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

[Andrew Murray]

On Sat, Dec 21, 2019 at 02:13:25PM +0000, Marc Zyngier wrote:
> On Fri, 20 Dec 2019 14:30:16 +0000
> Andrew Murray <andrew.murray@xxxxxxx> wrote:
> 
> [somehow managed not to do a reply all, re-sending]
> 
> > From: Sudeep Holla <sudeep.holla@xxxxxxx>
> > 
> > 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@xxxxxxx>
> > Signed-off-by: Andrew Murray <andrew.murray@xxxxxxx>
> > ---
> >  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.
> 

On VHE where SPE is used in the guest only - we save/restore in vcpu_load/put.

On VHE where SPE is used in the host only - we save/restore in the run loop.

On VHE where SPE is used in guest and host - we save/restore in the run loop.

As the guest can't trace EL2 it doesn't matter if we restore guest SPE early
in the vcpu_load/put functions. (I assume it doesn't matter that we restore
an EL0/EL1 profiling buffer address at this point and enable tracing given
that there is nothing to trace until entering the guest).

However the reason for moving save/restore to vcpu_load/put when the host is
using SPE is to minimise the host EL2 black-out window.


On nVHE we always save/restore in the run loop. For the SPE guest-use-only
use-case we can't save/restore in vcpu_load/put - because the guest runs at
the same ELx level as the host - and thus doing so would result in the guest
tracing part of the host.

Though if we determine that (for nVHE systems) the guest SPE is profiling only
EL0 - then we could also save/restore in vcpu_load/put where SPE is only being
used in the guest.

Does that make sense, are my reasons correct?

Thanks,

Andrew Murray


> >  
> >  	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...