Re: [PATCH 2/3] KVM: arm64: Allow the guest to change the OS Lock status

[Marc Zyngier <maz@kernel.org>]

Hi Oliver,

On Fri, 29 Oct 2021 01:32:01 +0100,
Oliver Upton <oupton@google.com> wrote:
> 
> KVM diverges from the architecture in the way it handles the OSLAR_EL1
> register. While the architecture requires that the register be WO and
> that the OSLK bit is 1 out of reset, KVM implements the register as
> RAZ/WI.
> 
> Align KVM with the architecture by permitting writes to OSLAR_EL1. Since
> the register is WO, stash the OS Lock status bit in OSLSR_EL1 and
> context switch the status between host/guest. Additionally, change the
> reset value of the OSLK bit to 1.
> 
> Suggested-by: Marc Zyngier <maz@kernel.org>
> Signed-off-by: Oliver Upton <oupton@google.com>
> ---
>  arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h |  5 +++++
>  arch/arm64/kvm/sys_regs.c                  | 22 +++++++++++++++++++---
>  2 files changed, 24 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h b/arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h
> index de7e14c862e6..a65dab34f85b 100644
> --- a/arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h
> +++ b/arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h
> @@ -65,6 +65,8 @@ static inline void __sysreg_save_el1_state(struct kvm_cpu_context *ctxt)
>  	ctxt_sys_reg(ctxt, SP_EL1)	= read_sysreg(sp_el1);
>  	ctxt_sys_reg(ctxt, ELR_EL1)	= read_sysreg_el1(SYS_ELR);
>  	ctxt_sys_reg(ctxt, SPSR_EL1)	= read_sysreg_el1(SYS_SPSR);
> +
> +	ctxt_sys_reg(ctxt, OSLSR_EL1)	= read_sysreg(oslsr_el1);

Why do we need to save/restore this outside of the debug context? It
seems to me that this is only needed if debug has been enabled by the
guest (KVM_ARM64_DEBUG_DIRTY being set), as we will have trapped the
OSLAR_EL1 access otherwise. I don't think we need to deal with this
register outside of this context, as debug exceptions cannot happen
otherwise (BRK excepted, of course).

>  }
>  
>  static inline void __sysreg_save_el2_return_state(struct kvm_cpu_context *ctxt)
> @@ -149,6 +151,9 @@ static inline void __sysreg_restore_el1_state(struct kvm_cpu_context *ctxt)
>  	write_sysreg(ctxt_sys_reg(ctxt, SP_EL1),	sp_el1);
>  	write_sysreg_el1(ctxt_sys_reg(ctxt, ELR_EL1),	SYS_ELR);
>  	write_sysreg_el1(ctxt_sys_reg(ctxt, SPSR_EL1),	SYS_SPSR);
> +
> +	/* restore OSLSR_EL1 by writing the OSLK bit to OSLAR_EL1 */
> +	write_sysreg((ctxt_sys_reg(ctxt, OSLSR_EL1) >> 1) & 1, oslar_el1);

Please introduce some eye-pleasing bit definitions ("Here, there, and
everywhere", to quote someone famous).

>  }
>  
>  static inline void __sysreg_restore_el2_return_state(struct kvm_cpu_context *ctxt)
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 0eb03e7508fe..0840ae081290 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -298,6 +298,22 @@ static bool trap_oslsr_el1(struct kvm_vcpu *vcpu,
>  	return true;
>  }
>  
> +static bool trap_oslar_el1(struct kvm_vcpu *vcpu,
> +			   struct sys_reg_params *p,
> +			   const struct sys_reg_desc *r)
> +{
> +	u64 oslsr;
> +
> +	if (!p->is_write)
> +		return read_zero(vcpu, p);

This really should be an UNDEF (and it really should UNDEF in HW, but
we are being, maybe pointlessly, cautious).

> +
> +	/* preserve all but the OSLK bit */
> +	oslsr = vcpu_read_sys_reg(vcpu, OSLSR_EL1) & ~0x2ull;
> +	vcpu_write_sys_reg(vcpu, OSLSR_EL1, oslsr | ((p->regval & 1) << 1));
> +	return true;
> +}
> +
> +

Extra newline.

>  static bool trap_dbgauthstatus_el1(struct kvm_vcpu *vcpu,
>  				   struct sys_reg_params *p,
>  				   const struct sys_reg_desc *r)
> @@ -1439,8 +1455,8 @@ static const struct sys_reg_desc sys_reg_descs[] = {
>  	DBG_BCR_BVR_WCR_WVR_EL1(15),
>  
>  	{ SYS_DESC(SYS_MDRAR_EL1), trap_raz_wi },
> -	{ SYS_DESC(SYS_OSLAR_EL1), trap_raz_wi },
> -	{ SYS_DESC(SYS_OSLSR_EL1), trap_oslsr_el1, reset_val, OSLSR_EL1, 0x00000008 },
> +	{ SYS_DESC(SYS_OSLAR_EL1), trap_oslar_el1 },
> +	{ SYS_DESC(SYS_OSLSR_EL1), trap_oslsr_el1, reset_val, OSLSR_EL1, 0x0000000A },
>  	{ SYS_DESC(SYS_OSDLR_EL1), trap_raz_wi },
>  	{ SYS_DESC(SYS_DBGPRCR_EL1), trap_raz_wi },
>  	{ SYS_DESC(SYS_DBGCLAIMSET_EL1), trap_raz_wi },
> @@ -1912,7 +1928,7 @@ static const struct sys_reg_desc cp14_regs[] = {
>  
>  	DBGBXVR(0),
>  	/* DBGOSLAR */
> -	{ Op1( 0), CRn( 1), CRm( 0), Op2( 4), trap_raz_wi },
> +	{ Op1( 0), CRn( 1), CRm( 0), Op2( 4), trap_oslar_el1 },
>  	DBGBXVR(1),
>  	/* DBGOSLSR */
>  	{ Op1( 0), CRn( 1), CRm( 1), Op2( 4), trap_oslsr_el1, NULL, OSLSR_EL1 },

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.
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