Re: [PATCH v4 24/40] KVM: arm64: Rewrite system register accessors to read/write functions

[Marc Zyngier <marc.zyngier@arm.com>]

On Thu, 15 Feb 2018 21:03:16 +0000,
Christoffer Dall wrote:
> 
> From: Christoffer Dall <cdall@cs.columbia.edu>
> 
> Currently we access the system registers array via the vcpu_sys_reg()
> macro.  However, we are about to change the behavior to some times
> modify the register file directly, so let's change this to two
> primitives:
> 
>  * Accessor macros vcpu_write_sys_reg() and vcpu_read_sys_reg()
>  * Direct array access macro __vcpu_sys_reg()
> 
> The first primitive should be used in places where the code needs to
> access the currently loaded VCPU's state as observed by the guest.  For
> example, when trapping on cache related registers, a write to a system
> register should go directly to the VCPU version of the register.
> 
> The second primitive can be used in places where the VCPU is known to
> never be running (for example userspace access) or for registers which
> are never context switched (for example all the PMU system registers).
> 
> This rewrites all users of vcpu_sys_regs to one of the two primitives
> above.
> 
> No functional change.
> 
> Signed-off-by: Christoffer Dall <cdall@cs.columbia.edu>
> ---
> 
> Notes:
>     Changes since v2:
>      - New patch (deferred register handling has been reworked)
> 
>  arch/arm64/include/asm/kvm_emulate.h | 13 ++++---
>  arch/arm64/include/asm/kvm_host.h    | 13 ++++++-
>  arch/arm64/include/asm/kvm_mmu.h     |  2 +-
>  arch/arm64/kvm/debug.c               | 27 +++++++++-----
>  arch/arm64/kvm/inject_fault.c        |  8 ++--
>  arch/arm64/kvm/sys_regs.c            | 71 ++++++++++++++++++------------------
>  arch/arm64/kvm/sys_regs.h            |  4 +-
>  arch/arm64/kvm/sys_regs_generic_v8.c |  4 +-
>  virt/kvm/arm/pmu.c                   | 37 ++++++++++---------
>  9 files changed, 102 insertions(+), 77 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_emulate.h b/arch/arm64/include/asm/kvm_emulate.h
> index 3cc535591bdf..d313aaae5c38 100644
> --- a/arch/arm64/include/asm/kvm_emulate.h
> +++ b/arch/arm64/include/asm/kvm_emulate.h
> @@ -290,15 +290,18 @@ static inline int kvm_vcpu_sys_get_rt(struct kvm_vcpu *vcpu)
>  
>  static inline unsigned long kvm_vcpu_get_mpidr_aff(struct kvm_vcpu *vcpu)
>  {
> -	return vcpu_sys_reg(vcpu, MPIDR_EL1) & MPIDR_HWID_BITMASK;
> +	return vcpu_read_sys_reg(vcpu, MPIDR_EL1) & MPIDR_HWID_BITMASK;
>  }
>  
>  static inline void kvm_vcpu_set_be(struct kvm_vcpu *vcpu)
>  {
> -	if (vcpu_mode_is_32bit(vcpu))
> +	if (vcpu_mode_is_32bit(vcpu)) {
>  		*vcpu_cpsr(vcpu) |= COMPAT_PSR_E_BIT;
> -	else
> -		vcpu_sys_reg(vcpu, SCTLR_EL1) |= (1 << 25);
> +	} else {
> +		u64 sctlr = vcpu_read_sys_reg(vcpu, SCTLR_EL1);
> +		sctlr |= (1 << 25);
> +		vcpu_write_sys_reg(vcpu, SCTLR_EL1, sctlr);

General comment: it is slightly annoying that vcpu_write_sys_reg takes
its parameters in an order different from that of write_sysreg
(register followed with value, instead of value followed with
register). Not a deal breaker, but slightly confusing.

> +	}
>  }
>  
>  static inline bool kvm_vcpu_is_be(struct kvm_vcpu *vcpu)
> @@ -306,7 +309,7 @@ static inline bool kvm_vcpu_is_be(struct kvm_vcpu *vcpu)
>  	if (vcpu_mode_is_32bit(vcpu))
>  		return !!(*vcpu_cpsr(vcpu) & COMPAT_PSR_E_BIT);
>  
> -	return !!(vcpu_sys_reg(vcpu, SCTLR_EL1) & (1 << 25));
> +	return !!(vcpu_read_sys_reg(vcpu, SCTLR_EL1) & (1 << 25));
>  }
>  
>  static inline unsigned long vcpu_data_guest_to_host(struct kvm_vcpu *vcpu,
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index f2a6f39aec87..68398bf7882f 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -287,7 +287,18 @@ struct kvm_vcpu_arch {
>  };
>  
>  #define vcpu_gp_regs(v)		(&(v)->arch.ctxt.gp_regs)
> -#define vcpu_sys_reg(v,r)	((v)->arch.ctxt.sys_regs[(r)])
> +
> +/*
> + * Only use __vcpu_sys_reg if you know you want the memory backed version of a
> + * register, and not the one most recently accessed by a runnning VCPU.  For
> + * example, for userpace access or for system registers that are never context
> + * switched, but only emulated.
> + */
> +#define __vcpu_sys_reg(v,r)	((v)->arch.ctxt.sys_regs[(r)])
> +
> +#define vcpu_read_sys_reg(v,r)	__vcpu_sys_reg(v,r)
> +#define vcpu_write_sys_reg(v,r,n)	do { __vcpu_sys_reg(v,r) = n; } while (0)
> +
>  /*
>   * CP14 and CP15 live in the same array, as they are backed by the
>   * same system registers.
> diff --git a/arch/arm64/include/asm/kvm_mmu.h b/arch/arm64/include/asm/kvm_mmu.h
> index 9679067a1574..95f46e73c4dc 100644
> --- a/arch/arm64/include/asm/kvm_mmu.h
> +++ b/arch/arm64/include/asm/kvm_mmu.h
> @@ -249,7 +249,7 @@ struct kvm;
>  
>  static inline bool vcpu_has_cache_enabled(struct kvm_vcpu *vcpu)
>  {
> -	return (vcpu_sys_reg(vcpu, SCTLR_EL1) & 0b101) == 0b101;
> +	return (vcpu_read_sys_reg(vcpu, SCTLR_EL1) & 0b101) == 0b101;
>  }
>  
>  static inline void __clean_dcache_guest_page(kvm_pfn_t pfn, unsigned long size)
> diff --git a/arch/arm64/kvm/debug.c b/arch/arm64/kvm/debug.c
> index feedb877cff8..db32d10a56a1 100644
> --- a/arch/arm64/kvm/debug.c
> +++ b/arch/arm64/kvm/debug.c
> @@ -46,7 +46,8 @@ static DEFINE_PER_CPU(u32, mdcr_el2);
>   */
>  static void save_guest_debug_regs(struct kvm_vcpu *vcpu)
>  {
> -	vcpu->arch.guest_debug_preserved.mdscr_el1 = vcpu_sys_reg(vcpu, MDSCR_EL1);
> +	vcpu->arch.guest_debug_preserved.mdscr_el1 =
> +		vcpu_read_sys_reg(vcpu, MDSCR_EL1);
>  
>  	trace_kvm_arm_set_dreg32("Saved MDSCR_EL1",
>  				vcpu->arch.guest_debug_preserved.mdscr_el1);
> @@ -54,10 +55,11 @@ static void save_guest_debug_regs(struct kvm_vcpu *vcpu)
>  
>  static void restore_guest_debug_regs(struct kvm_vcpu *vcpu)
>  {
> -	vcpu_sys_reg(vcpu, MDSCR_EL1) = vcpu->arch.guest_debug_preserved.mdscr_el1;
> +	vcpu_write_sys_reg(vcpu, MDSCR_EL1,
> +			   vcpu->arch.guest_debug_preserved.mdscr_el1);
>  
>  	trace_kvm_arm_set_dreg32("Restored MDSCR_EL1",
> -				vcpu_sys_reg(vcpu, MDSCR_EL1));
> +				vcpu_read_sys_reg(vcpu, MDSCR_EL1));
>  }
>  
>  /**
> @@ -108,6 +110,7 @@ void kvm_arm_reset_debug_ptr(struct kvm_vcpu *vcpu)
>  void kvm_arm_setup_debug(struct kvm_vcpu *vcpu)
>  {
>  	bool trap_debug = !(vcpu->arch.debug_flags & KVM_ARM64_DEBUG_DIRTY);
> +	unsigned long mdscr;
>  
>  	trace_kvm_arm_setup_debug(vcpu, vcpu->guest_debug);
>  
> @@ -152,9 +155,13 @@ void kvm_arm_setup_debug(struct kvm_vcpu *vcpu)
>  		 */
>  		if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) {
>  			*vcpu_cpsr(vcpu) |=  DBG_SPSR_SS;
> -			vcpu_sys_reg(vcpu, MDSCR_EL1) |= DBG_MDSCR_SS;
> +			mdscr = vcpu_read_sys_reg(vcpu, MDSCR_EL1);
> +			mdscr |= DBG_MDSCR_SS;
> +			vcpu_write_sys_reg(vcpu, MDSCR_EL1, mdscr);

I have the feeling that we're going to need some clearbits/setbits
variants of vcpu_write_sysreg at some point.

It otherwise looks correct to me.

Acked-by: Marc Zyngier <marc.zyngier@arm.com>

	M.

-- 
Jazz is not dead, it just smell funny.