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

[Steven Price <steven.price@arm.com>]

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?

>> +
>> +.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:

  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.

[1] https://lore.kernel.org/kvmarm/b16b65b5-d27f-7f86-fe0c-38a951e7d3ae@arm.com/

Thanks,

Steve

>> +
>>  	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.
> 


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