Re: [RFC PATCH v2 03/10] arm64: KVM: Save/restore the host SPE state when entering/leaving a VM

From: Marc Zyngier
Date: Mon Jan 16 2017 - 06:25:46 EST

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Hi Will,

On 13/01/17 16:03, Will Deacon wrote:
> The SPE buffer is virtually addressed, using the page tables of the CPU
> MMU. Unusually, this means that the EL0/1 page table may be live whilst
> we're executing at EL2 on non-VHE configurations. When VHE is in use,
> we can use the same property to profile the guest behind its back.
>
> This patch adds the relevant disabling and flushing code to KVM so that
> the host can make use of SPE without corrupting guest memory, and any
> attempts by a guest to use SPE will result in a trap.
>
> Cc: Marc Zyngier <marc.zyngier@xxxxxxx>
> Cc: Alex BennÃe <alex.bennee@xxxxxxxxxx>
> Cc: Christoffer Dall <christoffer.dall@xxxxxxxxxx>
> Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
> ---
> arch/arm64/include/asm/kvm_arm.h | 3 ++
> arch/arm64/include/asm/kvm_host.h | 7 ++++-
> arch/arm64/kvm/debug.c | 6 ++++
> arch/arm64/kvm/hyp/debug-sr.c | 66 +++++++++++++++++++++++++++++++++++++--
> arch/arm64/kvm/hyp/switch.c | 13 +++++++-
> 5 files changed, 91 insertions(+), 4 deletions(-)
>
> diff --git a/arch/arm64/include/asm/kvm_arm.h b/arch/arm64/include/asm/kvm_arm.h
> index 2a2752b5b6aa..6e99978e83bd 100644
> --- a/arch/arm64/include/asm/kvm_arm.h
> +++ b/arch/arm64/include/asm/kvm_arm.h
> @@ -188,6 +188,9 @@
> #define CPTR_EL2_DEFAULT 0x000033ff
>
> /* Hyp Debug Configuration Register bits */
> +#define MDCR_EL2_TPMS (1 << 14)
> +#define MDCR_EL2_E2PB_MASK (UL(0x3))
> +#define MDCR_EL2_E2PB_SHIFT (UL(12))
> #define MDCR_EL2_TDRA (1 << 11)
> #define MDCR_EL2_TDOSA (1 << 10)
> #define MDCR_EL2_TDA (1 << 9)
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index e5050388e062..443b387021f2 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -229,7 +229,12 @@ struct kvm_vcpu_arch {
>
> /* Pointer to host CPU context */
> kvm_cpu_context_t *host_cpu_context;
> - struct kvm_guest_debug_arch host_debug_state;
> + struct {
> + /* {Break,watch}point registers */
> + struct kvm_guest_debug_arch regs;
> + /* Statistical profiling extension */
> + u64 pmscr_el1;
> + } host_debug_state;
>
> /* VGIC state */
> struct vgic_cpu vgic_cpu;
> diff --git a/arch/arm64/kvm/debug.c b/arch/arm64/kvm/debug.c
> index 47e5f0feaee8..dbadfaf850a7 100644
> --- a/arch/arm64/kvm/debug.c
> +++ b/arch/arm64/kvm/debug.c
> @@ -95,6 +95,7 @@ void kvm_arm_reset_debug_ptr(struct kvm_vcpu *vcpu)
> * - Performance monitors (MDCR_EL2_TPM/MDCR_EL2_TPMCR)
> * - Debug ROM Address (MDCR_EL2_TDRA)
> * - OS related registers (MDCR_EL2_TDOSA)
> + * - Statistical profiler (MDCR_EL2_TPMS/MDCR_EL2_E2PB)
> *
> * Additionally, KVM only traps guest accesses to the debug registers if
> * the guest is not actively using them (see the KVM_ARM64_DEBUG_DIRTY
> @@ -110,8 +111,13 @@ void kvm_arm_setup_debug(struct kvm_vcpu *vcpu)
>
> trace_kvm_arm_setup_debug(vcpu, vcpu->guest_debug);
>
> + /*
> + * This also clears MDCR_EL2_E2PB_MASK to disable guest access
> + * to the profiling buffer.
> + */
> vcpu->arch.mdcr_el2 = __this_cpu_read(mdcr_el2) & MDCR_EL2_HPMN_MASK;
> vcpu->arch.mdcr_el2 |= (MDCR_EL2_TPM |
> + MDCR_EL2_TPMS |

This is RES0 on CPUs that do not implement SPE. Can we make this
conditional on SPE being available?

> MDCR_EL2_TPMCR |
> MDCR_EL2_TDRA |
> MDCR_EL2_TDOSA);
> diff --git a/arch/arm64/kvm/hyp/debug-sr.c b/arch/arm64/kvm/hyp/debug-sr.c
> index 4ba5c9095d03..f5154ed3da6c 100644
> --- a/arch/arm64/kvm/hyp/debug-sr.c
> +++ b/arch/arm64/kvm/hyp/debug-sr.c
> @@ -65,6 +65,66 @@
> default: write_debug(ptr[0], reg, 0); \
> }
>
> +#define PMSCR_EL1 sys_reg(3, 0, 9, 9, 0)
> +
> +#define PMBLIMITR_EL1 sys_reg(3, 0, 9, 10, 0)
> +#define PMBLIMITR_EL1_E BIT(0)
> +
> +#define PMBIDR_EL1 sys_reg(3, 0, 9, 10, 7)
> +#define PMBIDR_EL1_P BIT(4)
> +
> +#define psb_csync() asm volatile("hint #17")

Cute... ;-)

> +
> +static void __hyp_text __debug_save_spe_vhe(u64 *pmscr_el1)
> +{
> + /* The vcpu can run. but it can't hide. */
> +}
> +
> +static void __hyp_text __debug_save_spe_nvhe(u64 *pmscr_el1)
> +{
> + u64 reg;
> +
> + /* SPE present on this CPU? */
> + if (!cpuid_feature_extract_unsigned_field(read_sysreg(id_aa64dfr0_el1),
> + ID_AA64DFR0_PMSVER_SHIFT))
> + return;
> +
> + /* Yes; is it owned by EL3? */
> + reg = read_sysreg_s(PMBIDR_EL1);
> + if (reg & PMBIDR_EL1_P)
> + return;
> +
> + /* No; is the host actually using the thing? */
> + reg = read_sysreg_s(PMBLIMITR_EL1);
> + if (!(reg & PMBLIMITR_EL1_E))
> + return;
> +
> + /* Yes; save the control register and disable data generation */
> + *pmscr_el1 = read_sysreg_s(PMSCR_EL1);
> + write_sysreg_s(0, PMSCR_EL1);
> + isb();
> +
> + /* Now drain all buffered data to memory */
> + psb_csync();
> + dsb(nsh);

So let's hope that nobody will build a non-VHE, SPE capable system.
Because this feels like hitting the brakes each time we enter the guest
(if SPE is in use, that is...).

> +}
> +
> +static hyp_alternate_select(__debug_save_spe,
> + __debug_save_spe_nvhe, __debug_save_spe_vhe,
> + ARM64_HAS_VIRT_HOST_EXTN);
> +
> +static void __hyp_text __debug_restore_spe(u64 pmscr_el1)
> +{
> + if (!pmscr_el1)
> + return;
> +
> + /* The host page table is installed, but not yet synchronised */
> + isb();

I believe you can avoid that on VHE (host page tables haven't changed).
Also, if you now depend on the debug restore being run after the rest of
the sysregs, please add a comment in switch.c::__kvm_vcpu_run so that we
remember about that dependency.

> +
> + /* Re-enable data generation */
> + write_sysreg_s(pmscr_el1, PMSCR_EL1);

Same thing here, as we never disabled SPE the first place on VHE (and
we're probably only writing zeroes there).

> +}
> +
> void __hyp_text __debug_save_state(struct kvm_vcpu *vcpu,
> struct kvm_guest_debug_arch *dbg,
> struct kvm_cpu_context *ctxt)
> @@ -118,13 +178,15 @@ void __hyp_text __debug_cond_save_host_state(struct kvm_vcpu *vcpu)
> (vcpu->arch.ctxt.sys_regs[MDSCR_EL1] & DBG_MDSCR_MDE))
> vcpu->arch.debug_flags |= KVM_ARM64_DEBUG_DIRTY;
>
> - __debug_save_state(vcpu, &vcpu->arch.host_debug_state,
> + __debug_save_state(vcpu, &vcpu->arch.host_debug_state.regs,
> kern_hyp_va(vcpu->arch.host_cpu_context));
> + __debug_save_spe()(&vcpu->arch.host_debug_state.pmscr_el1);
> }
>
> void __hyp_text __debug_cond_restore_host_state(struct kvm_vcpu *vcpu)
> {
> - __debug_restore_state(vcpu, &vcpu->arch.host_debug_state,
> + __debug_restore_spe(vcpu->arch.host_debug_state.pmscr_el1);
> + __debug_restore_state(vcpu, &vcpu->arch.host_debug_state.regs,
> kern_hyp_va(vcpu->arch.host_cpu_context));
>
> if (vcpu->arch.debug_flags & KVM_ARM64_DEBUG_DIRTY)
> diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
> index 75e83dd40d43..1ee5b06d81e5 100644
> --- a/arch/arm64/kvm/hyp/switch.c
> +++ b/arch/arm64/kvm/hyp/switch.c
> @@ -103,7 +103,13 @@ static void __hyp_text __activate_traps(struct kvm_vcpu *vcpu)
> static void __hyp_text __deactivate_traps_vhe(void)
> {
> extern char vectors[]; /* kernel exception vectors */
> + u64 mdcr_el2 = read_sysreg(mdcr_el2);
>
> + mdcr_el2 &= MDCR_EL2_HPMN_MASK |
> + MDCR_EL2_E2PB_MASK << MDCR_EL2_E2PB_SHIFT |
> + MDCR_EL2_TPMS;
> +
> + write_sysreg(mdcr_el2, mdcr_el2);
> write_sysreg(HCR_HOST_VHE_FLAGS, hcr_el2);
> write_sysreg(CPACR_EL1_FPEN, cpacr_el1);
> write_sysreg(vectors, vbar_el1);
> @@ -111,6 +117,12 @@ static void __hyp_text __deactivate_traps_vhe(void)
>
> static void __hyp_text __deactivate_traps_nvhe(void)
> {
> + u64 mdcr_el2 = read_sysreg(mdcr_el2);
> +
> + mdcr_el2 &= MDCR_EL2_HPMN_MASK;
> + mdcr_el2 |= MDCR_EL2_E2PB_MASK << MDCR_EL2_E2PB_SHIFT;

Same remark as above. I'm not completely confident writing new bits to
registers that do not necessarily implement the feature.

> +
> + write_sysreg(mdcr_el2, mdcr_el2);
> write_sysreg(HCR_RW, hcr_el2);
> write_sysreg(CPTR_EL2_DEFAULT, cptr_el2);
> }
> @@ -132,7 +144,6 @@ static void __hyp_text __deactivate_traps(struct kvm_vcpu *vcpu)
>
> __deactivate_traps_arch()();
> write_sysreg(0, hstr_el2);
> - write_sysreg(read_sysreg(mdcr_el2) & MDCR_EL2_HPMN_MASK, mdcr_el2);
> write_sysreg(0, pmuserenr_el0);
> }
>
>

Thanks,

M.
--
Jazz is not dead. It just smells funny...

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Next in thread: Will Deacon: "Re: [RFC PATCH v2 03/10] arm64: KVM: Save/restore the host SPE state when entering/leaving a VM"
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