Subject: [PATCH 25/37] KVM: arm64: Prepare to handle traps on remaining deferred EL1 sysregs

From: [email-masked] (Christoffer Dall)
Date: Sun, 3 Dec 2017 21:29:37 +0100

On Mon, Nov 13, 2017 at 07:56:14PM +0100, Andrew Jones wrote:
> On Thu, Oct 12, 2017 at 12:41:29PM +0200, Christoffer Dall wrote:
> > Handle accesses during traps to any remaining EL1 registers which can be
> > deferred to vcpu_load and vcpu_put, by either accessing them directly on
> > the physical CPU when the latest version is stored there, or by
> > synchronizing the memory representation with the CPU state.
> > 
> > Signed-off-by: Christoffer Dall <[email-masked]>
> > ---
> >  arch/arm64/include/asm/kvm_emulate.h | 14 -------
> >  arch/arm64/kvm/inject_fault.c        | 79 ++++++++++++++++++++++++++++++++----
> >  arch/arm64/kvm/sys_regs.c            |  6 ++-
> >  3 files changed, 76 insertions(+), 23 deletions(-)
> > 
> > diff --git a/arch/arm64/include/asm/kvm_emulate.h b/arch/arm64/include/asm/kvm_emulate.h
> > index 630dd60..69bb40d 100644
> > --- a/arch/arm64/include/asm/kvm_emulate.h
> > +++ b/arch/arm64/include/asm/kvm_emulate.h
> > @@ -66,11 +66,6 @@ static inline unsigned long *vcpu_pc(const struct kvm_vcpu *vcpu)
> >  	return (unsigned long *)&vcpu_gp_regs(vcpu)->regs.pc;
> >  }
> >  
> > -static inline unsigned long *vcpu_elr_el1(const struct kvm_vcpu *vcpu)
> > -{
> > -	return (unsigned long *)&vcpu_gp_regs(vcpu)->elr_el1;
> > -}
> > -
> >  static inline unsigned long *vcpu_cpsr(const struct kvm_vcpu *vcpu)
> >  {
> >  	return (unsigned long *)&vcpu_gp_regs(vcpu)->regs.pstate;
> > @@ -120,15 +115,6 @@ static inline void vcpu_set_reg(struct kvm_vcpu *vcpu, u8 reg_num,
> >  		vcpu_gp_regs(vcpu)->regs.regs[reg_num] = val;
> >  }
> >  
> > -/* Get vcpu SPSR for current mode */
> > -static inline unsigned long *vcpu_spsr(const struct kvm_vcpu *vcpu)
> > -{
> > -	if (vcpu_mode_is_32bit(vcpu))
> > -		return vcpu_spsr32(vcpu);
> > -
> > -	return (unsigned long *)&vcpu_gp_regs(vcpu)->spsr[KVM_SPSR_EL1];
> > -}
> > -
> >  static inline bool vcpu_mode_priv(const struct kvm_vcpu *vcpu)
> >  {
> >  	u32 mode;
> > diff --git a/arch/arm64/kvm/inject_fault.c b/arch/arm64/kvm/inject_fault.c
> > index 45c7026..f4513fc 100644
> > --- a/arch/arm64/kvm/inject_fault.c
> > +++ b/arch/arm64/kvm/inject_fault.c
> > @@ -23,6 +23,7 @@
> >  
> >  #include <linux/kvm_host.h>
> >  #include <asm/kvm_emulate.h>
> > +#include <asm/kvm_hyp.h>
> >  #include <asm/esr.h>
> >  
> >  #define PSTATE_FAULT_BITS_64 	(PSR_MODE_EL1h | PSR_A_BIT | PSR_F_BIT | \
> > @@ -33,13 +34,55 @@
> >  #define LOWER_EL_AArch64_VECTOR		0x400
> >  #define LOWER_EL_AArch32_VECTOR		0x600
> >  
> > +static u64 vcpu_get_vbar_el1(struct kvm_vcpu *vcpu)
> > +{
> > +	unsigned long vbar;
> > +
> > +	if (vcpu->arch.sysregs_loaded_on_cpu)
> > +		vbar = read_sysreg_el1(vbar);
> > +	else
> > +		vbar = vcpu_sys_reg(vcpu, VBAR_EL1);
> > +
> > +	if (vcpu_el1_is_32bit(vcpu))
> > +		return lower_32_bits(vbar);
> > +	return vbar;
> > +}
> > +
> > +static void vcpu_set_elr_el1(struct kvm_vcpu *vcpu, u64 val)
> > +{
> > +	if (vcpu->arch.sysregs_loaded_on_cpu)
> > +		write_sysreg_el1(val, elr);
> > +	else
> > +		vcpu_gp_regs(vcpu)->elr_el1 = val;
> > +}
> > +
> > +/* Set the SPSR for the current mode */
> > +static void vcpu_set_spsr(struct kvm_vcpu *vcpu, u64 val)
> > +{
> > +	if (vcpu_mode_is_32bit(vcpu))
> > +		*vcpu_spsr32(vcpu) = val;
> > +
> > +	if (vcpu->arch.sysregs_loaded_on_cpu)
> > +		write_sysreg_el1(val, spsr);
> > +	else
> > +		vcpu_gp_regs(vcpu)->spsr[KVM_SPSR_EL1] = val;
> > +}
> > +
> > +static u32 vcpu_get_c1_sctlr(struct kvm_vcpu *vcpu)
> > +{
> > +	if (vcpu->arch.sysregs_loaded_on_cpu)
> > +		return lower_32_bits(read_sysreg_el1(sctlr));
> > +	else
> > +		return vcpu_cp15(vcpu, c1_SCTLR);
> > +}
> > +
> >  static void prepare_fault32(struct kvm_vcpu *vcpu, u32 mode, u32 vect_offset)
> >  {
> >  	unsigned long cpsr;
> >  	unsigned long new_spsr_value = *vcpu_cpsr(vcpu);
> >  	bool is_thumb = (new_spsr_value & COMPAT_PSR_T_BIT);
> >  	u32 return_offset = (is_thumb) ? 4 : 0;
> > -	u32 sctlr = vcpu_cp15(vcpu, c1_SCTLR);
> > +	u32 sctlr = vcpu_get_c1_sctlr(vcpu);
> >  
> >  	cpsr = mode | COMPAT_PSR_I_BIT;
> >  
> > @@ -51,14 +94,14 @@ static void prepare_fault32(struct kvm_vcpu *vcpu, u32 mode, u32 vect_offset)
> >  	*vcpu_cpsr(vcpu) = cpsr;
> >  
> >  	/* Note: These now point to the banked copies */
> > -	*vcpu_spsr(vcpu) = new_spsr_value;
> > +	vcpu_set_spsr(vcpu, new_spsr_value);
> >  	*vcpu_reg32(vcpu, 14) = *vcpu_pc(vcpu) + return_offset;
> >  
> >  	/* Branch to exception vector */
> >  	if (sctlr & (1 << 13))
> >  		vect_offset += 0xffff0000;
> >  	else /* always have security exceptions */
> > -		vect_offset += vcpu_cp15(vcpu, c12_VBAR);
> > +		vect_offset += vcpu_get_vbar_el1(vcpu);
> >  
> >  	*vcpu_pc(vcpu) = vect_offset;
> >  }
> > @@ -79,6 +122,20 @@ static void inject_abt32(struct kvm_vcpu *vcpu, bool is_pabt,
> >  	u32 *far, *fsr;
> >  	bool is_lpae;
> >  
> > +	/*
> > +	 * We are going to need the latest values of the following system
> > +	 * regiters:
> 
> registers
> 
> > +	 *   DFAR:  mapped to FAR_EL1
> 
> FAR_EL1[31:0]
> 
> > +	 *   IFAR:  mapped to FAR_EL1
> 
> FAR_EL1[63:32]
> 

I had to rework this after the 32-bit fault injection code has been
reworked, so I got rid of this comment (as it's now being shared with
the 32-bit side, and I use a bigger hammer to solve the problem).

> > +	 *   DFSR:  mapped to ESR_EL1
> > +	 *   TTBCR: mapped to TCR_EL1
> > +	 */
> > +	if (vcpu->arch.sysregs_loaded_on_cpu) {
> > +		vcpu->arch.ctxt.sys_regs[FAR_EL1] = read_sysreg_el1(far);
> > +		vcpu->arch.ctxt.sys_regs[ESR_EL1] = read_sysreg_el1(esr);
> > +		vcpu->arch.ctxt.sys_regs[TCR_EL1] = read_sysreg_el1(tcr);
> > +	}
> > +
> >  	if (is_pabt) {
> >  		vect_offset = 12;
> >  		far = &vcpu_cp15(vcpu, c6_IFAR);
> > @@ -99,6 +156,12 @@ static void inject_abt32(struct kvm_vcpu *vcpu, bool is_pabt,
> >  		*fsr = 1 << 9 | 0x34;
> >  	else
> >  		*fsr = 0x14;
> > +
> > +	/* Sync back any registers we may have changed */
> > +	if (vcpu->arch.sysregs_loaded_on_cpu) {
> > +		write_sysreg_el1(vcpu->arch.ctxt.sys_regs[FAR_EL1], far);
> > +		write_sysreg_el1(vcpu->arch.ctxt.sys_regs[ESR_EL1], esr);
> > +	}
> >  }
> >  
> >  enum exception_type {
> > @@ -126,7 +189,7 @@ static u64 get_except_vector(struct kvm_vcpu *vcpu, enum exception_type type)
> >  		exc_offset = LOWER_EL_AArch32_VECTOR;
> >  	}
> >  
> > -	return vcpu_sys_reg(vcpu, VBAR_EL1) + exc_offset + type;
> > +	return vcpu_get_vbar_el1(vcpu) + exc_offset + type;
> >  }
> >  
> >  static void inject_abt64(struct kvm_vcpu *vcpu, bool is_iabt, unsigned long addr)
> > @@ -135,11 +198,11 @@ static void inject_abt64(struct kvm_vcpu *vcpu, bool is_iabt, unsigned long addr
> >  	bool is_aarch32 = vcpu_mode_is_32bit(vcpu);
> >  	u32 esr = 0;
> >  
> > -	*vcpu_elr_el1(vcpu) = *vcpu_pc(vcpu);
> > +	vcpu_set_elr_el1(vcpu, *vcpu_pc(vcpu));
> >  	*vcpu_pc(vcpu) = get_except_vector(vcpu, except_type_sync);
> >  
> >  	*vcpu_cpsr(vcpu) = PSTATE_FAULT_BITS_64;
> > -	*vcpu_spsr(vcpu) = cpsr;
> > +	vcpu_set_spsr(vcpu, cpsr);
> >  
> >  	vcpu_sys_reg(vcpu, FAR_EL1) = addr;
> >  
> > @@ -170,11 +233,11 @@ static void inject_undef64(struct kvm_vcpu *vcpu)
> >  	unsigned long cpsr = *vcpu_cpsr(vcpu);
> >  	u32 esr = (ESR_ELx_EC_UNKNOWN << ESR_ELx_EC_SHIFT);
> >  
> > -	*vcpu_elr_el1(vcpu) = *vcpu_pc(vcpu);
> > +	vcpu_set_elr_el1(vcpu, *vcpu_pc(vcpu));
> >  	*vcpu_pc(vcpu) = get_except_vector(vcpu, except_type_sync);
> >  
> >  	*vcpu_cpsr(vcpu) = PSTATE_FAULT_BITS_64;
> > -	*vcpu_spsr(vcpu) = cpsr;
> > +	vcpu_set_spsr(vcpu, cpsr);
> >  
> 
> I'm concerned the maintenance of emulation code will become more
> difficult now that some registers have special accessors, while
> others don't, and some functions have save/restore lists, that
> will need to stay maintained with the emulation. The only alternative
> that pops to mind, though, is adding get/set members to the register
> descriptors and encapsulating the decision in them, but that might be
> overkill.
> 

I was concerned initially as well, and had rewritten the entire code to
use get/set accessors everywhere, and that was not a pretty set of
patches to review, and in fact seemed less intuitive and easy to
maintain.

This is an interesting discussion; my original standpoint was very
similar to yours and I had similar concernes, but this rework has left
me feeling that maintaining this code base requires an intimate
understanding of the flow and state the CPU/VM can be in at any one
point, and it's therefore actually easier to maintain something that
does the required things in the right places, as opposed to creating
larger abstractions, at least to some degree.

I'm curious what Marc thinks here, and I'm willing to produce an
alternative patch with everything abstracted as get/set functions, with
giant switch statements to access hardware registers (which will only
actually hit in a few of the cases), but I think I know what the final
answer will be already.

> >  	/*
> >  	 * Build an unknown exception, depending on the instruction
> > diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> > index f7887dd..60d1660 100644
> > --- a/arch/arm64/kvm/sys_regs.c
> > +++ b/arch/arm64/kvm/sys_regs.c
> > @@ -86,12 +86,16 @@ static u32 cache_levels;
> >  static u32 get_ccsidr(u32 csselr)
> >  {
> >  	u32 ccsidr;
> > +	u32 csselr_preserve;
> >  
> > -	/* Make sure noone else changes CSSELR during this! */
> > +	/* Make sure noone else changes CSSELR during this and preserve any
> > +	 * existing value in the CSSELR! */
> >  	local_irq_disable();
> > +	csselr_preserve = read_sysreg(csselr_el1);
> >  	write_sysreg(csselr, csselr_el1);
> >  	isb();
> >  	ccsidr = read_sysreg(ccsidr_el1);
> > +	write_sysreg(csselr_preserve, csselr_el1);
> >  	local_irq_enable();
> >  
> >  	return ccsidr;
> 
> This looks like an unrelated fix.
> 

Not really.  The point is that the host kernel doesn't make any
assumptions about the csselr and will set it to select the value it
wants to read when using this feature.

Until we start deferring el1 registers for the guest, we can therefore
do the same, as the guest will always get its values restore.

However, when we introduce lazy save/restore of guest state, we cannot
just step on the guest state, but we have to preserve the guest state.

We could make it conditional on the sysregs loaded flag, but that
doesn't get us much, and we might as well do this.

Thanks,
-Christoffer