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

[Steven Price]

On 20/05/2021 10:46, Marc Zyngier wrote:
> 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).

Ah, I clearly didn't read the code (or comment) carefully enough -
indeed with RAS we're potentially skipping the ISB.

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

Agreed, I'll stick an ISB at the end of mte_switch_to_hyp. Although
there's clearly room for optimisation here as ptrauth_switch_to_hyp has
a similar ISB.

>>
>>>> +
>>>> +.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.

No problem!

Thanks,

Steve