Re: [PATCH V11 10/10] arm/arm64: KVM: add guest SEA support

[James Morse <james.morse@arm.com>]

Hi Tyler,

On 21/02/17 21:22, Tyler Baicar wrote:
> Currently external aborts are unsupported by the guest abort
> handling. Add handling for SEAs so that the host kernel reports
> SEAs which occur in the guest kernel.

> diff --git a/arch/arm/include/asm/kvm_arm.h b/arch/arm/include/asm/kvm_arm.h
> index e22089f..33a77509 100644
> --- a/arch/arm/include/asm/kvm_arm.h
> +++ b/arch/arm/include/asm/kvm_arm.h
> @@ -187,6 +187,7 @@
>  #define FSC_FAULT	(0x04)
>  #define FSC_ACCESS	(0x08)
>  #define FSC_PERM	(0x0c)
> +#define FSC_EXTABT	(0x10)

arm64 has ESR_ELx_FSC_EXTABT which is used in inject_abt64(), but for matching
an external abort coming from hardware the range is wider.

Looking at the ARM-ARMs 'ISS encoding for an exception from an Instruction
Abort' in 'D7.2.27 ESR_ELx, Exception Syndrome Register (ELx)' (page D7-1954 of
version 'k'...iss10775), the ten flavours of you Synchronous abort you hooked
with do_sea() in patch 4 occupy 0x10 to 0x1f...


> diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
> index a5265ed..04f1dd50 100644
> --- a/arch/arm/kvm/mmu.c
> +++ b/arch/arm/kvm/mmu.c
> @@ -29,6 +29,7 @@
>  #include <asm/kvm_asm.h>
>  #include <asm/kvm_emulate.h>
>  #include <asm/virt.h>
> +#include <asm/system_misc.h>
>  
>  #include "trace.h"
>  
> @@ -1444,8 +1445,21 @@ int kvm_handle_guest_abort(struct kvm_vcpu *vcpu, struct kvm_run *run)
>  
>  	/* Check the stage-2 fault is trans. fault or write fault */
>  	fault_status = kvm_vcpu_trap_get_fault_type(vcpu);

... kvm_vcpu_trap_get_fault_type() on both arm and arm64 masks the HSR/ESR_EL2
with 0x3c ...


> -	if (fault_status != FSC_FAULT && fault_status != FSC_PERM &&
> -	    fault_status != FSC_ACCESS) {
> +
> +	/* The host kernel will handle the synchronous external abort. There
> +	 * is no need to pass the error into the guest.
> +	 */
> +	if (fault_status == FSC_EXTABT) {

... but here we only check for 'Synchronous external abort, not on a translation
table walk'. Are the other types relevant?

If so we need some helper as this range is sparse and 'all other values are
reserved'. The aarch32 HSR format is slightly different. (G6-4411 ISS encoding
from an exception from a Data Abort).

If not, can we change patch 4 to check this type too so we don't call out to
APEI for a fault type we know isn't relevant.


> +		if(handle_guest_sea((unsigned long)fault_ipa,
> +				    kvm_vcpu_get_hsr(vcpu))) {
> +			kvm_err("Failed to handle guest SEA, FSC: EC=%#x xFSC=%#lx ESR_EL2=%#lx\n",
> +				kvm_vcpu_trap_get_class(vcpu),
> +				(unsigned long)kvm_vcpu_trap_get_fault(vcpu),
> +				(unsigned long)kvm_vcpu_get_hsr(vcpu));
> +			return -EFAULT;
> +		}
> +	} else if (fault_status != FSC_FAULT && fault_status != FSC_PERM &&
> +		   fault_status != FSC_ACCESS) {
>  		kvm_err("Unsupported FSC: EC=%#x xFSC=%#lx ESR_EL2=%#lx\n",
>  			kvm_vcpu_trap_get_class(vcpu),
>  			(unsigned long)kvm_vcpu_trap_get_fault(vcpu),

> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> index b2d57fc..403277b 100644
> --- a/arch/arm64/mm/fault.c
> +++ b/arch/arm64/mm/fault.c
> @@ -602,6 +602,24 @@ static const char *fault_name(unsigned int esr)
>  }
>
>  /*
> + * Handle Synchronous External Aborts that occur in a guest kernel.
> + */
> +int handle_guest_sea(unsigned long addr, unsigned int esr)
> +{

> +	if(IS_ENABLED(HAVE_ACPI_APEI_SEA)) {
> +		nmi_enter();
> +		ghes_notify_sea();
> +		nmi_exit();

This nmi stuff was needed for synchronous aborts that may have interrupted
APEI's interrupts-masked code. We want to avoid trying to take the same set of
locks, hence taking the in_nmi() path through APEI. Here we know we interrupted
a guest, so there is no risk that we have interrupted APEI on the host.
ghes_notify_sea() can safely take the normal path.


Thanks,

James