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

[Marc Zyngier]

On Wed, 19 May 2021 14:04:20 +0100,
Steven Price <steven.price@arm.com> wrote:
> 
> On 17/05/2021 18:17, Marc Zyngier wrote:
> > 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.
> 
> I don't follow. This is part of the __guest_exit path and there's an ISB
> at the end of that - is that not sufficient? I don't see any possibility
> for allocating memory before that. What am I missing?

Which ISB?  We have a few in the SError handling code, but that's
conditioned on not having RAS. With any RAS-enabled CPU, we return to
C code early, since we don't need any extra synchronisation (see the
comment about the absence of ISB on this path).

I would really like to ensure that we return to C code in the exact
state we left it.

> 
> >> +
> >> +.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?
> 
> Indeed, I will drop it.
> 
> >>    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?
> 
> The comment[1] I made before was:

Ah, I totally missed this email (or can't remember reading it, which
amounts to the same thing). Apologies for that.

>   For TFSR_EL1 + VHE I believe it is synchronised only on vcpu_load/put -
>   __sysreg_save_el1_state() is called from kvm_vcpu_load_sysregs_vhe().
> 
>   TFSRE0_EL1 potentially could be improved. I have to admit I was unsure
>   if it should be in __sysreg_save_user_state() instead. However AFAICT
>   that is called at the same time as __sysreg_save_el1_state() and there's
>   no optimisation for nVHE. And given it's an _EL1 register this seemed
>   like the logic place.
>
>   Am I missing something here? Potentially there are other registers to be
>   optimised (TPIDRRO_EL0 looks like a possiblity), but IMHO that doesn't
>   belong in this series.
> 
> For VHE TFSR_EL1 is already only saved/restored on load/put
> (__sysreg_save_el1_state() is called from kvm_vcpu_put_sysregs_vhe()).
> 
> TFSRE0_EL1 could be moved, but I'm not sure where it should live as I
> mentioned above.

Yeah, this looks fine, please ignore my rambling.

Thanks,

        M.

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