Re: [PATCH 03/13] arm64: Convert hcalls to use ISS field

[Mark Rutland <mark.rutland@arm.com>]

Hi Geoff,

The general approach looks good to me, using the HVC immediate makes
this look far nicer to me. Hopefully Marc and Christoffer agree on that.

That said, I have some comments on the mechanics below.

On Tue, Sep 09, 2014 at 11:49:04PM +0100, Geoff Levand wrote:
> To allow for additional hcalls to be defined and to make the arm64 hcall API
> more consistent across exception vector routines change the hcall implementations
> to use the ISS field of the ESR_EL2 register to specify the hcall type.
> 
> The existing arm64 hcall implementations are limited in that they only allow
> for two distinct hcalls; with the x0 register either zero, or not zero.  Also,
> the API of the hyp-stub exception vector routines and the KVM exception vector
> routines differ; hyp-stub uses a non-zero value in x0 to implement
> __hyp_set_vectors, whereas KVM uses it to implement kvm_call_hyp.
> 
> Define three new preprocessor macros HVC_GET_VECTORS, HVC_SET_VECTORS and
> HVC_KVM_CALL_HYP and to be used as hcall type specifiers and convert the
> existing __hyp_get_vectors(), __hyp_set_vectors() and kvm_call_hyp() routines
> to use these new macros when executing and HVC call.  Also change the
> corresponding hyp-stub and KVM el1_sync exception vector routines to use these
> new macros.
> 
> Signed-off-by: Geoff Levand <geoff@infradead.org>
> ---
>  arch/arm64/include/asm/virt.h | 20 ++++++++++++++++++++
>  arch/arm64/kernel/hyp-stub.S  | 38 ++++++++++++++++++++++++++------------
>  arch/arm64/kvm/hyp.S          | 19 ++++++++++++-------
>  3 files changed, 58 insertions(+), 19 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/virt.h b/arch/arm64/include/asm/virt.h
> index 7a5df52..894fe53 100644
> --- a/arch/arm64/include/asm/virt.h
> +++ b/arch/arm64/include/asm/virt.h
> @@ -21,6 +21,26 @@
>  #define BOOT_CPU_MODE_EL1	(0xe11)
>  #define BOOT_CPU_MODE_EL2	(0xe12)
>  
> +/*
> + * HVC_GET_VECTORS - Return the value of the vbar_el2 register.
> + */
> +
> +#define HVC_GET_VECTORS 1
> +
> +/*
> + * HVC_SET_VECTORS - Set the value of the vbar_el2 register.
> + *
> + * @x0: Physical address of the new vector table.
> + */
> +
> +#define HVC_SET_VECTORS 2
> +
> +/*
> + * HVC_KVM_CALL_HYP - Execute kvm_call_hyp routine.
> + */
> +
> +#define HVC_KVM_CALL_HYP 3

If this can be used without KVM (e.g. in the hyp stub) I'd just call
this HVC_CALL_HYP, or the name will be a little misleading.

> +
>  #ifndef __ASSEMBLY__
>  
>  /*
> diff --git a/arch/arm64/kernel/hyp-stub.S b/arch/arm64/kernel/hyp-stub.S
> index 2d960a9..9ab5f70 100644
> --- a/arch/arm64/kernel/hyp-stub.S
> +++ b/arch/arm64/kernel/hyp-stub.S
> @@ -54,16 +54,29 @@ ENDPROC(__hyp_stub_vectors)
>  
>  #define ESR_EL2_EC_SHIFT	26
>  #define ESR_EL2_EC_HVC64	0x16
> +#define ESR_EL2_ISS		0xffff

The last patch tried to add an identical macro to a header file. Can we
use that header please?

>  
>  el1_sync:
> -	mrs	x1, esr_el2
> -	lsr	x1, x1, #ESR_EL2_EC_SHIFT
> -	cmp	x1, #ESR_EL2_EC_HVC64
> -	b.ne	2f				// Not an HVC trap
> -	cbz	x0, 1f
> -	msr	vbar_el2, x0			// Set vbar_el2
> +	mrs	x10, esr_el2

Any reason for using x10?

If we want to preserve the lowest register numbers, start with the
highest caller-saved register numbers (i.e. x18). At least for me it
makes the code far easier to read; it doesn't make it look like x10 is
special.

> +	lsr	x9, x10, #ESR_EL2_EC_SHIFT	// x9=EC
> +	and	x10, x10, #ESR_EL2_ISS		// x10=ISS

The mnemonics make these comments redundant.

> +	cmp     x9, #ESR_EL2_EC_HVC64
> +	b.ne    2f                              // Not a host HVC trap

Now that we have the nice mnemonic, we could get rid of the comment
here. I'd drop the 'host' from the comment; it wasn't there orginally
and it's somewhat meaningless for the stub (KVM isn't up yet, and the
only the native OS can make a HVC).

> +	mrs     x9, vttbr_el2
> +	cbnz    x9, 2f                          // Not a host HVC trap

I don't understand this. When is vttbr_el2 non-zero, and why do we want
to silently return from a HVC in that case? That didn't seem to be the
case in the original code.

> +
> +	cmp	x10, #HVC_GET_VECTORS
> +	b.ne	1f
> +	mrs	x0, vbar_el2
>  	b	2f
> -1:	mrs	x0, vbar_el2			// Return vbar_el2
> +
> +1:	cmp	x10, #HVC_SET_VECTORS
> +	b.ne	1f
> +	msr	vbar_el2, x0
> +
> +1:

It feels like we should explode if we ever reach here from the host --
if we've made an unsupported HVC wereally want to know that we've done
so.

>  2:	eret
>  ENDPROC(el1_sync)
>  
> @@ -103,11 +116,12 @@ ENDPROC(\label)
>   * initialisation entry point.
>   */
>  
> -ENTRY(__hyp_get_vectors)
> -	mov	x0, xzr
> -	// fall through
>  ENTRY(__hyp_set_vectors)
> -	hvc	#0
> +	hvc	#HVC_SET_VECTORS
>  	ret
> -ENDPROC(__hyp_get_vectors)
>  ENDPROC(__hyp_set_vectors)
> +
> +ENTRY(__hyp_get_vectors)
> +	hvc	#HVC_GET_VECTORS
> +	ret
> +ENDPROC(__hyp_get_vectors)
> diff --git a/arch/arm64/kvm/hyp.S b/arch/arm64/kvm/hyp.S
> index b72aa9f..3972ee9 100644
> --- a/arch/arm64/kvm/hyp.S
> +++ b/arch/arm64/kvm/hyp.S
> @@ -26,6 +26,7 @@
>  #include <asm/kvm_asm.h>
>  #include <asm/kvm_arm.h>
>  #include <asm/kvm_mmu.h>
> +#include <asm/virt.h>
>  
>  #define CPU_GP_REG_OFFSET(x)	(CPU_GP_REGS + x)
>  #define CPU_XREG_OFFSET(x)	CPU_GP_REG_OFFSET(CPU_USER_PT_REGS + 8*x)
> @@ -1105,12 +1106,9 @@ __hyp_panic_str:
>   * in Hyp mode (see init_hyp_mode in arch/arm/kvm/arm.c).  Return values are
>   * passed in r0 and r1.
>   *
> - * A function pointer with a value of 0 has a special meaning, and is
> - * used to implement __hyp_get_vectors in the same way as in
> - * arch/arm64/kernel/hyp_stub.S.
>   */
>  ENTRY(kvm_call_hyp)
> -	hvc	#0
> +	hvc	#HVC_KVM_CALL_HYP
>  	ret
>  ENDPROC(kvm_call_hyp)
>  
> @@ -1140,6 +1138,7 @@ el1_sync:					// Guest trapped into EL2
>  	push	x2, x3
>  
>  	mrs	x1, esr_el2
> +	and	x0, x1, #ESR_EL2_ISS
>  	lsr	x2, x1, #ESR_EL2_EC_SHIFT
>  
>  	cmp	x2, #ESR_EL2_EC_HVC64
> @@ -1149,15 +1148,19 @@ el1_sync:					// Guest trapped into EL2
>  	cbnz	x3, el1_trap			// called HVC
>  
>  	/* Here, we're pretty sure the host called HVC. */
> +	mov	x10, x0

As above, please use the highest numbered caller-saved register so as to
not make the register numbering look special.

>  	pop	x2, x3
>  	pop	x0, x1
>  
> -	/* Check for __hyp_get_vectors */
> -	cbnz	x0, 1f
> +	cmp	x10, #HVC_GET_VECTORS
> +	b.ne	1f
>  	mrs	x0, vbar_el2
>  	b	2f
>  
> -1:	push	lr, xzr
> +1:	cmp	x10, #HVC_KVM_CALL_HYP
> +	b.ne	1f
> +
> +	push	lr, xzr
>  
>  	/*
>  	 * Compute the function address in EL2, and shuffle the parameters.
> @@ -1170,6 +1173,8 @@ el1_sync:					// Guest trapped into EL2
>  	blr	lr
>  
>  	pop	lr, xzr
> +
> +1:
>  2:	eret

Any reason we need two labels here?

If we've got here with an invalid HVC immediate, I think we should
explode loudly.

Mark.

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