Subject: [PATCH 08/37] KVM: arm64: Defer restoring host VFP state to vcpu_put

From: [email-masked] (Christoffer Dall)
Date: Sun, 26 Nov 2017 17:24:55 +0100

On Tue, Nov 07, 2017 at 02:15:50PM +0100, Andrew Jones wrote:
> On Thu, Oct 12, 2017 at 12:41:12PM +0200, Christoffer Dall wrote:
> > Avoid saving the guest VFP registers and restoring the host VFP
> > registers on every exit from the VM.  Only when we're about to run
> > userspace or other threads in the kernel do we really have to switch the
> > state back to the host state.
> > 
> > We still initially configure the VFP registers to trap when entering the
> > VM, but the difference is that we now leave the guest state in the
> > hardware registers while running the VM.
> 
> running the host.
> 

I actually did mean the VM, but I should clarify to mean as long as
we're running the VCPU on this physical CPU, even if we trap to the
host.

> > 
> > Signed-off-by: Christoffer Dall <[email-masked]>
> > ---
> >  arch/arm64/include/asm/kvm_emulate.h |  5 ++++
> >  arch/arm64/include/asm/kvm_host.h    |  3 +++
> >  arch/arm64/kernel/asm-offsets.c      |  1 +
> >  arch/arm64/kvm/hyp/entry.S           |  3 +++
> >  arch/arm64/kvm/hyp/switch.c          | 47 +++++++++++-------------------------
> >  arch/arm64/kvm/hyp/sysreg-sr.c       | 21 +++++++++++++---
> >  6 files changed, 44 insertions(+), 36 deletions(-)
> > 
> > diff --git a/arch/arm64/include/asm/kvm_emulate.h b/arch/arm64/include/asm/kvm_emulate.h
> > index 1fbfe96..630dd60 100644
> > --- a/arch/arm64/include/asm/kvm_emulate.h
> > +++ b/arch/arm64/include/asm/kvm_emulate.h
> > @@ -56,6 +56,11 @@ static inline unsigned long *vcpu_hcr(struct kvm_vcpu *vcpu)
> >  	return (unsigned long *)&vcpu->arch.hcr_el2;
> >  }
> >  
> > +static inline bool vcpu_el1_is_32bit(struct kvm_vcpu *vcpu)
> > +{
> > +	return (!(vcpu->arch.hcr_el2 & HCR_RW));
> 
> nit: no need for the outer ().
> 
> > +}
> > +
> >  static inline unsigned long *vcpu_pc(const struct kvm_vcpu *vcpu)
> >  {
> >  	return (unsigned long *)&vcpu_gp_regs(vcpu)->regs.pc;
> > diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> > index 7d3bfa7..5e09eb9 100644
> > --- a/arch/arm64/include/asm/kvm_host.h
> > +++ b/arch/arm64/include/asm/kvm_host.h
> > @@ -210,6 +210,9 @@ struct kvm_vcpu_arch {
> >  	/* Guest debug state */
> >  	u64 debug_flags;
> >  
> > +	/* 1 if the guest VFP state is loaded into the hardware */
> > +	u64 guest_vfp_loaded;
> > +
> 
> Is there a chance we'll want other flags like this? Should we just make
> this a lazy state flags field with the (currently only) flag VFP? If not,
> then a bool would be nicer, although I see below the u64 was chosen in
> order for the 'str' to be used.
> 

See my reply to Yury.  In terms of merging flags I thought about merging
it with the debug flags, but I didn't think it would look very nice, and
I couldn't come up with a name for the variable that would describe the
logic.

Honestly, I didn't care about the few extra bytes per CPU, and much prefer
clarity, but it may make sense to combine this with for example the
sysreg and timer state later, I'll have a look.

> >  	/*
> >  	 * We maintain more than a single set of debug registers to support
> >  	 * debugging the guest from the host and to maintain separate host and
> > diff --git a/arch/arm64/kernel/asm-offsets.c b/arch/arm64/kernel/asm-offsets.c
> > index 612021d..9946732 100644
> > --- a/arch/arm64/kernel/asm-offsets.c
> > +++ b/arch/arm64/kernel/asm-offsets.c
> > @@ -133,6 +133,7 @@ int main(void)
> >    DEFINE(CPU_GP_REGS,		offsetof(struct kvm_cpu_context, gp_regs));
> >    DEFINE(CPU_USER_PT_REGS,	offsetof(struct kvm_regs, regs));
> >    DEFINE(CPU_FP_REGS,		offsetof(struct kvm_regs, fp_regs));
> > +  DEFINE(VCPU_GUEST_VFP_LOADED,	offsetof(struct kvm_vcpu, arch.guest_vfp_loaded));
> >    DEFINE(VCPU_FPEXC32_EL2,	offsetof(struct kvm_vcpu, arch.ctxt.sys_regs[FPEXC32_EL2]));
> >    DEFINE(VCPU_HOST_CONTEXT,	offsetof(struct kvm_vcpu, arch.host_cpu_context));
> >    DEFINE(HOST_CONTEXT_VCPU,	offsetof(struct kvm_cpu_context, __hyp_running_vcpu));
> > diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S
> > index 76cd48f..b3e7191 100644
> > --- a/arch/arm64/kvm/hyp/entry.S
> > +++ b/arch/arm64/kvm/hyp/entry.S
> > @@ -185,6 +185,9 @@ alternative_endif
> >  	add	x0, x2, #CPU_GP_REG_OFFSET(CPU_FP_REGS)
> >  	bl	__fpsimd_restore_state
> >  
> > +	mov	x0, #1
> > +	str	x0, [x3, #VCPU_GUEST_VFP_LOADED]
> > +
> >  	// Skip restoring fpexc32 for AArch64 guests
> >  	mrs	x1, hcr_el2
> >  	tbnz	x1, #HCR_RW_SHIFT, 1f
> > diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
> > index 7703d63..ef05c59 100644
> > --- a/arch/arm64/kvm/hyp/switch.c
> > +++ b/arch/arm64/kvm/hyp/switch.c
> > @@ -23,43 +23,31 @@
> >  #include <asm/kvm_hyp.h>
> >  #include <asm/fpsimd.h>
> >  
> > -static bool __hyp_text __fpsimd_enabled_nvhe(void)
> > -{
> > -	return !(read_sysreg(cptr_el2) & CPTR_EL2_TFP);
> > -}
> > -
> > -static bool __hyp_text __fpsimd_enabled_vhe(void)
> > -{
> > -	return !!(read_sysreg(cpacr_el1) & CPACR_EL1_FPEN);
> > -}
> > -
> > -static hyp_alternate_select(__fpsimd_is_enabled,
> > -			    __fpsimd_enabled_nvhe, __fpsimd_enabled_vhe,
> > -			    ARM64_HAS_VIRT_HOST_EXTN);
> > -
> > -bool __hyp_text __fpsimd_enabled(void)
> > -{
> > -	return __fpsimd_is_enabled()();
> > -}
> > -
> > -static void __hyp_text __activate_traps_vhe(void)
> > +static void __hyp_text __activate_traps_vhe(struct kvm_vcpu *vcpu)
> >  {
> >  	u64 val;
> >  
> >  	val = read_sysreg(cpacr_el1);
> >  	val |= CPACR_EL1_TTA;
> > -	val &= ~CPACR_EL1_FPEN;
> > +	if (vcpu->arch.guest_vfp_loaded)
> > +		val |= CPACR_EL1_FPEN;
> > +	else
> > +		val &= ~CPACR_EL1_FPEN;
> >  	write_sysreg(val, cpacr_el1);
> >  
> >  	write_sysreg(__kvm_hyp_vector, vbar_el1);
> >  }
> >  
> > -static void __hyp_text __activate_traps_nvhe(void)
> > +static void __hyp_text __activate_traps_nvhe(struct kvm_vcpu *vcpu)
> >  {
> >  	u64 val;
> >  
> >  	val = CPTR_EL2_DEFAULT;
> > -	val |= CPTR_EL2_TTA | CPTR_EL2_TFP;
> > +	val |= CPTR_EL2_TTA;
> > +	if (vcpu->arch.guest_vfp_loaded)
> > +		val &= ~CPTR_EL2_TFP;
> > +	else
> > +		val |= CPTR_EL2_TFP;
> >  	write_sysreg(val, cptr_el2);
> >  }
> >  
> > @@ -81,7 +69,8 @@ static void __hyp_text __activate_traps(struct kvm_vcpu *vcpu)
> >  	 * it will cause an exception.
> >  	 */
> >  	val = vcpu->arch.hcr_el2;
> > -	if (!(val & HCR_RW) && system_supports_fpsimd()) {
> > +	if (vcpu_el1_is_32bit(vcpu) && system_supports_fpsimd() &&
> > +	    !vcpu->arch.guest_vfp_loaded) {
> >  		write_sysreg(1 << 30, fpexc32_el2);
> >  		isb();
> >  	}
> > @@ -97,7 +86,7 @@ static void __hyp_text __activate_traps(struct kvm_vcpu *vcpu)
> >  	write_sysreg(0, pmselr_el0);
> >  	write_sysreg(ARMV8_PMU_USERENR_MASK, pmuserenr_el0);
> >  	write_sysreg(vcpu->arch.mdcr_el2, mdcr_el2);
> > -	__activate_traps_arch()();
> > +	__activate_traps_arch()(vcpu);
> >  }
> >  
> >  static void __hyp_text __deactivate_traps_vhe(void)
> > @@ -273,7 +262,6 @@ int __hyp_text __kvm_vcpu_run(struct kvm_vcpu *vcpu)
> >  {
> >  	struct kvm_cpu_context *host_ctxt;
> >  	struct kvm_cpu_context *guest_ctxt;
> > -	bool fp_enabled;
> >  	u64 exit_code;
> >  
> >  	vcpu = kern_hyp_va(vcpu);
> > @@ -355,8 +343,6 @@ int __hyp_text __kvm_vcpu_run(struct kvm_vcpu *vcpu)
> >  		/* 0 falls through to be handled out of EL2 */
> >  	}
> >  
> > -	fp_enabled = __fpsimd_enabled();
> > -
> >  	__sysreg_save_guest_state(guest_ctxt);
> >  	__sysreg32_save_state(vcpu);
> >  	__timer_disable_traps(vcpu);
> > @@ -367,11 +353,6 @@ int __hyp_text __kvm_vcpu_run(struct kvm_vcpu *vcpu)
> >  
> >  	__sysreg_restore_host_state(host_ctxt);
> >  
> > -	if (fp_enabled) {
> > -		__fpsimd_save_state(&guest_ctxt->gp_regs.fp_regs);
> > -		__fpsimd_restore_state(&host_ctxt->gp_regs.fp_regs);
> > -	}
> > -
> >  	__debug_save_state(vcpu, kern_hyp_va(vcpu->arch.debug_ptr), guest_ctxt);
> >  	/*
> >  	 * This must come after restoring the host sysregs, since a non-VHE
> > diff --git a/arch/arm64/kvm/hyp/sysreg-sr.c b/arch/arm64/kvm/hyp/sysreg-sr.c
> > index b7438c8..c4a3714 100644
> > --- a/arch/arm64/kvm/hyp/sysreg-sr.c
> > +++ b/arch/arm64/kvm/hyp/sysreg-sr.c
> > @@ -19,6 +19,7 @@
> >  #include <linux/kvm_host.h>
> >  
> >  #include <asm/kvm_asm.h>
> > +#include <asm/kvm_emulate.h>
> >  #include <asm/kvm_hyp.h>
> >  
> >  /* Yes, this does nothing, on purpose */
> > @@ -137,6 +138,11 @@ void __hyp_text __sysreg_restore_guest_state(struct kvm_cpu_context *ctxt)
> >  	__sysreg_restore_common_state(ctxt);
> >  }
> >  
> > +static void __hyp_text __fpsimd32_save_state(struct kvm_cpu_context *ctxt)
> > +{
> > +	ctxt->sys_regs[FPEXC32_EL2] = read_sysreg(fpexc32_el2);
> > +}
> > +
> >  void __hyp_text __sysreg32_save_state(struct kvm_vcpu *vcpu)
> >  {
> >  	u64 *spsr, *sysreg;
> > @@ -155,9 +161,6 @@ void __hyp_text __sysreg32_save_state(struct kvm_vcpu *vcpu)
> >  	sysreg[DACR32_EL2] = read_sysreg(dacr32_el2);
> >  	sysreg[IFSR32_EL2] = read_sysreg(ifsr32_el2);
> >  
> > -	if (__fpsimd_enabled())
> > -		sysreg[FPEXC32_EL2] = read_sysreg(fpexc32_el2);
> > -
> >  	if (vcpu->arch.debug_flags & KVM_ARM64_DEBUG_DIRTY)
> >  		sysreg[DBGVCR32_EL2] = read_sysreg(dbgvcr32_el2);
> >  }
> > @@ -209,4 +212,16 @@ void kvm_vcpu_load_sysregs(struct kvm_vcpu *vcpu)
> >   */
> >  void kvm_vcpu_put_sysregs(struct kvm_vcpu *vcpu)
> >  {
> > +	struct kvm_cpu_context *host_ctxt = vcpu->arch.host_cpu_context;
> > +	struct kvm_cpu_context *guest_ctxt = &vcpu->arch.ctxt;
> > +
> > +	/* Restore host FP/SIMD state */
> > +	if (vcpu->arch.guest_vfp_loaded) {
> > +		if (vcpu_el1_is_32bit(vcpu))
> > +			kvm_call_hyp(__fpsimd32_save_state,
> > +				     kern_hyp_va(guest_ctxt));
> 
> nit: might be nice to use {} since we need two lines.
> 

sure.

> > +		__fpsimd_save_state(&guest_ctxt->gp_regs.fp_regs);
> > +		__fpsimd_restore_state(&host_ctxt->gp_regs.fp_regs);
> > +		vcpu->arch.guest_vfp_loaded = 0;
> > +	}
> >  }
> > -- 
> > 2.9.0
> >
> 
> Otherwise,
> 
> Reviewed-by: Andrew Jones <[email-masked]>

Thanks,
-Christoffer