Re: [PATCHv2 09/12] arm64/kvm: preserve host HCR_EL2 value

[Christoffer Dall]

On Mon, Nov 27, 2017 at 04:38:03PM +0000, Mark Rutland 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.
> 
> For __{activate,deactivate}_traps(), the HCR save/restore is made common
> across the !VHE and VHE paths. As the host and guest HCR values must
> have E2H set when VHE is in use, register redirection should always be
> in effect at EL2, and this change should not adversely affect the VHE
> code.
> 
> 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 now unused HCR_HOST_VHE_FLAGS definition is removed.
> 
> Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
> Reviewed-by: Christoffer Dall <cdall@xxxxxxxxxx>
> Cc: Marc Zyngier <marc.zyngier@xxxxxxx>
> Cc: kvmarm@xxxxxxxxxxxxxxxxxxxxx
> ---
>  arch/arm64/include/asm/kvm_arm.h  | 1 -
>  arch/arm64/include/asm/kvm_host.h | 5 ++++-
>  arch/arm64/kvm/hyp/switch.c       | 5 +++--
>  arch/arm64/kvm/hyp/tlb.c          | 6 +++++-
>  4 files changed, 12 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_arm.h b/arch/arm64/include/asm/kvm_arm.h
> index 62854d5d1d3b..aa02b05430e8 100644
> --- a/arch/arm64/include/asm/kvm_arm.h
> +++ b/arch/arm64/include/asm/kvm_arm.h
> @@ -84,7 +84,6 @@
>  			 HCR_AMO | HCR_SWIO | HCR_TIDCP | HCR_RW)
>  #define HCR_VIRT_EXCP_MASK (HCR_VSE | HCR_VI | HCR_VF)
>  #define HCR_INT_OVERRIDE   (HCR_FMO | HCR_IMO)
> -#define HCR_HOST_VHE_FLAGS (HCR_RW | HCR_TGE | HCR_E2H)
>  
>  /* TCR_EL2 Registers bits */
>  #define TCR_EL2_RES1		((1 << 31) | (1 << 23))
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 674912d7a571..39184aa3e2f2 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -199,10 +199,13 @@ typedef struct kvm_cpu_context kvm_cpu_context_t;
>  struct kvm_vcpu_arch {
>  	struct kvm_cpu_context ctxt;
>  
> -	/* HYP configuration */
> +	/* Guest HYP configuration */
>  	u64 hcr_el2;
>  	u32 mdcr_el2;
>  
> +	/* Host HYP configuration */
> +	u64 host_hcr_el2;
> +
>  	/* Exception Information */
>  	struct kvm_vcpu_fault_info fault;
>  
> diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
> index 525c01f48867..2205f0be3ced 100644
> --- a/arch/arm64/kvm/hyp/switch.c
> +++ b/arch/arm64/kvm/hyp/switch.c
> @@ -71,6 +71,8 @@ static void __hyp_text __activate_traps(struct kvm_vcpu *vcpu)
>  {
>  	u64 val;
>  
> +	vcpu->arch.host_hcr_el2 = read_sysreg(hcr_el2);
> +

Looking back at this, it seems excessive to switch this at every
round-trip.  I think it should be possible to have this as a single
global (or per-CPU) variable that gets restored directly when returning
from the VM.

Thanks,
-Christoffer

>  	/*
>  	 * We are about to set CPTR_EL2.TFP to trap all floating point
>  	 * register accesses to EL2, however, the ARM ARM clearly states that
> @@ -116,7 +118,6 @@ static void __hyp_text __deactivate_traps_vhe(void)
>  		    MDCR_EL2_TPMS;
>  
>  	write_sysreg(mdcr_el2, mdcr_el2);
> -	write_sysreg(HCR_HOST_VHE_FLAGS, hcr_el2);
>  	write_sysreg(CPACR_EL1_DEFAULT, cpacr_el1);
>  	write_sysreg(vectors, vbar_el1);
>  }
> @@ -129,7 +130,6 @@ static void __hyp_text __deactivate_traps_nvhe(void)
>  	mdcr_el2 |= MDCR_EL2_E2PB_MASK << MDCR_EL2_E2PB_SHIFT;
>  
>  	write_sysreg(mdcr_el2, mdcr_el2);
> -	write_sysreg(HCR_RW, hcr_el2);
>  	write_sysreg(CPTR_EL2_DEFAULT, cptr_el2);
>  }
>  
> @@ -151,6 +151,7 @@ static void __hyp_text __deactivate_traps(struct kvm_vcpu *vcpu)
>  	__deactivate_traps_arch()();
>  	write_sysreg(0, hstr_el2);
>  	write_sysreg(0, pmuserenr_el0);
> +	write_sysreg(vcpu->arch.host_hcr_el2, hcr_el2);
>  }
>  
>  static void __hyp_text __activate_vm(struct kvm_vcpu *vcpu)
> diff --git a/arch/arm64/kvm/hyp/tlb.c b/arch/arm64/kvm/hyp/tlb.c
> index 73464a96c365..c2b0680efa2c 100644
> --- a/arch/arm64/kvm/hyp/tlb.c
> +++ b/arch/arm64/kvm/hyp/tlb.c
> @@ -49,12 +49,16 @@ static hyp_alternate_select(__tlb_switch_to_guest,
>  
>  static void __hyp_text __tlb_switch_to_host_vhe(struct kvm *kvm)
>  {
> +	u64 val;
> +
>  	/*
>  	 * We're done with the TLB operation, let's restore the host's
>  	 * view of HCR_EL2.
>  	 */
>  	write_sysreg(0, vttbr_el2);
> -	write_sysreg(HCR_HOST_VHE_FLAGS, hcr_el2);
> +	val = read_sysreg(hcr_el2);
> +	val |= HCR_TGE;
> +	write_sysreg(val, hcr_el2);
>  }
>  
>  static void __hyp_text __tlb_switch_to_host_nvhe(struct kvm *kvm)
> -- 
> 2.11.0
>