Re: [PATCH v10 14/18] KVM: arm64: Save host SVE context as appropriate

From: "Alex Bennée" <alex.bennee@linaro.org>
To: Dave Martin <Dave.Martin@arm.com>
Cc: Christoffer Dall <cdall@kernel.org>,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	Marc Zyngier <marc.zyngier@arm.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	kvmarm@lists.cs.columbia.edu,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v10 14/18] KVM: arm64: Save host SVE context as appropriate
Date: Thu, 24 May 2018 15:49:51 +0100	[thread overview]
Message-ID: <871se1t8fk.fsf@linaro.org> (raw)
In-Reply-To: <1527005119-6842-15-git-send-email-Dave.Martin@arm.com>

Dave Martin <Dave.Martin@arm.com> writes:

> This patch adds SVE context saving to the hyp FPSIMD context switch
> path.  This means that it is no longer necessary to save the host
> SVE state in advance of entering the guest, when in use.
>
> In order to avoid adding pointless complexity to the code, VHE is
> assumed if SVE is in use.  VHE is an architectural prerequisite for
> SVE, so there is no good reason to turn CONFIG_ARM64_VHE off in
> kernels that support both SVE and KVM.
>
> Historically, software models exist that can expose the
> architecturally invalid configuration of SVE without VHE, so if
> this situation is detected at kvm_init() time then KVM will be
> disabled.
>
> Signed-off-by: Dave Martin <Dave.Martin@arm.com>
>
> ---
>
>  * Tags stripped since v8, please reconfirm if possible:
>
> Formerly-Reviewed-by: Christoffer Dall <christoffer.dall at arm.com>
> Formerly-Acked-by: Marc Zyngier <marc.zyngier at arm.com>
> Formerly-Acked-by: Catalin Marinas <catalin.marinas at arm.com>
>
> Changes since v9:
>
> Requested by Marc Zyngier:
>
>  * Inline check for VHE if SVE is present into kvm_host.h.
>
>    The check has been renamed to the more specific
>    kvm_arch_check_sve_has_vhe(), and the kvm_pr_unimpl() moved back to
>    arm.c (to avoid circular include issues).
>
>    arm.c is not single-arch code, but it is all Arm-specific, so
>    adding a hook like this doesn't seem too unreasonable.
>
> Changes since v8:
>
>  * Add kvm_arch_check_supported() hook, and move arm64-specific check
>    for SVE-implies-VHE into arch/arm64/.
>
>    Due to circular header dependency problems, it is difficult to get
>    the prototype for kvm_pr_*() functions in <asm/kvm_host.h>, so this
>    patch puts arm64's kvm_arch_check_supported() hook out of line.
>    This is not a hot function.
> ---
>  arch/arm/include/asm/kvm_host.h   |  1 +
>  arch/arm64/Kconfig                |  7 +++++++
>  arch/arm64/include/asm/kvm_host.h | 13 +++++++++++++
>  arch/arm64/kvm/fpsimd.c           |  1 -
>  arch/arm64/kvm/hyp/switch.c       | 20 +++++++++++++++++++-
>  virt/kvm/arm/arm.c                |  7 +++++++
>  6 files changed, 47 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h
> index ac870b2..3b85bbb 100644
> --- a/arch/arm/include/asm/kvm_host.h
> +++ b/arch/arm/include/asm/kvm_host.h
> @@ -280,6 +280,7 @@ void kvm_mmu_wp_memory_region(struct kvm *kvm, int slot);
>
>  struct kvm_vcpu *kvm_mpidr_to_vcpu(struct kvm *kvm, unsigned long mpidr);
>
> +static inline bool kvm_arch_check_sve_has_vhe(void) { return true; }
>  static inline void kvm_arch_hardware_unsetup(void) {}
>  static inline void kvm_arch_sync_events(struct kvm *kvm) {}
>  static inline void kvm_arch_vcpu_uninit(struct kvm_vcpu *vcpu) {}
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index eb2cf49..b0d3820 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -1130,6 +1130,7 @@ endmenu
>  config ARM64_SVE
>  	bool "ARM Scalable Vector Extension support"
>  	default y
> +	depends on !KVM || ARM64_VHE
>  	help
>  	  The Scalable Vector Extension (SVE) is an extension to the AArch64
>  	  execution state which complements and extends the SIMD functionality
> @@ -1155,6 +1156,12 @@ config ARM64_SVE
>  	  booting the kernel.  If unsure and you are not observing these
>  	  symptoms, you should assume that it is safe to say Y.
>
> +	  CPUs that support SVE are architecturally required to support the
> +	  Virtualization Host Extensions (VHE), so the kernel makes no
> +	  provision for supporting SVE alongside KVM without VHE enabled.
> +	  Thus, you will need to enable CONFIG_ARM64_VHE if you want to support
> +	  KVM in the same kernel image.
> +
>  config ARM64_MODULE_PLTS
>  	bool
>  	select HAVE_MOD_ARCH_SPECIFIC
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index b3fe730..06d5a61 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -405,6 +405,19 @@ static inline void __cpu_init_hyp_mode(phys_addr_t pgd_ptr,
>  	kvm_call_hyp(__kvm_set_tpidr_el2, tpidr_el2);
>  }
>
> +static inline bool kvm_arch_check_sve_has_vhe(void)
> +{
> +	/*
> +	 * The Arm architecture specifies that imlpementation of SVE
> +	 * requires VHE also to be implemented.  The KVM code for arm64
> +	 * relies on this when SVE is present:
> +	 */
> +	if (system_supports_sve())
> +		return has_vhe();
> +	else
> +		return true;
> +}
> +
>  static inline void kvm_arch_hardware_unsetup(void) {}
>  static inline void kvm_arch_sync_events(struct kvm *kvm) {}
>  static inline void kvm_arch_vcpu_uninit(struct kvm_vcpu *vcpu) {}
> diff --git a/arch/arm64/kvm/fpsimd.c b/arch/arm64/kvm/fpsimd.c
> index 365933a..dc6ecfa 100644
> --- a/arch/arm64/kvm/fpsimd.c
> +++ b/arch/arm64/kvm/fpsimd.c
> @@ -59,7 +59,6 @@ int kvm_arch_vcpu_run_map_fp(struct kvm_vcpu *vcpu)
>   */
>  void kvm_arch_vcpu_load_fp(struct kvm_vcpu *vcpu)
>  {
> -	BUG_ON(system_supports_sve());
>  	BUG_ON(!current->mm);
>
>  	vcpu->arch.flags &= ~(KVM_ARM64_FP_ENABLED | KVM_ARM64_HOST_SVE_IN_USE);
> diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
> index 118f300..a6a8c7d 100644
> --- a/arch/arm64/kvm/hyp/switch.c
> +++ b/arch/arm64/kvm/hyp/switch.c
> @@ -21,6 +21,7 @@
>
>  #include <kvm/arm_psci.h>
>
> +#include <asm/cpufeature.h>
>  #include <asm/kvm_asm.h>
>  #include <asm/kvm_emulate.h>
>  #include <asm/kvm_host.h>
> @@ -28,6 +29,7 @@
>  #include <asm/kvm_mmu.h>
>  #include <asm/fpsimd.h>
>  #include <asm/debug-monitors.h>
> +#include <asm/processor.h>
>  #include <asm/thread_info.h>
>
>  /* Check whether the FP regs were dirtied while in the host-side run loop: */
> @@ -329,6 +331,8 @@ static bool __hyp_text __skip_instr(struct kvm_vcpu *vcpu)
>  void __hyp_text __hyp_switch_fpsimd(u64 esr __always_unused,
>  				    struct kvm_vcpu *vcpu)
>  {
> +	struct user_fpsimd_state *host_fpsimd = vcpu->arch.host_fpsimd_state;
> +
>  	if (has_vhe())
>  		write_sysreg(read_sysreg(cpacr_el1) | CPACR_EL1_FPEN,
>  			     cpacr_el1);
> @@ -339,7 +343,21 @@ void __hyp_text __hyp_switch_fpsimd(u64 esr __always_unused,
>  	isb();
>
>  	if (vcpu->arch.flags & KVM_ARM64_FP_HOST) {
> -		__fpsimd_save_state(vcpu->arch.host_fpsimd_state);
> +		/*
> +		 * In the SVE case, VHE is assumed: it is enforced by
> +		 * Kconfig and kvm_arch_init().
> +		 */
> +		if (system_supports_sve() &&
> +		    (vcpu->arch.flags & KVM_ARM64_HOST_SVE_IN_USE)) {
> +			struct thread_struct *thread = container_of(
> +				host_fpsimd,
> +				struct thread_struct, uw.fpsimd_state);
> +
> +			sve_save_state(sve_pffr(thread), &host_fpsimd->fpsr);
> +		} else {
> +			__fpsimd_save_state(host_fpsimd);
> +		}
> +
>  		vcpu->arch.flags &= ~KVM_ARM64_FP_HOST;
>  	}
>
> diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
> index bee226c..ce7c6f3 100644
> --- a/virt/kvm/arm/arm.c
> +++ b/virt/kvm/arm/arm.c
> @@ -16,6 +16,7 @@
>   * Foundation, 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
>   */
>
> +#include <linux/bug.h>
>  #include <linux/cpu_pm.h>
>  #include <linux/errno.h>
>  #include <linux/err.h>
> @@ -41,6 +42,7 @@
>  #include <asm/mman.h>
>  #include <asm/tlbflush.h>
>  #include <asm/cacheflush.h>
> +#include <asm/cpufeature.h>
>  #include <asm/virt.h>
>  #include <asm/kvm_arm.h>
>  #include <asm/kvm_asm.h>
> @@ -1574,6 +1576,11 @@ int kvm_arch_init(void *opaque)
>  		return -ENODEV;
>  	}
>
> +	if (!kvm_arch_check_sve_has_vhe()) {
> +		kvm_pr_unimpl("SVE system without VHE unsupported.  Broken cpu?");
> +		return -ENODEV;
> +	}
> +

Ahh this is going to be a pain when people want to enable system
emulation for SVE in QEMU given our patchy feature implementation (i.e.
we haven't done VHE yet). However that's totally our problem not yours
;-)

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>

>  	for_each_online_cpu(cpu) {
>  		smp_call_function_single(cpu, check_kvm_target_cpu, &ret, 1);
>  		if (ret < 0) {

--
Alex Bennée

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