[PATCH v3 19/20] KVM: arm64: Handle RAS SErrors from EL2 on guest exit

[Marc Zyngier]

On Thu, Oct 05 2017 at  8:18:11 pm BST, James Morse <james.morse at arm.com> wrote:
> We expect to have firmware-first handling of RAS SErrors, with errors
> notified via an APEI method. For systems without firmware-first, add
> some minimal handling to KVM.
>
> There are two ways KVM can take an SError due to a guest, either may be a
> RAS error: we exit the guest due to an SError routed to EL2 by HCR_EL2.AMO,
> or we take an SError from EL2 when we unmask PSTATE.A from __guest_exit.
>
> The current SError from EL2 code unmasks SError and tries to fence any
> pending SError into a single instruction window. It then leaves SError
> unmasked.
>
> With the v8.2 RAS Extensions we may take an SError for a 'corrected'
> error, but KVM is only able to handle SError from EL2 if they occur
> during this single instruction window...
>
> The RAS Extensions give us a new instruction to synchronise and
> consume SErrors. The RAS Extensions document (ARM DDI0587),
> '2.4.1 ESB and Unrecoverable errors' describes ESB as synchronising
> SError interrupts generated by 'instructions, translation table walks,
> hardware updates to the translation tables, and instruction fetches on
> the same PE'. This makes ESB equivalent to KVMs existing
> 'dsb, mrs-daifclr, isb' sequence.
>
> Use the alternatives to synchronise and consume any SError using ESB
> instead of unmasking and taking the SError. Set ARM_EXIT_WITH_SERROR_BIT
> in the exit_code so that we can restart the vcpu if it turns out this
> SError has no impact on the vcpu.
>
> Signed-off-by: James Morse <james.morse at arm.com>
> ---
>  arch/arm64/include/asm/kvm_emulate.h |  5 +++++
>  arch/arm64/include/asm/kvm_host.h    |  1 +
>  arch/arm64/kernel/asm-offsets.c      |  1 +
>  arch/arm64/kvm/handle_exit.c         | 10 +++++++++-
>  arch/arm64/kvm/hyp/entry.S           | 13 +++++++++++++
>  5 files changed, 29 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/include/asm/kvm_emulate.h b/arch/arm64/include/asm/kvm_emulate.h
> index 8a7a838eb17a..8274d16df3cd 100644
> --- a/arch/arm64/include/asm/kvm_emulate.h
> +++ b/arch/arm64/include/asm/kvm_emulate.h
> @@ -173,6 +173,11 @@ static inline phys_addr_t kvm_vcpu_get_fault_ipa(const struct kvm_vcpu *vcpu)
>  	return ((phys_addr_t)vcpu->arch.fault.hpfar_el2 & HPFAR_MASK) << 8;
>  }
>  
> +static inline u64 kvm_vcpu_get_disr(const struct kvm_vcpu *vcpu)
> +{
> +	return vcpu->arch.fault.disr_el1;
> +}
> +
>  static inline u32 kvm_vcpu_hvc_get_imm(const struct kvm_vcpu *vcpu)
>  {
>  	return kvm_vcpu_get_hsr(vcpu) & ESR_ELx_xVC_IMM_MASK;
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 9564e6d56b79..b3c53d8b6a60 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -89,6 +89,7 @@ struct kvm_vcpu_fault_info {
>  	u32 esr_el2;		/* Hyp Syndrom Register */
>  	u64 far_el2;		/* Hyp Fault Address Register */
>  	u64 hpfar_el2;		/* Hyp IPA Fault Address Register */
> +	u64 disr_el1;		/* Deferred [SError] Status Register */
>  };
>  
>  /*
> diff --git a/arch/arm64/kernel/asm-offsets.c b/arch/arm64/kernel/asm-offsets.c
> index 71bf088f1e4b..121889c49542 100644
> --- a/arch/arm64/kernel/asm-offsets.c
> +++ b/arch/arm64/kernel/asm-offsets.c
> @@ -130,6 +130,7 @@ int main(void)
>    BLANK();
>  #ifdef CONFIG_KVM_ARM_HOST
>    DEFINE(VCPU_CONTEXT,		offsetof(struct kvm_vcpu, arch.ctxt));
> +  DEFINE(VCPU_FAULT_DISR,	offsetof(struct kvm_vcpu, arch.fault.disr_el1));
>    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));
> diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
> index 345fdbba6c2e..e1e6cfe7d4d9 100644
> --- a/arch/arm64/kvm/handle_exit.c
> +++ b/arch/arm64/kvm/handle_exit.c
> @@ -23,6 +23,7 @@
>  #include <linux/kvm_host.h>
>  
>  #include <asm/esr.h>
> +#include <asm/exception.h>
>  #include <asm/kvm_asm.h>
>  #include <asm/kvm_coproc.h>
>  #include <asm/kvm_emulate.h>
> @@ -208,7 +209,14 @@ int handle_exit(struct kvm_vcpu *vcpu, struct kvm_run *run,
>  			*vcpu_pc(vcpu) -= adj;
>  		}
>  
> -		kvm_inject_vabt(vcpu);
> +		if (cpus_have_const_cap(ARM64_HAS_RAS_EXTN)) {
> +			u64 disr = kvm_vcpu_get_disr(vcpu);
> +
> +			kvm_handle_guest_serror(vcpu, disr_to_esr(disr));
> +		} else {
> +			kvm_inject_vabt(vcpu);
> +		}
> +
>  		return 1;
>  	}
>  
> diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S
> index 12ee62d6d410..96caa5328b3a 100644
> --- a/arch/arm64/kvm/hyp/entry.S
> +++ b/arch/arm64/kvm/hyp/entry.S
> @@ -124,6 +124,17 @@ ENTRY(__guest_exit)
>  	// Now restore the host regs
>  	restore_callee_saved_regs x2
>  
> +alternative_if ARM64_HAS_RAS_EXTN
> +	// If we have the RAS extensions we can consume a pending error
> +	// without an unmask-SError and isb.
> +	esb
> +	mrs_s	x2, SYS_DISR_EL1
> +	str	x2, [x1, #(VCPU_FAULT_DISR - VCPU_CONTEXT)]
> +	cbz	x2, 1f
> +	msr_s	SYS_DISR_EL1, xzr
> +	orr	x0, x0, #(1<<ARM_EXIT_WITH_SERROR_BIT)
> +1:	ret
> +alternative_else
>  	// If we have a pending asynchronous abort, now is the
>  	// time to find out. From your VAXorcist book, page 666:
>  	// "Threaten me not, oh Evil one!  For I speak with
> @@ -135,6 +146,8 @@ ENTRY(__guest_exit)
>  
>  	dsb	sy		// Synchronize against in-flight ld/st
>  	msr	daifclr, #4	// Unmask aborts
> +	nop

Oops. You've now introduced an instruction in what was supposed to be a
single instruction window (the isb). It means that we may fail to
identify the Serror as having been generated by our synchronisation
mechanism, and we'll panic for no good reason.

Moving the nop up will solve this.

> +alternative_endif
>  
>  	// This is our single instruction exception window. A pending
>  	// SError is guaranteed to occur at the earliest when we unmask

Thanks,

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