Subject: [PATCH] arm/arm64: KVM: Handle out-of-RAM cache maintenance as a NOP

From: [email-masked] (Christoffer Dall)
Date: Thu, 11 Feb 2016 09:43:47 +0100

On Mon, Feb 08, 2016 at 02:37:25PM +0000, Marc Zyngier wrote:
> So far, our handling of cache maintenance by VA has been pretty
> simple: Either the access is in the guest RAM and generates a S2
> fault, which results in the page being mapped RW, or we go down
> the io_mem_abort() path, and nuke the guest.
> 
> The first one is fine, but the second one is extremely weird.
> Treating the CM as an I/O is wrong, and nothing in the ARM ARM
> indicates that we should generate a fault for something that
> cannot end-up in the cache anyway (even if the guest maps it,
> it will keep on faulting at stage-2 for emulation).
> 
> So let's just skip this instruction, and let the guest get away
> with it.
> 
> Signed-off-by: Marc Zyngier <[email-masked]>
> ---
>  arch/arm/include/asm/kvm_emulate.h   |  5 +++++
>  arch/arm/kvm/mmu.c                   | 17 +++++++++++++++++
>  arch/arm64/include/asm/kvm_emulate.h |  5 +++++
>  3 files changed, 27 insertions(+)
> 
> diff --git a/arch/arm/include/asm/kvm_emulate.h b/arch/arm/include/asm/kvm_emulate.h
> index 3095df0..f768797 100644
> --- a/arch/arm/include/asm/kvm_emulate.h
> +++ b/arch/arm/include/asm/kvm_emulate.h
> @@ -143,6 +143,11 @@ static inline bool kvm_vcpu_dabt_iss1tw(struct kvm_vcpu *vcpu)
>  	return kvm_vcpu_get_hsr(vcpu) & HSR_DABT_S1PTW;
>  }
>  
> +static inline bool kvm_vcpu_dabt_iscm(struct kvm_vcpu *vcpu)
> +{
> +	return !!(kvm_vcpu_get_hsr(vcpu) & HSR_DABT_CM);
> +}
> +
>  /* Get Access Size from a data abort */
>  static inline int kvm_vcpu_dabt_get_as(struct kvm_vcpu *vcpu)
>  {
> diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
> index aba61fd..1a5f2ea 100644
> --- a/arch/arm/kvm/mmu.c
> +++ b/arch/arm/kvm/mmu.c
> @@ -1431,6 +1431,23 @@ int kvm_handle_guest_abort(struct kvm_vcpu *vcpu, struct kvm_run *run)
>  		}
>  
>  		/*
> +		 * Check for a cache maintenance operation. Since we
> +		 * ended-up here, we know it is outside of any memory
> +		 * slot. But we can't find out if that is for a device,
> +		 * or if the guest is just being stupid (going all the
> +		 * way to userspace is not an option - what would you
> +		 * write?).

I'm not sure what the stuff about "what would you write?" means, I'm
assuming it means that the ISV bit is clear.

I think the point is more that if there's no S2 mapping, then there
could never be any cache entries as a result of memory accesses using
the GVA in question, so there's nothing to invalidate (like you state in
your commit message).

> +		 *
> +		 * So let's assume that the guest is just being
> +		 * cautious, and skip the instruction.
> +		 */
> +		if (kvm_vcpu_dabt_iscm(vcpu)) {
> +			kvm_skip_instr(vcpu, kvm_vcpu_trap_il_is32bit(vcpu));
> +			ret = 1;
> +			goto out_unlock;
> +		}
> +
> +		/*
>  		 * The IPA is reported as [MAX:12], so we need to
>  		 * complement it with the bottom 12 bits from the
>  		 * faulting VA. This is always 12 bits, irrespective
> diff --git a/arch/arm64/include/asm/kvm_emulate.h b/arch/arm64/include/asm/kvm_emulate.h
> index 3066328..01cdf5f 100644
> --- a/arch/arm64/include/asm/kvm_emulate.h
> +++ b/arch/arm64/include/asm/kvm_emulate.h
> @@ -185,6 +185,11 @@ static inline bool kvm_vcpu_dabt_iss1tw(const struct kvm_vcpu *vcpu)
>  	return !!(kvm_vcpu_get_hsr(vcpu) & ESR_ELx_S1PTW);
>  }
>  
> +static inline bool kvm_vcpu_dabt_iscm(const struct kvm_vcpu *vcpu)
> +{
> +	return !!(kvm_vcpu_get_hsr(vcpu) & ESR_ELx_CM);
> +}
> +
>  static inline int kvm_vcpu_dabt_get_as(const struct kvm_vcpu *vcpu)
>  {
>  	return 1 << ((kvm_vcpu_get_hsr(vcpu) & ESR_ELx_SAS) >> ESR_ELx_SAS_SHIFT);
> -- 
> 2.1.4
> 

some nitpicking: if you modify anything here, I think is_cm is more clear than iscm.

But besides the cosmetics:

Reviewed-by: Christoffer Dall <[email-masked]>