Re: [PATCH v4 03/40] KVM: arm64: Avoid storing the vcpu pointer on the stack

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

On Thu, 15 Feb 2018 21:02:55 +0000,
Christoffer Dall wrote:
> 
> We already have the percpu area for the host cpu state, which points to
> the VCPU, so there's no need to store the VCPU pointer on the stack on
> every context switch.  We can be a little more clever and just use
> tpidr_el2 for the percpu offset and load the VCPU pointer from the host
> context.
> 
> This does require us to calculate the percpu offset without including
> the offset from the kernel mapping of the percpu array to the linear
> mapping of the array (which is what we store in tpidr_el1), because a
> PC-relative generated address in EL2 is already giving us the hyp alias
> of the linear mapping of a kernel address.  We do this in
> __cpu_init_hyp_mode() by using kvm_ksym_ref().
> 
> This change also requires us to have a scratch register, so we take the
> chance to rearrange some of the el1_sync code to only look at the
> vttbr_el2 to determine if this is a trap from the guest or an HVC from
> the host.  We do add an extra check to call the panic code if the kernel
> is configured with debugging enabled and we saw a trap from the host
> which wasn't an HVC, indicating that we left some EL2 trap configured by
> mistake.
> 
> The code that accesses ESR_EL2 was previously using an alternative to
> use the _EL1 accessor on VHE systems, but this was actually unnecessary
> as the _EL1 accessor aliases the ESR_EL2 register on VHE, and the _EL2
> accessor does the same thing on both systems.
> 
> Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
> ---
> 
> Notes:
>     Changes since v3:
>      - Reworked the assembly part of the patch after rebasing on v4.16-rc1
>        which created a conflict with the variant 2 mitigations.
>      - Removed Marc's reviewed-by due to the rework.
>      - Removed unneeded extern keyword in declaration in header file
>     
>     Changes since v1:
>      - Use PC-relative addressing to access per-cpu variables instead of
>        using a load from the literal pool.
>      - Remove stale comments as pointed out by Marc
>      - Reworded the commit message as suggested by Drew
> 
>  arch/arm64/include/asm/kvm_asm.h  | 14 ++++++++++++++
>  arch/arm64/include/asm/kvm_host.h | 15 +++++++++++++++
>  arch/arm64/kernel/asm-offsets.c   |  1 +
>  arch/arm64/kvm/hyp/entry.S        |  6 +-----
>  arch/arm64/kvm/hyp/hyp-entry.S    | 31 +++++++++++++------------------
>  arch/arm64/kvm/hyp/switch.c       |  5 +----
>  arch/arm64/kvm/hyp/sysreg-sr.c    |  5 +++++
>  7 files changed, 50 insertions(+), 27 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h
> index 24961b732e65..6b626750b0a1 100644
> --- a/arch/arm64/include/asm/kvm_asm.h
> +++ b/arch/arm64/include/asm/kvm_asm.h
> @@ -33,6 +33,7 @@
>  #define KVM_ARM64_DEBUG_DIRTY_SHIFT	0
>  #define KVM_ARM64_DEBUG_DIRTY		(1 << KVM_ARM64_DEBUG_DIRTY_SHIFT)
>  
> +/* Translate a kernel address of @sym into its equivalent linear mapping */
>  #define kvm_ksym_ref(sym)						\
>  	({								\
>  		void *val = &sym;					\
> @@ -70,6 +71,19 @@ extern u32 __init_stage2_translation(void);
>  
>  extern void __qcom_hyp_sanitize_btac_predictors(void);
>  
> +#else /* __ASSEMBLY__ */
> +
> +.macro get_host_ctxt reg, tmp
> +	adr_l	\reg, kvm_host_cpu_state
> +	mrs	\tmp, tpidr_el2
> +	add	\reg, \reg, \tmp
> +.endm
> +
> +.macro get_vcpu vcpu, ctxt
> +	ldr	\vcpu, [\ctxt, #HOST_CONTEXT_VCPU]
> +	kern_hyp_va	\vcpu
> +.endm
> +
>  #endif
>  
>  #endif /* __ARM_KVM_ASM_H__ */
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 596f8e414a4c..618cfee7206a 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -358,10 +358,15 @@ int kvm_perf_teardown(void);
>  
>  struct kvm_vcpu *kvm_mpidr_to_vcpu(struct kvm *kvm, unsigned long mpidr);
>  
> +void __kvm_set_tpidr_el2(u64 tpidr_el2);
> +DECLARE_PER_CPU(kvm_cpu_context_t, kvm_host_cpu_state);
> +
>  static inline void __cpu_init_hyp_mode(phys_addr_t pgd_ptr,
>  				       unsigned long hyp_stack_ptr,
>  				       unsigned long vector_ptr)
>  {
> +	u64 tpidr_el2;
> +
>  	/*
>  	 * Call initialization code, and switch to the full blown HYP code.
>  	 * If the cpucaps haven't been finalized yet, something has gone very
> @@ -370,6 +375,16 @@ static inline void __cpu_init_hyp_mode(phys_addr_t pgd_ptr,
>  	 */
>  	BUG_ON(!static_branch_likely(&arm64_const_caps_ready));
>  	__kvm_call_hyp((void *)pgd_ptr, hyp_stack_ptr, vector_ptr);
> +
> +	/*
> +	 * Calculate the raw per-cpu offset without a translation from the
> +	 * kernel's mapping to the linear mapping, and store it in tpidr_el2
> +	 * so that we can use adr_l to access per-cpu variables in EL2.
> +	 */
> +	tpidr_el2 = (u64)this_cpu_ptr(&kvm_host_cpu_state)
> +		- (u64)kvm_ksym_ref(kvm_host_cpu_state);
> +
> +	kvm_call_hyp(__kvm_set_tpidr_el2, tpidr_el2);
>  }
>  
>  static inline void kvm_arch_hardware_unsetup(void) {}
> diff --git a/arch/arm64/kernel/asm-offsets.c b/arch/arm64/kernel/asm-offsets.c
> index 1303e04110cd..78e1b0a70aaf 100644
> --- a/arch/arm64/kernel/asm-offsets.c
> +++ b/arch/arm64/kernel/asm-offsets.c
> @@ -138,6 +138,7 @@ int main(void)
>    DEFINE(CPU_FP_REGS,		offsetof(struct kvm_regs, fp_regs));
>    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));
>  #endif
>  #ifdef CONFIG_CPU_PM
>    DEFINE(CPU_SUSPEND_SZ,	sizeof(struct cpu_suspend_ctx));
> diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S
> index fdd1068ee3a5..1f458f7c3b44 100644
> --- a/arch/arm64/kvm/hyp/entry.S
> +++ b/arch/arm64/kvm/hyp/entry.S
> @@ -62,9 +62,6 @@ ENTRY(__guest_enter)
>  	// Store the host regs
>  	save_callee_saved_regs x1
>  
> -	// Store host_ctxt and vcpu for use at exit time
> -	stp	x1, x0, [sp, #-16]!
> -
>  	add	x18, x0, #VCPU_CONTEXT
>  
>  	// Restore guest regs x0-x17
> @@ -118,8 +115,7 @@ ENTRY(__guest_exit)
>  	// Store the guest regs x19-x29, lr
>  	save_callee_saved_regs x1
>  
> -	// Restore the host_ctxt from the stack
> -	ldr	x2, [sp], #16
> +	get_host_ctxt	x2, x3
>  
>  	// Now restore the host regs
>  	restore_callee_saved_regs x2
> diff --git a/arch/arm64/kvm/hyp/hyp-entry.S b/arch/arm64/kvm/hyp/hyp-entry.S
> index f36464bd57c5..559b4d54bc42 100644
> --- a/arch/arm64/kvm/hyp/hyp-entry.S
> +++ b/arch/arm64/kvm/hyp/hyp-entry.S
> @@ -57,13 +57,8 @@ ENDPROC(__vhe_hyp_call)
>  el1_sync:				// Guest trapped into EL2
>  	stp	x0, x1, [sp, #-16]!
>  
> -alternative_if_not ARM64_HAS_VIRT_HOST_EXTN
> -	mrs	x1, esr_el2
> -alternative_else
> -	mrs	x1, esr_el1
> -alternative_endif
> -	lsr	x0, x1, #ESR_ELx_EC_SHIFT
> -
> +	mrs	x0, esr_el2
> +	lsr	x0, x0, #ESR_ELx_EC_SHIFT
>  	cmp	x0, #ESR_ELx_EC_HVC64
>  	ccmp	x0, #ESR_ELx_EC_HVC32, #4, ne
>  	b.ne	el1_trap
> @@ -117,10 +112,15 @@ el1_hvc_guest:
>  	eret
>  
>  el1_trap:
> +	get_host_ctxt	x0, x1
> +	get_vcpu	x1, x0
> +
> +	mrs		x0, esr_el2
> +	lsr		x0, x0, #ESR_ELx_EC_SHIFT
>  	/*
>  	 * x0: ESR_EC
> +	 * x1: vcpu pointer
>  	 */
> -	ldr	x1, [sp, #16 + 8]	// vcpu stored by __guest_enter
>  
>  	/*
>  	 * We trap the first access to the FP/SIMD to save the host context
> @@ -138,13 +138,15 @@ alternative_else_nop_endif
>  
>  el1_irq:
>  	stp     x0, x1, [sp, #-16]!
> -	ldr	x1, [sp, #16 + 8]
> +	get_host_ctxt	x0, x1
> +	get_vcpu	x1, x0
>  	mov	x0, #ARM_EXCEPTION_IRQ
>  	b	__guest_exit
>  
>  el1_error:
>  	stp     x0, x1, [sp, #-16]!
> -	ldr	x1, [sp, #16 + 8]
> +	get_host_ctxt	x0, x1
> +	get_vcpu	x1, x0

Given how frequent this construct is, would there be a benefit in
having something like "get_vcpu_ptr" that conflates the two macros? We
don't seem to have a single case of using get_vcpu on its own.

>  	mov	x0, #ARM_EXCEPTION_EL1_SERROR
>  	b	__guest_exit
>  
> @@ -180,14 +182,7 @@ ENTRY(__hyp_do_panic)
>  ENDPROC(__hyp_do_panic)
>  
>  ENTRY(__hyp_panic)
> -	/*
> -	 * '=kvm_host_cpu_state' is a host VA from the constant pool, it may
> -	 * not be accessible by this address from EL2, hyp_panic() converts
> -	 * it with kern_hyp_va() before use.
> -	 */
> -	ldr	x0, =kvm_host_cpu_state
> -	mrs	x1, tpidr_el2
> -	add	x0, x0, x1
> +	get_host_ctxt x0, x1
>  	b	hyp_panic
>  ENDPROC(__hyp_panic)
>  
> diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
> index b991f85c1133..d1749fa0bfc3 100644
> --- a/arch/arm64/kvm/hyp/switch.c
> +++ b/arch/arm64/kvm/hyp/switch.c
> @@ -467,7 +467,7 @@ static hyp_alternate_select(__hyp_call_panic,
>  			    __hyp_call_panic_nvhe, __hyp_call_panic_vhe,
>  			    ARM64_HAS_VIRT_HOST_EXTN);
>  
> -void __hyp_text __noreturn hyp_panic(struct kvm_cpu_context *__host_ctxt)
> +void __hyp_text __noreturn hyp_panic(struct kvm_cpu_context *host_ctxt)
>  {
>  	struct kvm_vcpu *vcpu = NULL;
>  
> @@ -476,9 +476,6 @@ void __hyp_text __noreturn hyp_panic(struct kvm_cpu_context *__host_ctxt)
>  	u64 par = read_sysreg(par_el1);
>  
>  	if (read_sysreg(vttbr_el2)) {
> -		struct kvm_cpu_context *host_ctxt;
> -
> -		host_ctxt = kern_hyp_va(__host_ctxt);
>  		vcpu = host_ctxt->__hyp_running_vcpu;
>  		__timer_disable_traps(vcpu);
>  		__deactivate_traps(vcpu);
> diff --git a/arch/arm64/kvm/hyp/sysreg-sr.c b/arch/arm64/kvm/hyp/sysreg-sr.c
> index 2c17afd2be96..43b7dd65e3e6 100644
> --- a/arch/arm64/kvm/hyp/sysreg-sr.c
> +++ b/arch/arm64/kvm/hyp/sysreg-sr.c
> @@ -189,3 +189,8 @@ void __hyp_text __sysreg32_restore_state(struct kvm_vcpu *vcpu)
>  	if (vcpu->arch.debug_flags & KVM_ARM64_DEBUG_DIRTY)
>  		write_sysreg(sysreg[DBGVCR32_EL2], dbgvcr32_el2);
>  }
> +
> +void __hyp_text __kvm_set_tpidr_el2(u64 tpidr_el2)
> +{
> +	asm("msr tpidr_el2, %0": : "r" (tpidr_el2));

The paranoid in me says that we'd want an ISB here if we can possibly
use tpidr_el2 on this path. If we had to manic, for example...

> +}
> -- 
> 2.14.2
> 

The above notwithstanding,

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

	M.

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