Re: [PATCH v4 04/21] arm64: Provide an 'upgrade to VHE' stub hypercall

[David Brazdil <dbrazdil@google.com>]

On Mon, Jan 18, 2021 at 09:45:16AM +0000, Marc Zyngier wrote:
> As we are about to change the way a VHE system boots, let's
> provide the core helper, in the form of a stub hypercall that
> enables VHE and replicates the full EL1 context at EL2, thanks
> to EL1 and VHE-EL2 being extremely similar.
> 
> On exception return, the kernel carries on at EL2. Fancy!
> 
> Nothing calls this new hypercall yet, so no functional change.
> 
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> ---
>  arch/arm64/include/asm/virt.h |  7 +++-
>  arch/arm64/kernel/hyp-stub.S  | 67 +++++++++++++++++++++++++++++++++--
>  2 files changed, 71 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/virt.h b/arch/arm64/include/asm/virt.h
> index ee6a48df89d9..7379f35ae2c6 100644
> --- a/arch/arm64/include/asm/virt.h
> +++ b/arch/arm64/include/asm/virt.h
> @@ -35,8 +35,13 @@
>   */
>  #define HVC_RESET_VECTORS 2
>  
> +/*
> + * HVC_VHE_RESTART - Upgrade the CPU from EL1 to EL2, if possible
> + */
> +#define HVC_VHE_RESTART	3
> +
>  /* Max number of HYP stub hypercalls */
> -#define HVC_STUB_HCALL_NR 3
> +#define HVC_STUB_HCALL_NR 4
>  
>  /* Error returned when an invalid stub number is passed into x0 */
>  #define HVC_STUB_ERR	0xbadca11
> diff --git a/arch/arm64/kernel/hyp-stub.S b/arch/arm64/kernel/hyp-stub.S
> index 160f5881a0b7..fb12398b5c28 100644
> --- a/arch/arm64/kernel/hyp-stub.S
> +++ b/arch/arm64/kernel/hyp-stub.S
> @@ -8,9 +8,9 @@
>  
>  #include <linux/init.h>
>  #include <linux/linkage.h>
> -#include <linux/irqchip/arm-gic-v3.h>
>  
>  #include <asm/assembler.h>
> +#include <asm/el2_setup.h>
>  #include <asm/kvm_arm.h>
>  #include <asm/kvm_asm.h>
>  #include <asm/ptrace.h>
> @@ -47,10 +47,13 @@ SYM_CODE_END(__hyp_stub_vectors)
>  
>  SYM_CODE_START_LOCAL(el1_sync)
>  	cmp	x0, #HVC_SET_VECTORS
> -	b.ne	2f
> +	b.ne	1f
>  	msr	vbar_el2, x1
>  	b	9f
>  
> +1:	cmp	x0, #HVC_VHE_RESTART
> +	b.eq	mutate_to_vhe
> +
>  2:	cmp	x0, #HVC_SOFT_RESTART
>  	b.ne	3f
>  	mov	x0, x2
> @@ -70,6 +73,66 @@ SYM_CODE_START_LOCAL(el1_sync)
>  	eret
>  SYM_CODE_END(el1_sync)
>  
> +// nVHE? No way! Give me the real thing!
> +SYM_CODE_START_LOCAL(mutate_to_vhe)
> +	// Sanity check: MMU *must* be off
> +	mrs	x0, sctlr_el2
> +	tbnz	x0, #0, 1f
> +
> +	// Needs to be VHE capable, obviously
> +	mrs	x0, id_aa64mmfr1_el1
> +	ubfx	x0, x0, #ID_AA64MMFR1_VHE_SHIFT, #4
> +	cbz	x0, 1f

nit: There is a HVC_STUB_ERR that you could return if these sanity checks fail.
The documentation also states that it should be returned on error.

> +
> +	// Engage the VHE magic!
> +	mov_q	x0, HCR_HOST_VHE_FLAGS
> +	msr	hcr_el2, x0
> +	isb
> +
> +	// Doesn't do much on VHE, but still, worth a shot
> +	init_el2_state vhe
> +
> +	// Use the EL1 allocated stack, per-cpu offset
> +	mrs	x0, sp_el1
> +	mov	sp, x0
> +	mrs	x0, tpidr_el1
> +	msr	tpidr_el2, x0
> +
> +	// FP configuration, vectors
> +	mrs_s	x0, SYS_CPACR_EL12
> +	msr	cpacr_el1, x0
> +	mrs_s	x0, SYS_VBAR_EL12
> +	msr	vbar_el1, x0
> +
> +	// Transfer the MM state from EL1 to EL2
> +	mrs_s	x0, SYS_TCR_EL12
> +	msr	tcr_el1, x0
> +	mrs_s	x0, SYS_TTBR0_EL12
> +	msr	ttbr0_el1, x0
> +	mrs_s	x0, SYS_TTBR1_EL12
> +	msr	ttbr1_el1, x0
> +	mrs_s	x0, SYS_MAIR_EL12
> +	msr	mair_el1, x0
> +	isb
> +
> +	// Invalidate TLBs before enabling the MMU
> +	tlbi	vmalle1
> +	dsb	nsh
> +
> +	// Enable the EL2 S1 MMU, as set up from EL1
> +	mrs_s	x0, SYS_SCTLR_EL12
> +	set_sctlr_el1	x0
> +
> +	// Hack the exception return to stay at EL2
> +	mrs	x0, spsr_el1
> +	and	x0, x0, #~PSR_MODE_MASK
> +	mov	x1, #PSR_MODE_EL2h
> +	orr	x0, x0, x1
> +	msr	spsr_el1, x0
> +
> +1:	eret
> +SYM_CODE_END(mutate_to_vhe)
> +
>  .macro invalid_vector	label
>  SYM_CODE_START_LOCAL(\label)
>  	b \label
> -- 
> 2.29.2
> 
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