cpu_copy_hyp_conf, hcr_el2 field in struct kvm_cpu_context]
> Signed-off-by: Amit Daniel Kachhap <amit.kachhap@....com>
> Cc: Marc Zyngier <marc.zyngier@....com>
> Cc: Christoffer Dall <christoffer.dall@....com>
> Cc: kvmarm@...ts.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