Subject: [PATCH v3 14/41] KVM: arm64: Introduce VHE-specific kvm_vcpu_run

From: [email-masked] (Christoffer Dall)
Date: Thu, 25 Jan 2018 09:45:46 +0100

On Wed, Jan 24, 2018 at 04:13:26PM +0000, Dave Martin wrote:
> On Fri, Jan 12, 2018 at 01:07:20PM +0100, Christoffer Dall wrote:
> > So far this is just a copy of the legacy non-VHE switch function, but we
> > will start reworking these functions in separate directions to work on
> > VHE and non-VHE in the most optimal way in later patches.
> 
> I'd be concerned that now that these are separate, they will accumulate
> pointless forkage over time and become hard to maintain.  Yet they are
> supposed to do the same thing to within abstractable details.

I actually think they are conceptually quite different on a VHE vs.
non-VHE system.  On a non-VHE system, you really are talking about a
world-switch, because you're switching the entire world as seen from the
kernel mode (EL1) and up.  On a VHE-system, you're just running
something in a less privileged CPU mode, like a special user space, and
not really changing the world; part of the world as you know it (EL2,
your own mode) remains unaffected.

> 
> To a first approximation, the _vhe() case is the same as _nhve() except
> for the omission of those things that are deferred to load()/put().
> Doubtless I'm glossing over some tricky details though.
> 
> Do you expect more fundamental divergence in the future?
> 

So this is just the way I tried to lay out the patches to make them easy
to follow and easy to bisect.  I may have failed somewhat for the
former, or at least I should have explained this more clearly in the
commit message.

If you look at switch.c after patch 35 you should observe that the VHE
version does significantly less work (very little work, in fact) than
the non-VHE function and is essentially a wrapper around the low-level
exception return code, with as much logic shared between the two
functions as possible.

Note that in my initial optimization attempts I did not pursue the
separate kvm_vcpu_run functions, but I wasn't able to achieve the same
level of performance improvements as I am with separate functions, and
my attempts to pin-point and measure that by instrumenting the code with
cycle counts along the paths only indicated that it was the cumulative
effect of having multiple conditionals and unnecessary function calls
that cause significant differences between the two approaches.

Let me know what you think.

Thanks,
-Christoffer

> 
> > Reviewed-by: Marc Zyngier <[email-masked]>
> > Signed-off-by: Christoffer Dall <[email-masked]>
> > ---
> >  arch/arm/include/asm/kvm_asm.h   |  5 +++-
> >  arch/arm/kvm/hyp/switch.c        |  2 +-
> >  arch/arm64/include/asm/kvm_asm.h |  4 ++-
> >  arch/arm64/kvm/hyp/switch.c      | 58 +++++++++++++++++++++++++++++++++++++++-
> >  virt/kvm/arm/arm.c               |  5 +++-
> >  5 files changed, 69 insertions(+), 5 deletions(-)
> > 
> > diff --git a/arch/arm/include/asm/kvm_asm.h b/arch/arm/include/asm/kvm_asm.h
> > index 36dd2962a42d..4ac717276543 100644
> > --- a/arch/arm/include/asm/kvm_asm.h
> > +++ b/arch/arm/include/asm/kvm_asm.h
> > @@ -70,7 +70,10 @@ extern void __kvm_tlb_flush_local_vmid(struct kvm_vcpu *vcpu);
> >  
> >  extern void __kvm_timer_set_cntvoff(u32 cntvoff_low, u32 cntvoff_high);
> >  
> > -extern int __kvm_vcpu_run(struct kvm_vcpu *vcpu);
> > +/* no VHE on 32-bit :( */
> > +static inline int kvm_vcpu_run_vhe(struct kvm_vcpu *vcpu) { return 0; }
> > +
> > +extern int __kvm_vcpu_run_nvhe(struct kvm_vcpu *vcpu);
> >  
> >  extern void __init_stage2_translation(void);
> >  
> > diff --git a/arch/arm/kvm/hyp/switch.c b/arch/arm/kvm/hyp/switch.c
> > index c3b9799e2e13..7b2bd25e3b10 100644
> > --- a/arch/arm/kvm/hyp/switch.c
> > +++ b/arch/arm/kvm/hyp/switch.c
> > @@ -153,7 +153,7 @@ static bool __hyp_text __populate_fault_info(struct kvm_vcpu *vcpu)
> >  	return true;
> >  }
> >  
> > -int __hyp_text __kvm_vcpu_run(struct kvm_vcpu *vcpu)
> > +int __hyp_text __kvm_vcpu_run_nvhe(struct kvm_vcpu *vcpu)
> >  {
> >  	struct kvm_cpu_context *host_ctxt;
> >  	struct kvm_cpu_context *guest_ctxt;
> > diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h
> > index 6c7599b5cb40..fb91e728207b 100644
> > --- a/arch/arm64/include/asm/kvm_asm.h
> > +++ b/arch/arm64/include/asm/kvm_asm.h
> > @@ -58,7 +58,9 @@ extern void __kvm_tlb_flush_local_vmid(struct kvm_vcpu *vcpu);
> >  
> >  extern void __kvm_timer_set_cntvoff(u32 cntvoff_low, u32 cntvoff_high);
> >  
> > -extern int __kvm_vcpu_run(struct kvm_vcpu *vcpu);
> > +extern int kvm_vcpu_run_vhe(struct kvm_vcpu *vcpu);
> > +
> > +extern int __kvm_vcpu_run_nvhe(struct kvm_vcpu *vcpu);
> >  
> >  extern u64 __vgic_v3_get_ich_vtr_el2(void);
> >  extern u64 __vgic_v3_read_vmcr(void);
> > diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
> > index 55ca2e3d42eb..accfe9a016f9 100644
> > --- a/arch/arm64/kvm/hyp/switch.c
> > +++ b/arch/arm64/kvm/hyp/switch.c
> > @@ -338,7 +338,63 @@ static bool __hyp_text fixup_guest_exit(struct kvm_vcpu *vcpu, u64 *exit_code)
> >  	return false;
> >  }
> >  
> > -int __hyp_text __kvm_vcpu_run(struct kvm_vcpu *vcpu)
> > +/* Switch to the guest for VHE systems running in EL2 */
> > +int kvm_vcpu_run_vhe(struct kvm_vcpu *vcpu)
> > +{
> > +	struct kvm_cpu_context *host_ctxt;
> > +	struct kvm_cpu_context *guest_ctxt;
> > +	u64 exit_code;
> > +
> > +	vcpu = kern_hyp_va(vcpu);
> > +
> > +	host_ctxt = kern_hyp_va(vcpu->arch.host_cpu_context);
> > +	host_ctxt->__hyp_running_vcpu = vcpu;
> > +	guest_ctxt = &vcpu->arch.ctxt;
> > +
> > +	__sysreg_save_host_state(host_ctxt);
> > +
> > +	__activate_traps(vcpu);
> > +	__activate_vm(vcpu);
> > +
> > +	__vgic_restore_state(vcpu);
> > +	__timer_enable_traps(vcpu);
> > +
> > +	/*
> > +	 * We must restore the 32-bit state before the sysregs, thanks
> > +	 * to erratum #852523 (Cortex-A57) or #853709 (Cortex-A72).
> > +	 */
> > +	__sysreg32_restore_state(vcpu);
> > +	__sysreg_restore_guest_state(guest_ctxt);
> > +	__debug_switch_to_guest(vcpu);
> > +
> > +	do {
> > +		/* Jump in the fire! */
> > +		exit_code = __guest_enter(vcpu, host_ctxt);
> > +
> > +		/* And we're baaack! */
> > +	} while (fixup_guest_exit(vcpu, &exit_code));
> > +
> > +	__sysreg_save_guest_state(guest_ctxt);
> > +	__sysreg32_save_state(vcpu);
> > +	__timer_disable_traps(vcpu);
> > +	__vgic_save_state(vcpu);
> > +
> > +	__deactivate_traps(vcpu);
> > +	__deactivate_vm(vcpu);
> > +
> > +	__sysreg_restore_host_state(host_ctxt);
> > +
> > +	/*
> > +	 * This must come after restoring the host sysregs, since a non-VHE
> > +	 * system may enable SPE here and make use of the TTBRs.
> > +	 */
> > +	__debug_switch_to_host(vcpu);
> > +
> > +	return exit_code;
> > +}
> > +
> > +/* Switch to the guest for legacy non-VHE systems */
> > +int __hyp_text __kvm_vcpu_run_nvhe(struct kvm_vcpu *vcpu)
> >  {
> >  	struct kvm_cpu_context *host_ctxt;
> >  	struct kvm_cpu_context *guest_ctxt;
> > diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
> > index 5b1487bd91e8..6bce8f9c55db 100644
> > --- a/virt/kvm/arm/arm.c
> > +++ b/virt/kvm/arm/arm.c
> > @@ -733,7 +733,10 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
> >  		trace_kvm_entry(*vcpu_pc(vcpu));
> >  		guest_enter_irqoff();
> >  
> > -		ret = kvm_call_hyp(__kvm_vcpu_run, vcpu);
> > +		if (has_vhe())
> > +			ret = kvm_vcpu_run_vhe(vcpu);
> > +		else
> > +			ret = kvm_call_hyp(__kvm_vcpu_run_nvhe, vcpu);
> >  
> >  		vcpu->mode = OUTSIDE_GUEST_MODE;
> >  		vcpu->stat.exits++;
> > -- 
> > 2.14.2
> > 
> > _______________________________________________
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> > kvmarm at lists.cs.columbia.edu
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