Re: [PATCH v2 09/18] arm64: KVM: enable conditional save/restore full SPE profiling buffer controls

Marc Zyngier
2019-12-21T14:13:32Z

On Fri, 20 Dec 2019 14:30:16 +0000
Andrew Murray <andrew.murray@arm.com> wrote:

[somehow managed not to do a reply all, re-sending]

> From: Sudeep Holla <sudeep.holla@arm.com>
>
> Now that we can save/restore the full SPE controls, we can enable it
> if SPE is setup and ready to use in KVM. It's supported in KVM only if
> all the CPUs in the system supports SPE.
>
> However to support heterogenous systems, we need to move the check if
> host supports SPE and do a partial save/restore.

No. Let's just not go down that path. For now, KVM on heterogeneous
systems do not get SPE. If SPE has been enabled on a guest and a CPU
comes up without SPE, this CPU should fail to boot (same as exposing a
feature to userspace).

>
> Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
> Signed-off-by: Andrew Murray <andrew.murray@arm.com>
> ---
> arch/arm64/kvm/hyp/debug-sr.c | 33 ++++++++++++++++-----------------
> include/kvm/arm_spe.h | 6 ++++++
> 2 files changed, 22 insertions(+), 17 deletions(-)
>
> diff --git a/arch/arm64/kvm/hyp/debug-sr.c b/arch/arm64/kvm/hyp/debug-sr.c
> index 12429b212a3a..d8d857067e6d 100644
> --- a/arch/arm64/kvm/hyp/debug-sr.c
> +++ b/arch/arm64/kvm/hyp/debug-sr.c
> @@ -86,18 +86,13 @@
> }
>
> static void __hyp_text
> -__debug_save_spe_nvhe(struct kvm_cpu_context *ctxt, bool full_ctxt)
> +__debug_save_spe_context(struct kvm_cpu_context *ctxt, bool full_ctxt)
> {
> u64 reg;
>
> /* Clear pmscr in case of early return */
> ctxt->sys_regs[PMSCR_EL1] = 0;
>
> - /* 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 higher EL? */
> reg = read_sysreg_s(SYS_PMBIDR_EL1);
> if (reg & BIT(SYS_PMBIDR_EL1_P_SHIFT))
> @@ -142,7 +137,7 @@ __debug_save_spe_nvhe(struct kvm_cpu_context *ctxt, bool full_ctxt)
> }
>
> static void __hyp_text
> -__debug_restore_spe_nvhe(struct kvm_cpu_context *ctxt, bool full_ctxt)
> +__debug_restore_spe_context(struct kvm_cpu_context *ctxt, bool full_ctxt)
> {
> if (!ctxt->sys_regs[PMSCR_EL1])
> return;
> @@ -210,11 +205,14 @@ void __hyp_text __debug_restore_guest_context(struct kvm_vcpu *vcpu)
> struct kvm_guest_debug_arch *host_dbg;
> struct kvm_guest_debug_arch *guest_dbg;
>
> + host_ctxt = kern_hyp_va(vcpu->arch.host_cpu_context);
> + guest_ctxt = &vcpu->arch.ctxt;
> +
> + __debug_restore_spe_context(guest_ctxt, kvm_arm_spe_v1_ready(vcpu));
> +
> if (!(vcpu->arch.flags & KVM_ARM64_DEBUG_DIRTY))
> return;
>
> - host_ctxt = kern_hyp_va(vcpu->arch.host_cpu_context);
> - guest_ctxt = &vcpu->arch.ctxt;
> host_dbg = &vcpu->arch.host_debug_state.regs;
> guest_dbg = kern_hyp_va(vcpu->arch.debug_ptr);
>
> @@ -232,8 +230,7 @@ void __hyp_text __debug_restore_host_context(struct kvm_vcpu *vcpu)
> host_ctxt = kern_hyp_va(vcpu->arch.host_cpu_context);
> guest_ctxt = &vcpu->arch.ctxt;
>
> - if (!has_vhe())
> - __debug_restore_spe_nvhe(host_ctxt, false);
> + __debug_restore_spe_context(host_ctxt, kvm_arm_spe_v1_ready(vcpu));

So you now do an unconditional save/restore on the exit path for VHE as
well? Even if the host isn't using the SPE HW? That's not acceptable
as, in most cases, only the host /or/ the guest will use SPE. Here, you
put a measurable overhead on each exit.

If the host is not using SPE, then the restore/save should happen in
vcpu_load/vcpu_put. Only if the host is using SPE should you do
something in the run loop. Of course, this only applies to VHE and
non-VHE must switch eagerly.

>
> if (!(vcpu->arch.flags & KVM_ARM64_DEBUG_DIRTY))
> return;
> @@ -249,19 +246,21 @@ void __hyp_text __debug_restore_host_context(struct kvm_vcpu *vcpu)
>
> void __hyp_text __debug_save_host_context(struct kvm_vcpu *vcpu)
> {
> - /*
> - * Non-VHE: Disable and flush SPE data generation
> - * VHE: The vcpu can run, but it can't hide.
> - */
> struct kvm_cpu_context *host_ctxt;
>
> host_ctxt = kern_hyp_va(vcpu->arch.host_cpu_context);
> - if (!has_vhe())
> - __debug_save_spe_nvhe(host_ctxt, false);
> + if (cpuid_feature_extract_unsigned_field(read_sysreg(id_aa64dfr0_el1),
> + ID_AA64DFR0_PMSVER_SHIFT))
> + __debug_save_spe_context(host_ctxt, kvm_arm_spe_v1_ready(vcpu));
> }
>
> void __hyp_text __debug_save_guest_context(struct kvm_vcpu *vcpu)
> {
> + bool kvm_spe_ready = kvm_arm_spe_v1_ready(vcpu);
> +
> + /* SPE present on this vCPU? */
> + if (kvm_spe_ready)
> + __debug_save_spe_context(&vcpu->arch.ctxt, kvm_spe_ready);
> }
>
> u32 __hyp_text __kvm_get_mdcr_el2(void)
> diff --git a/include/kvm/arm_spe.h b/include/kvm/arm_spe.h
> index 48d118fdb174..30c40b1bc385 100644
> --- a/include/kvm/arm_spe.h
> +++ b/include/kvm/arm_spe.h
> @@ -16,4 +16,10 @@ struct kvm_spe {
> bool irq_level;
> };
>
> +#ifdef CONFIG_KVM_ARM_SPE
> +#define kvm_arm_spe_v1_ready(v) ((v)->arch.spe.ready)
> +#else
> +#define kvm_arm_spe_v1_ready(v) (false)
> +#endif /* CONFIG_KVM_ARM_SPE */
> +
> #endif /* __ASM_ARM_KVM_SPE_H */

Thanks,

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