Re: [PATCH v2 19/21] arm64: KVM: Move most of the fault decoding to C

[Christoffer Dall]

On Mon, Jan 25, 2016 at 03:53:53PM +0000, Marc Zyngier wrote:
> The fault decoding process (including computing the IPA in the case
> of a permission fault) would be much better done in C code, as we
> have a reasonable infrastructure to deal with the VHE/non-VHE
> differences.
> 
> Let's move the whole thing to C, including the workaround for
> erratum 834220, and just patch the odd ESR_EL2 access remaining
> in hyp-entry.S.
> 
> Signed-off-by: Marc Zyngier <marc.zyngier@xxxxxxx>
> ---
>  arch/arm64/kernel/asm-offsets.c |  3 --
>  arch/arm64/kvm/hyp/hyp-entry.S  | 69 +++--------------------------------------
>  arch/arm64/kvm/hyp/switch.c     | 54 ++++++++++++++++++++++++++++++++
>  3 files changed, 59 insertions(+), 67 deletions(-)
> 
> diff --git a/arch/arm64/kernel/asm-offsets.c b/arch/arm64/kernel/asm-offsets.c
> index fffa4ac6..b0ab4e9 100644
> --- a/arch/arm64/kernel/asm-offsets.c
> +++ b/arch/arm64/kernel/asm-offsets.c
> @@ -110,9 +110,6 @@ int main(void)
>    DEFINE(CPU_USER_PT_REGS,	offsetof(struct kvm_regs, regs));
>    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_ESR_EL2,		offsetof(struct kvm_vcpu, arch.fault.esr_el2));
> -  DEFINE(VCPU_FAR_EL2,		offsetof(struct kvm_vcpu, arch.fault.far_el2));
> -  DEFINE(VCPU_HPFAR_EL2,	offsetof(struct kvm_vcpu, arch.fault.hpfar_el2));
>    DEFINE(VCPU_HOST_CONTEXT,	offsetof(struct kvm_vcpu, arch.host_cpu_context));
>  #endif
>  #ifdef CONFIG_CPU_PM
> diff --git a/arch/arm64/kvm/hyp/hyp-entry.S b/arch/arm64/kvm/hyp/hyp-entry.S
> index 9e0683f..213de52 100644
> --- a/arch/arm64/kvm/hyp/hyp-entry.S
> +++ b/arch/arm64/kvm/hyp/hyp-entry.S
> @@ -19,7 +19,6 @@
>  
>  #include <asm/alternative.h>
>  #include <asm/assembler.h>
> -#include <asm/asm-offsets.h>
>  #include <asm/cpufeature.h>
>  #include <asm/kvm_arm.h>
>  #include <asm/kvm_asm.h>
> @@ -67,7 +66,11 @@ ENDPROC(__vhe_hyp_call)
>  el1_sync:				// Guest trapped into EL2
>  	save_x0_to_x3
>  
> +alternative_if_not ARM64_HAS_VIRT_HOST_EXTN
>  	mrs	x1, esr_el2
> +alternative_else
> +	mrs	x1, esr_el1
> +alternative_endif

I suppose this is not technically part of what the patch description
says it does, but ok...

>  	lsr	x2, x1, #ESR_ELx_EC_SHIFT
>  
>  	cmp	x2, #ESR_ELx_EC_HVC64
> @@ -103,72 +106,10 @@ el1_trap:
>  	cmp	x2, #ESR_ELx_EC_FP_ASIMD
>  	b.eq	__fpsimd_guest_restore
>  
> -	cmp	x2, #ESR_ELx_EC_DABT_LOW
> -	mov	x0, #ESR_ELx_EC_IABT_LOW
> -	ccmp	x2, x0, #4, ne
> -	b.ne	1f		// Not an abort we care about
> -
> -	/* This is an abort. Check for permission fault */
> -alternative_if_not ARM64_WORKAROUND_834220
> -	and	x2, x1, #ESR_ELx_FSC_TYPE
> -	cmp	x2, #FSC_PERM
> -	b.ne	1f		// Not a permission fault
> -alternative_else
> -	nop			// Use the permission fault path to
> -	nop			// check for a valid S1 translation,
> -	nop			// regardless of the ESR value.
> -alternative_endif
> -
> -	/*
> -	 * Check for Stage-1 page table walk, which is guaranteed
> -	 * to give a valid HPFAR_EL2.
> -	 */
> -	tbnz	x1, #7, 1f	// S1PTW is set
> -
> -	/* Preserve PAR_EL1 */
> -	mrs	x3, par_el1
> -	stp	x3, xzr, [sp, #-16]!
> -
> -	/*
> -	 * Permission fault, HPFAR_EL2 is invalid.
> -	 * Resolve the IPA the hard way using the guest VA.
> -	 * Stage-1 translation already validated the memory access rights.
> -	 * As such, we can use the EL1 translation regime, and don't have
> -	 * to distinguish between EL0 and EL1 access.
> -	 */
> -	mrs	x2, far_el2
> -	at	s1e1r, x2
> -	isb
> -
> -	/* Read result */
> -	mrs	x3, par_el1
> -	ldp	x0, xzr, [sp], #16	// Restore PAR_EL1 from the stack
> -	msr	par_el1, x0
> -	tbnz	x3, #0, 3f		// Bail out if we failed the translation
> -	ubfx	x3, x3, #12, #36	// Extract IPA
> -	lsl	x3, x3, #4		// and present it like HPFAR
> -	b	2f
> -
> -1:	mrs	x3, hpfar_el2
> -	mrs	x2, far_el2
> -
> -2:	mrs	x0, tpidr_el2
> -	str	w1, [x0, #VCPU_ESR_EL2]
> -	str	x2, [x0, #VCPU_FAR_EL2]
> -	str	x3, [x0, #VCPU_HPFAR_EL2]
> -
> +	mrs	x0, tpidr_el2
>  	mov	x1, #ARM_EXCEPTION_TRAP
>  	b	__guest_exit
>  
> -	/*
> -	 * Translation failed. Just return to the guest and
> -	 * let it fault again. Another CPU is probably playing
> -	 * behind our back.
> -	 */
> -3:	restore_x0_to_x3
> -
> -	eret
> -
>  el1_irq:
>  	save_x0_to_x3
>  	mrs	x0, tpidr_el2
> diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
> index 0cadb7f..df2cce9 100644
> --- a/arch/arm64/kvm/hyp/switch.c
> +++ b/arch/arm64/kvm/hyp/switch.c
> @@ -15,6 +15,7 @@
>   * along with this program.  If not, see <http://www.gnu.org/licenses/>.
>   */
>  
> +#include <linux/types.h>
>  #include <asm/kvm_asm.h>
>  
>  #include "hyp.h"
> @@ -150,6 +151,55 @@ static void __hyp_text __vgic_restore_state(struct kvm_vcpu *vcpu)
>  	__vgic_call_restore_state()(vcpu);
>  }
>  
> +static hyp_alternate_value(__check_arm_834220,
> +			   false, true,
> +			   ARM64_WORKAROUND_834220);
> +
> +static bool __hyp_text __populate_fault_info(struct kvm_vcpu *vcpu)
> +{
> +	u64 esr = read_sysreg_el2(esr);
> +	u8 ec = esr >> ESR_ELx_EC_SHIFT;
> +	u64 hpfar, far;
> +
> +	vcpu->arch.fault.esr_el2 = esr;
> +
> +	if (ec != ESR_ELx_EC_DABT_LOW && ec != ESR_ELx_EC_IABT_LOW)
> +		return true;
> +
> +	far = read_sysreg_el2(far);
> +
> +	if (!(esr & ESR_ELx_S1PTW) &&
> +	    (__check_arm_834220() || (esr & ESR_ELx_FSC_TYPE) == FSC_PERM)) {

this is really hard to read.  How do you feel about putting the below
block into its own function and changing to something like this:

/*
 * The HPFAR can be invalid if the stage 2 fault did not happen during a
 * stage 1 page table walk (the ESR_EL2.S1PTW bit is clear) and one of
 * the two following cases are true:
 *   1. The fault was due to a permission fault
 *   2. The processor carries errata 834220
 *
 * Therefore, for all non S1PTW faults where we either have a permission
 * fault or the errata workaround is enabled, we resolve the IPA using
 * the AT instruction.
 */
if (!(esr & ESR_ELx_S1PTW) &&
    (__check_arm_834220() || (esr & ESR_ELx_FSC_TYPE) == FSC_PERM)) {
    	if (!__translate_far_to_ipa(&hpfar))
		return false; /* Translation failed, back to guest */
} else {
	hpfar = read_sysreg(hpfar_el2);
}

not sure if it helps that much, perhaps it's just complicated by nature.

> +		u64 par, tmp;
> +
> +		/*
> +		 * Permission fault, HPFAR_EL2 is invalid. Resolve the
> +		 * IPA the hard way using the guest VA.
> +		 * Stage-1 translation already validated the memory
> +		 * access rights. As such, we can use the EL1
> +		 * translation regime, and don't have to distinguish
> +		 * between EL0 and EL1 access.
> +		 */
> +		par = read_sysreg(par_el1);

in any cas I think we also need the comment about preserving par_el1
here, which is only something we do because we may return early, IIUC.

> +		asm volatile("at s1e1r, %0" : : "r" (far));
> +		isb();
> +
> +		tmp = read_sysreg(par_el1);
> +		write_sysreg(par, par_el1);
> +
> +		if (unlikely(tmp & 1))
> +			return false; /* Translation failed, back to guest */
> +

nit: add comment /* Convert PAR to HPFAR format */

> +		hpfar = ((tmp >> 12) & ((1UL << 36) - 1)) << 4;
> +	} else {
> +		hpfar = read_sysreg(hpfar_el2);
> +	}
> +
> +	vcpu->arch.fault.far_el2 = far;
> +	vcpu->arch.fault.hpfar_el2 = hpfar;
> +	return true;
> +}
> +
>  static int __hyp_text __guest_run(struct kvm_vcpu *vcpu)
>  {
>  	struct kvm_cpu_context *host_ctxt;
> @@ -181,9 +231,13 @@ static int __hyp_text __guest_run(struct kvm_vcpu *vcpu)
>  	__debug_restore_state(vcpu, kern_hyp_va(vcpu->arch.debug_ptr), guest_ctxt);
>  
>  	/* Jump in the fire! */
> +again:
>  	exit_code = __guest_enter(vcpu, host_ctxt);
>  	/* And we're baaack! */
>  
> +	if (exit_code == ARM_EXCEPTION_TRAP && !__populate_fault_info(vcpu))
> +		goto again;
> +
>  	fp_enabled = __fpsimd_enabled();
>  
>  	__sysreg_save_guest_state(guest_ctxt);
> -- 
> 2.1.4
> 
The good news are that I couldn't find any bugs in the code.

-Christoffer