Re: [PATCH v12 5/8] arm64: kvm: Save/restore MTE registers

[Marc Zyngier]

On Mon, 17 May 2021 13:32:36 +0100,
Steven Price <steven.price@arm.com> wrote:
> 
> Define the new system registers that MTE introduces and context switch
> them. The MTE feature is still hidden from the ID register as it isn't
> supported in a VM yet.
> 
> Signed-off-by: Steven Price <steven.price@arm.com>
> ---
>  arch/arm64/include/asm/kvm_host.h          |  6 ++
>  arch/arm64/include/asm/kvm_mte.h           | 66 ++++++++++++++++++++++
>  arch/arm64/include/asm/sysreg.h            |  3 +-
>  arch/arm64/kernel/asm-offsets.c            |  3 +
>  arch/arm64/kvm/hyp/entry.S                 |  7 +++
>  arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h | 21 +++++++
>  arch/arm64/kvm/sys_regs.c                  | 22 ++++++--
>  7 files changed, 123 insertions(+), 5 deletions(-)
>  create mode 100644 arch/arm64/include/asm/kvm_mte.h
> 
> diff --git a/arch/arm64/include/asm/kvm_host.h 
> b/arch/arm64/include/asm/kvm_host.h
> index afaa5333f0e4..309e36cc1b42 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -208,6 +208,12 @@ enum vcpu_sysreg {
>       CNTP_CVAL_EL0,
>       CNTP_CTL_EL0,
>  
> +     /* Memory Tagging Extension registers */
> +     RGSR_EL1,       /* Random Allocation Tag Seed Register */
> +     GCR_EL1,        /* Tag Control Register */
> +     TFSR_EL1,       /* Tag Fault Status Register (EL1) */
> +     TFSRE0_EL1,     /* Tag Fault Status Register (EL0) */
> +
>       /* 32bit specific registers. Keep them at the end of the range */
>       DACR32_EL2,     /* Domain Access Control Register */
>       IFSR32_EL2,     /* Instruction Fault Status Register */
> diff --git a/arch/arm64/include/asm/kvm_mte.h 
> b/arch/arm64/include/asm/kvm_mte.h
> new file mode 100644
> index 000000000000..6541c7d6ce06
> --- /dev/null
> +++ b/arch/arm64/include/asm/kvm_mte.h
> @@ -0,0 +1,66 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * Copyright (C) 2020 ARM Ltd.
> + */
> +#ifndef __ASM_KVM_MTE_H
> +#define __ASM_KVM_MTE_H
> +
> +#ifdef __ASSEMBLY__
> +
> +#include <asm/sysreg.h>
> +
> +#ifdef CONFIG_ARM64_MTE
> +
> +.macro mte_switch_to_guest g_ctxt, h_ctxt, reg1
> +alternative_if_not ARM64_MTE
> +     b       .L__skip_switch\@
> +alternative_else_nop_endif
> +     mrs     \reg1, hcr_el2
> +     and     \reg1, \reg1, #(HCR_ATA)
> +     cbz     \reg1, .L__skip_switch\@
> +
> +     mrs_s   \reg1, SYS_RGSR_EL1
> +     str     \reg1, [\h_ctxt, #CPU_RGSR_EL1]
> +     mrs_s   \reg1, SYS_GCR_EL1
> +     str     \reg1, [\h_ctxt, #CPU_GCR_EL1]
> +
> +     ldr     \reg1, [\g_ctxt, #CPU_RGSR_EL1]
> +     msr_s   SYS_RGSR_EL1, \reg1
> +     ldr     \reg1, [\g_ctxt, #CPU_GCR_EL1]
> +     msr_s   SYS_GCR_EL1, \reg1
> +
> +.L__skip_switch\@:
> +.endm
> +
> +.macro mte_switch_to_hyp g_ctxt, h_ctxt, reg1
> +alternative_if_not ARM64_MTE
> +     b       .L__skip_switch\@
> +alternative_else_nop_endif
> +     mrs     \reg1, hcr_el2
> +     and     \reg1, \reg1, #(HCR_ATA)
> +     cbz     \reg1, .L__skip_switch\@
> +
> +     mrs_s   \reg1, SYS_RGSR_EL1
> +     str     \reg1, [\g_ctxt, #CPU_RGSR_EL1]
> +     mrs_s   \reg1, SYS_GCR_EL1
> +     str     \reg1, [\g_ctxt, #CPU_GCR_EL1]
> +
> +     ldr     \reg1, [\h_ctxt, #CPU_RGSR_EL1]
> +     msr_s   SYS_RGSR_EL1, \reg1
> +     ldr     \reg1, [\h_ctxt, #CPU_GCR_EL1]
> +     msr_s   SYS_GCR_EL1, \reg1

What is the rational for not having any synchronisation here? It is
quite uncommon to allocate memory at EL2, but VHE can perform all kind
of tricks.

> +
> +.L__skip_switch\@:
> +.endm
> +
> +#else /* CONFIG_ARM64_MTE */
> +
> +.macro mte_switch_to_guest g_ctxt, h_ctxt, reg1
> +.endm
> +
> +.macro mte_switch_to_hyp g_ctxt, h_ctxt, reg1
> +.endm
> +
> +#endif /* CONFIG_ARM64_MTE */
> +#endif /* __ASSEMBLY__ */
> +#endif /* __ASM_KVM_MTE_H */
> diff --git a/arch/arm64/include/asm/sysreg.h b/arch/arm64/include/asm/sysreg.h
> index 65d15700a168..347ccac2341e 100644
> --- a/arch/arm64/include/asm/sysreg.h
> +++ b/arch/arm64/include/asm/sysreg.h
> @@ -651,7 +651,8 @@
>  
>  #define INIT_SCTLR_EL2_MMU_ON                                                
> \
>       (SCTLR_ELx_M  | SCTLR_ELx_C | SCTLR_ELx_SA | SCTLR_ELx_I |      \
> -      SCTLR_ELx_IESB | SCTLR_ELx_WXN | ENDIAN_SET_EL2 | SCTLR_EL2_RES1)
> +      SCTLR_ELx_IESB | SCTLR_ELx_WXN | ENDIAN_SET_EL2 |              \
> +      SCTLR_ELx_ITFSB | SCTLR_EL2_RES1)
>  
>  #define INIT_SCTLR_EL2_MMU_OFF \
>       (SCTLR_EL2_RES1 | ENDIAN_SET_EL2)
> diff --git a/arch/arm64/kernel/asm-offsets.c b/arch/arm64/kernel/asm-offsets.c
> index 0cb34ccb6e73..6b489a8462f0 100644
> --- a/arch/arm64/kernel/asm-offsets.c
> +++ b/arch/arm64/kernel/asm-offsets.c
> @@ -111,6 +111,9 @@ int main(void)
>    DEFINE(VCPU_WORKAROUND_FLAGS,      offsetof(struct kvm_vcpu, 
> arch.workaround_flags));
>    DEFINE(VCPU_HCR_EL2,               offsetof(struct kvm_vcpu, 
> arch.hcr_el2));
>    DEFINE(CPU_USER_PT_REGS,   offsetof(struct kvm_cpu_context, regs));
> +  DEFINE(CPU_RGSR_EL1,               offsetof(struct kvm_cpu_context, 
> sys_regs[RGSR_EL1]));
> +  DEFINE(CPU_GCR_EL1,                offsetof(struct kvm_cpu_context, 
> sys_regs[GCR_EL1]));
> +  DEFINE(CPU_TFSRE0_EL1,     offsetof(struct kvm_cpu_context, 
> sys_regs[TFSRE0_EL1]));

TFSRE0_EL1 is never accessed from assembly code. Leftover from a
previous version?

>    DEFINE(CPU_APIAKEYLO_EL1,  offsetof(struct kvm_cpu_context, 
> sys_regs[APIAKEYLO_EL1]));
>    DEFINE(CPU_APIBKEYLO_EL1,  offsetof(struct kvm_cpu_context, 
> sys_regs[APIBKEYLO_EL1]));
>    DEFINE(CPU_APDAKEYLO_EL1,  offsetof(struct kvm_cpu_context, 
> sys_regs[APDAKEYLO_EL1]));
> diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S
> index e831d3dfd50d..435346ea1504 100644
> --- a/arch/arm64/kvm/hyp/entry.S
> +++ b/arch/arm64/kvm/hyp/entry.S
> @@ -13,6 +13,7 @@
>  #include <asm/kvm_arm.h>
>  #include <asm/kvm_asm.h>
>  #include <asm/kvm_mmu.h>
> +#include <asm/kvm_mte.h>
>  #include <asm/kvm_ptrauth.h>
>  
>       .text
> @@ -51,6 +52,9 @@ alternative_else_nop_endif
>  
>       add     x29, x0, #VCPU_CONTEXT
>  
> +     // mte_switch_to_guest(g_ctxt, h_ctxt, tmp1)
> +     mte_switch_to_guest x29, x1, x2
> +
>       // Macro ptrauth_switch_to_guest format:
>       //      ptrauth_switch_to_guest(guest cxt, tmp1, tmp2, tmp3)
>       // The below macro to restore guest keys is not implemented in C code
> @@ -142,6 +146,9 @@ SYM_INNER_LABEL(__guest_exit, SYM_L_GLOBAL)
>       // when this feature is enabled for kernel code.
>       ptrauth_switch_to_hyp x1, x2, x3, x4, x5
>  
> +     // mte_switch_to_hyp(g_ctxt, h_ctxt, reg1)
> +     mte_switch_to_hyp x1, x2, x3
> +
>       // Restore hyp's sp_el0
>       restore_sp_el0 x2, x3
>  
> diff --git a/arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h 
> b/arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h
> index cce43bfe158f..de7e14c862e6 100644
> --- a/arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h
> +++ b/arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h
> @@ -14,6 +14,7 @@
>  #include <asm/kvm_asm.h>
>  #include <asm/kvm_emulate.h>
>  #include <asm/kvm_hyp.h>
> +#include <asm/kvm_mmu.h>
>  
>  static inline void __sysreg_save_common_state(struct kvm_cpu_context *ctxt)
>  {
> @@ -26,6 +27,16 @@ static inline void __sysreg_save_user_state(struct 
> kvm_cpu_context *ctxt)
>       ctxt_sys_reg(ctxt, TPIDRRO_EL0) = read_sysreg(tpidrro_el0);
>  }
>  
> +static inline bool ctxt_has_mte(struct kvm_cpu_context *ctxt)
> +{
> +     struct kvm_vcpu *vcpu = ctxt->__hyp_running_vcpu;
> +
> +     if (!vcpu)
> +             vcpu = container_of(ctxt, struct kvm_vcpu, arch.ctxt);
> +
> +     return kvm_has_mte(kern_hyp_va(vcpu->kvm));
> +}
> +
>  static inline void __sysreg_save_el1_state(struct kvm_cpu_context *ctxt)
>  {
>       ctxt_sys_reg(ctxt, CSSELR_EL1)  = read_sysreg(csselr_el1);
> @@ -46,6 +57,11 @@ static inline void __sysreg_save_el1_state(struct 
> kvm_cpu_context *ctxt)
>       ctxt_sys_reg(ctxt, PAR_EL1)     = read_sysreg_par();
>       ctxt_sys_reg(ctxt, TPIDR_EL1)   = read_sysreg(tpidr_el1);
>  
> +     if (ctxt_has_mte(ctxt)) {
> +             ctxt_sys_reg(ctxt, TFSR_EL1) = read_sysreg_el1(SYS_TFSR);
> +             ctxt_sys_reg(ctxt, TFSRE0_EL1) = read_sysreg_s(SYS_TFSRE0_EL1);
> +     }

I remember suggesting that this is slightly heavier than necessary.

On nVHE, TFSRE0_EL1 could be moved to load/put, as we never run
userspace with a vcpu loaded. The same holds of course for VHE, but we
also can move TFSR_EL1 to load/put, as the host uses TFSR_EL2.

Do you see any issue with that?

> +
>       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);
> @@ -107,6 +123,11 @@ static inline void __sysreg_restore_el1_state(struct 
> kvm_cpu_context *ctxt)
>       write_sysreg(ctxt_sys_reg(ctxt, PAR_EL1),       par_el1);
>       write_sysreg(ctxt_sys_reg(ctxt, TPIDR_EL1),     tpidr_el1);
>  
> +     if (ctxt_has_mte(ctxt)) {
> +             write_sysreg_el1(ctxt_sys_reg(ctxt, TFSR_EL1), SYS_TFSR);
> +             write_sysreg_s(ctxt_sys_reg(ctxt, TFSRE0_EL1), SYS_TFSRE0_EL1);
> +     }
> +
>       if (!has_vhe() &&
>           cpus_have_final_cap(ARM64_WORKAROUND_SPECULATIVE_AT) &&
>           ctxt->__hyp_running_vcpu) {
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 24a844cb79ca..88adbc2286f2 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -1305,6 +1305,20 @@ static bool access_ccsidr(struct kvm_vcpu *vcpu, 
> struct sys_reg_params *p,
>       return true;
>  }
>  
> +static unsigned int mte_visibility(const struct kvm_vcpu *vcpu,
> +                                const struct sys_reg_desc *rd)
> +{
> +     return REG_HIDDEN;
> +}
> +
> +#define MTE_REG(name) {                              \
> +     SYS_DESC(SYS_##name),                   \
> +     .access = undef_access,                 \
> +     .reset = reset_unknown,                 \
> +     .reg = name,                            \
> +     .visibility = mte_visibility,           \
> +}
> +
>  /* sys_reg_desc initialiser for known cpufeature ID registers */
>  #define ID_SANITISED(name) {                 \
>       SYS_DESC(SYS_##name),                   \
> @@ -1473,8 +1487,8 @@ static const struct sys_reg_desc sys_reg_descs[] = {
>       { SYS_DESC(SYS_ACTLR_EL1), access_actlr, reset_actlr, ACTLR_EL1 },
>       { SYS_DESC(SYS_CPACR_EL1), NULL, reset_val, CPACR_EL1, 0 },
>  
> -     { SYS_DESC(SYS_RGSR_EL1), undef_access },
> -     { SYS_DESC(SYS_GCR_EL1), undef_access },
> +     MTE_REG(RGSR_EL1),
> +     MTE_REG(GCR_EL1),
>  
>       { SYS_DESC(SYS_ZCR_EL1), NULL, reset_val, ZCR_EL1, 0, .visibility = 
> sve_visibility },
>       { SYS_DESC(SYS_TRFCR_EL1), undef_access },
> @@ -1501,8 +1515,8 @@ static const struct sys_reg_desc sys_reg_descs[] = {
>       { SYS_DESC(SYS_ERXMISC0_EL1), trap_raz_wi },
>       { SYS_DESC(SYS_ERXMISC1_EL1), trap_raz_wi },
>  
> -     { SYS_DESC(SYS_TFSR_EL1), undef_access },
> -     { SYS_DESC(SYS_TFSRE0_EL1), undef_access },
> +     MTE_REG(TFSR_EL1),
> +     MTE_REG(TFSRE0_EL1),
>  
>       { SYS_DESC(SYS_FAR_EL1), access_vm_reg, reset_unknown, FAR_EL1 },
>       { SYS_DESC(SYS_PAR_EL1), NULL, reset_unknown, PAR_EL1 },

Thanks,

        M.

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