Re: [PATCH v4 28/40] KVM: arm64: Defer saving/restoring 64-bit sysregs to vcpu load/put on VHE

[Marc Zyngier <marc.zyngier@arm.com>]

On Thu, 15 Feb 2018 21:03:20 +0000,
Christoffer Dall wrote:
> 
> Some system registers do not affect the host kernel's execution and can
> therefore be loaded when we are about to run a VCPU and we don't have to
> restore the host state to the hardware before the time when we are
> actually about to return to userspace or schedule out the VCPU thread.
> 
> The EL1 system registers and the userspace state registers only
> affecting EL0 execution do not need to be saved and restored on every
> switch between the VM and the host, because they don't affect the host
> kernel's execution.
> 
> We mark all registers which are now deffered as such in the
> vcpu_{read,write}_sys_reg accessors in sys-regs.c to ensure the most
> up-to-date copy is always accessed.
> 
> Note MPIDR_EL1 (controlled via VMPIDR_EL2) is accessed from other vcpu
> threads, for example via the GIC emulation, and therefore must be
> declared as immediate, which is fine as the guest cannot modify this
> value.
> 
> The 32-bit sysregs can also be deferred but we do this in a separate
> patch as it requires a bit more infrastructure.
> 
> Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
> ---
> 
> Notes:
>     Changes since v3:
>      - Changed to switch-based sysreg approach
> 
>  arch/arm64/kvm/hyp/sysreg-sr.c | 39 +++++++++++++++++++++++++++++++--------
>  arch/arm64/kvm/sys_regs.c      | 40 ++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 71 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/arm64/kvm/hyp/sysreg-sr.c b/arch/arm64/kvm/hyp/sysreg-sr.c
> index 906606dc4e2c..9c60b8062724 100644
> --- a/arch/arm64/kvm/hyp/sysreg-sr.c
> +++ b/arch/arm64/kvm/hyp/sysreg-sr.c
> @@ -25,8 +25,12 @@
>  /*
>   * Non-VHE: Both host and guest must save everything.
>   *
> - * VHE: Host must save tpidr*_el0, mdscr_el1, sp_el0,
> - * and guest must save everything.
> + * VHE: Host and guest must save mdscr_el1 and sp_el0 (and the PC and pstate,
> + * which are handled as part of the el2 return state) on every switch.
> + * tpidr_el0 and tpidrro_el0 only need to be switched when going

How about suspend/resume, which saves/restores both of these EL0
registers (see cpu_do_suspend)? We may not need to do anything (either
because vcpu_put will have happened, or because we'll come back
exactly where we were), but I'd like to make sure this hasn't been
overlooked.

> + * to host userspace or a different VCPU.  EL1 registers only need to be
> + * switched when potentially going to run a different VCPU.  The latter two
> + * classes are handled as part of kvm_arch_vcpu_load and kvm_arch_vcpu_put.
>   */
>  
>  static void __hyp_text __sysreg_save_common_state(struct kvm_cpu_context *ctxt)
> @@ -93,14 +97,11 @@ void __hyp_text __sysreg_save_state_nvhe(struct kvm_cpu_context *ctxt)
>  void sysreg_save_host_state_vhe(struct kvm_cpu_context *ctxt)
>  {
>  	__sysreg_save_common_state(ctxt);
> -	__sysreg_save_user_state(ctxt);
>  }
>  
>  void sysreg_save_guest_state_vhe(struct kvm_cpu_context *ctxt)
>  {
> -	__sysreg_save_el1_state(ctxt);
>  	__sysreg_save_common_state(ctxt);
> -	__sysreg_save_user_state(ctxt);
>  	__sysreg_save_el2_return_state(ctxt);
>  }
>  
> @@ -169,14 +170,11 @@ void __hyp_text __sysreg_restore_state_nvhe(struct kvm_cpu_context *ctxt)
>  void sysreg_restore_host_state_vhe(struct kvm_cpu_context *ctxt)
>  {
>  	__sysreg_restore_common_state(ctxt);
> -	__sysreg_restore_user_state(ctxt);
>  }
>  
>  void sysreg_restore_guest_state_vhe(struct kvm_cpu_context *ctxt)
>  {
> -	__sysreg_restore_el1_state(ctxt);
>  	__sysreg_restore_common_state(ctxt);
> -	__sysreg_restore_user_state(ctxt);
>  	__sysreg_restore_el2_return_state(ctxt);
>  }
>  
> @@ -240,6 +238,18 @@ void __hyp_text __sysreg32_restore_state(struct kvm_vcpu *vcpu)
>   */
>  void kvm_vcpu_load_sysregs(struct kvm_vcpu *vcpu)
>  {
> +	struct kvm_cpu_context *host_ctxt = vcpu->arch.host_cpu_context;
> +	struct kvm_cpu_context *guest_ctxt = &vcpu->arch.ctxt;
> +
> +	if (!has_vhe())
> +		return;
> +
> +	__sysreg_save_user_state(host_ctxt);
> +
> +	__sysreg_restore_user_state(guest_ctxt);
> +	__sysreg_restore_el1_state(guest_ctxt);
> +
> +	vcpu->arch.sysregs_loaded_on_cpu = true;
>  }
>  
>  /**
> @@ -255,6 +265,19 @@ void kvm_vcpu_load_sysregs(struct kvm_vcpu *vcpu)
>   */
>  void kvm_vcpu_put_sysregs(struct kvm_vcpu *vcpu)
>  {
> +	struct kvm_cpu_context *host_ctxt = vcpu->arch.host_cpu_context;
> +	struct kvm_cpu_context *guest_ctxt = &vcpu->arch.ctxt;
> +
> +	if (!has_vhe())
> +		return;
> +
> +	__sysreg_save_el1_state(guest_ctxt);
> +	__sysreg_save_user_state(guest_ctxt);
> +
> +	/* Restore host user state */
> +	__sysreg_restore_user_state(host_ctxt);
> +
> +	vcpu->arch.sysregs_loaded_on_cpu = false;
>  }
>  
>  void __hyp_text __kvm_set_tpidr_el2(u64 tpidr_el2)
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index b3c3f014aa61..f060309337aa 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -87,6 +87,26 @@ u64 vcpu_read_sys_reg(struct kvm_vcpu *vcpu, int reg)
>  	 * exit from the guest but are only saved on vcpu_put.
>  	 */
>  	switch (reg) {
> +	case CSSELR_EL1:	return read_sysreg_s(SYS_CSSELR_EL1);
> +	case SCTLR_EL1:		return read_sysreg_s(sctlr_EL12);
> +	case ACTLR_EL1:		return read_sysreg_s(SYS_ACTLR_EL1);
> +	case CPACR_EL1:		return read_sysreg_s(cpacr_EL12);
> +	case TTBR0_EL1:		return read_sysreg_s(ttbr0_EL12);
> +	case TTBR1_EL1:		return read_sysreg_s(ttbr1_EL12);
> +	case TCR_EL1:		return read_sysreg_s(tcr_EL12);
> +	case ESR_EL1:		return read_sysreg_s(esr_EL12);
> +	case AFSR0_EL1:		return read_sysreg_s(afsr0_EL12);
> +	case AFSR1_EL1:		return read_sysreg_s(afsr1_EL12);
> +	case FAR_EL1:		return read_sysreg_s(far_EL12);
> +	case MAIR_EL1:		return read_sysreg_s(mair_EL12);
> +	case VBAR_EL1:		return read_sysreg_s(vbar_EL12);
> +	case CONTEXTIDR_EL1:	return read_sysreg_s(contextidr_EL12);
> +	case TPIDR_EL0:		return read_sysreg_s(SYS_TPIDR_EL0);
> +	case TPIDRRO_EL0:	return read_sysreg_s(SYS_TPIDRRO_EL0);
> +	case TPIDR_EL1:		return read_sysreg_s(SYS_TPIDR_EL1);
> +	case AMAIR_EL1:		return read_sysreg_s(amair_EL12);
> +	case CNTKCTL_EL1:	return read_sysreg_s(cntkctl_EL12);
> +	case PAR_EL1:		return read_sysreg_s(SYS_PAR_EL1);
>  	}
>  
>  immediate_read:
> @@ -103,6 +123,26 @@ void vcpu_write_sys_reg(struct kvm_vcpu *vcpu, int reg, u64 val)
>  	 * entry to the guest but are only restored on vcpu_load.
>  	 */
>  	switch (reg) {
> +	case CSSELR_EL1:	write_sysreg_s(val, SYS_CSSELR_EL1);	return;
> +	case SCTLR_EL1:		write_sysreg_s(val, sctlr_EL12);	return;
> +	case ACTLR_EL1:		write_sysreg_s(val, SYS_ACTLR_EL1);	return;
> +	case CPACR_EL1:		write_sysreg_s(val, cpacr_EL12);	return;
> +	case TTBR0_EL1:		write_sysreg_s(val, ttbr0_EL12);	return;
> +	case TTBR1_EL1:		write_sysreg_s(val, ttbr1_EL12);	return;
> +	case TCR_EL1:		write_sysreg_s(val, tcr_EL12);		return;
> +	case ESR_EL1:		write_sysreg_s(val, esr_EL12);		return;
> +	case AFSR0_EL1:		write_sysreg_s(val, afsr0_EL12);	return;
> +	case AFSR1_EL1:		write_sysreg_s(val, afsr1_EL12);	return;
> +	case FAR_EL1:		write_sysreg_s(val, far_EL12);		return;
> +	case MAIR_EL1:		write_sysreg_s(val, mair_EL12);		return;
> +	case VBAR_EL1:		write_sysreg_s(val, vbar_EL12);		return;
> +	case CONTEXTIDR_EL1:	write_sysreg_s(val, contextidr_EL12);	return;
> +	case TPIDR_EL0:		write_sysreg_s(val, SYS_TPIDR_EL0);	return;
> +	case TPIDRRO_EL0:	write_sysreg_s(val, SYS_TPIDRRO_EL0);	return;
> +	case TPIDR_EL1:		write_sysreg_s(val, SYS_TPIDR_EL1);	return;
> +	case AMAIR_EL1:		write_sysreg_s(val, amair_EL12);	return;
> +	case CNTKCTL_EL1:	write_sysreg_s(val, cntkctl_EL12);	return;
> +	case PAR_EL1:		write_sysreg_s(val, SYS_PAR_EL1);	return;
>  	}
>  
>  immediate_write:
> -- 
> 2.14.2
> 

Looks good to me otherwise.

	M.

-- 
Jazz is not dead, it just smell funny.