Re: [PATCH v6 13/38] KVM: arm64: Enable access to sanitized CPU features at EL2

[Marc Zyngier <maz@kernel.org>]

Hi Quentin,

On Fri, 19 Mar 2021 10:01:21 +0000,
Quentin Perret <qperret@google.com> wrote:
> 
> Introduce the infrastructure in KVM enabling to copy CPU feature
> registers into EL2-owned data-structures, to allow reading sanitised
> values directly at EL2 in nVHE.
> 
> Given that only a subset of these features are being read by the
> hypervisor, the ones that need to be copied are to be listed under
> <asm/kvm_cpufeature.h> together with the name of the nVHE variable that
> will hold the copy. This introduces only the infrastructure enabling
> this copy. The first users will follow shortly.
> 
> Signed-off-by: Quentin Perret <qperret@google.com>
> ---
>  arch/arm64/include/asm/cpufeature.h     |  1 +
>  arch/arm64/include/asm/kvm_cpufeature.h | 22 ++++++++++++++++++++++
>  arch/arm64/include/asm/kvm_host.h       |  4 ++++
>  arch/arm64/kernel/cpufeature.c          | 13 +++++++++++++
>  arch/arm64/kvm/sys_regs.c               | 19 +++++++++++++++++++
>  5 files changed, 59 insertions(+)
>  create mode 100644 arch/arm64/include/asm/kvm_cpufeature.h
> 
> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> index 61177bac49fa..a85cea2cac57 100644
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -607,6 +607,7 @@ void check_local_cpu_capabilities(void);
>  
>  u64 read_sanitised_ftr_reg(u32 id);
>  u64 __read_sysreg_by_encoding(u32 sys_id);
> +int copy_ftr_reg(u32 id, struct arm64_ftr_reg *dst);
>  
>  static inline bool cpu_supports_mixed_endian_el0(void)
>  {
> diff --git a/arch/arm64/include/asm/kvm_cpufeature.h b/arch/arm64/include/asm/kvm_cpufeature.h
> new file mode 100644
> index 000000000000..3d245f96a9fe
> --- /dev/null
> +++ b/arch/arm64/include/asm/kvm_cpufeature.h
> @@ -0,0 +1,22 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +/*
> + * Copyright (C) 2020 - Google LLC
> + * Author: Quentin Perret <qperret@google.com>
> + */
> +
> +#ifndef __ARM64_KVM_CPUFEATURE_H__
> +#define __ARM64_KVM_CPUFEATURE_H__
> +
> +#include <asm/cpufeature.h>
> +
> +#include <linux/build_bug.h>
> +
> +#if defined(__KVM_NVHE_HYPERVISOR__)
> +#define DECLARE_KVM_HYP_CPU_FTR_REG(name) extern struct arm64_ftr_reg name
> +#define DEFINE_KVM_HYP_CPU_FTR_REG(name) struct arm64_ftr_reg name
> +#else
> +#define DECLARE_KVM_HYP_CPU_FTR_REG(name) extern struct arm64_ftr_reg kvm_nvhe_sym(name)
> +#define DEFINE_KVM_HYP_CPU_FTR_REG(name) BUILD_BUG()
> +#endif
> +
> +#endif
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 6a2031af9562..02e172dc5087 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -740,9 +740,13 @@ void kvm_clr_pmu_events(u32 clr);
>  
>  void kvm_vcpu_pmu_restore_guest(struct kvm_vcpu *vcpu);
>  void kvm_vcpu_pmu_restore_host(struct kvm_vcpu *vcpu);
> +
> +void setup_kvm_el2_caps(void);
>  #else
>  static inline void kvm_set_pmu_events(u32 set, struct perf_event_attr *attr) {}
>  static inline void kvm_clr_pmu_events(u32 clr) {}
> +
> +static inline void setup_kvm_el2_caps(void) {}
>  #endif
>  
>  void kvm_vcpu_load_sysregs_vhe(struct kvm_vcpu *vcpu);
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 066030717a4c..6252476e4e73 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -1154,6 +1154,18 @@ u64 read_sanitised_ftr_reg(u32 id)
>  }
>  EXPORT_SYMBOL_GPL(read_sanitised_ftr_reg);
>  
> +int copy_ftr_reg(u32 id, struct arm64_ftr_reg *dst)
> +{
> +	struct arm64_ftr_reg *regp = get_arm64_ftr_reg(id);
> +
> +	if (!regp)
> +		return -EINVAL;
> +
> +	*dst = *regp;
> +
> +	return 0;
> +}
> +
>  #define read_sysreg_case(r)	\
>  	case r:		val = read_sysreg_s(r); break;
>  
> @@ -2773,6 +2785,7 @@ void __init setup_cpu_features(void)
>  
>  	setup_system_capabilities();
>  	setup_elf_hwcaps(arm64_elf_hwcaps);
> +	setup_kvm_el2_caps();
>  
>  	if (system_supports_32bit_el0())
>  		setup_elf_hwcaps(compat_elf_hwcaps);
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 4f2f1e3145de..6c5d133689ae 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -21,6 +21,7 @@
>  #include <asm/debug-monitors.h>
>  #include <asm/esr.h>
>  #include <asm/kvm_arm.h>
> +#include <asm/kvm_cpufeature.h>
>  #include <asm/kvm_emulate.h>
>  #include <asm/kvm_hyp.h>
>  #include <asm/kvm_mmu.h>
> @@ -2775,3 +2776,21 @@ void kvm_sys_reg_table_init(void)
>  	/* Clear all higher bits. */
>  	cache_levels &= (1 << (i*3))-1;
>  }
> +
> +#define CPU_FTR_REG_HYP_COPY(id, name) \
> +	{ .sys_id = id, .dst = (struct arm64_ftr_reg *)&kvm_nvhe_sym(name) }
> +struct __ftr_reg_copy_entry {
> +	u32			sys_id;
> +	struct arm64_ftr_reg	*dst;
> +} hyp_ftr_regs[] __initdata = {
> +};
> +
> +void __init setup_kvm_el2_caps(void)
> +{
> +	int i;
> +
> +	for (i = 0; i < ARRAY_SIZE(hyp_ftr_regs); i++) {
> +		WARN(copy_ftr_reg(hyp_ftr_regs[i].sys_id, hyp_ftr_regs[i].dst),
> +		     "%u feature register not found\n", hyp_ftr_regs[i].sys_id);
> +	}
> +}
> -- 
> 2.31.0.rc2.261.g7f71774620-goog
> 
> 

I can't say I'm thrilled with this. Actually, it is fair to say that I
don't like it at all! ;-) Copying whole structures with pointers that
make no sense at EL2 feels... wrong.

As we discussed offline, the main reason for this infrastructure is
that the read_ctr macro directly uses arm64_ftr_reg_ctrel0.sys_val
when ARM64_MISMATCHED_CACHE_TYPE is set.

One thing to realise is that with the protected mode, we can rely on
patching as there is no such thing as a "late" CPU. So by specialising
read_ctr when compiled for nVHE, we can just make it give us the final
value, provided that KVM's own __flush_dcache_area() is limited to
protected mode.

Once this problem is solved, this whole patch can mostly go, as we are
left with exactly *two* u64 quantities to be populated, something that
we can probably do in kvm_sys_reg_table_init().

I'll post some patches later today to try and explain what I have in
mind.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.
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