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

Marc Zyngier
2021-10-29T11:17:00Z

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.