Re: [PATCH v8 13/38] KVM: arm64: Manage GCS registers for guests

[Marc Zyngier]

On Sat, 03 Feb 2024 12:25:39 +0000,
Mark Brown <broonie@xxxxxxxxxx> wrote:
> 
> GCS introduces a number of system registers for EL1 and EL0, on systems

and EL2.

> with GCS we need to context switch them and expose them to VMMs to allow
> guests to use GCS, as well as describe their fine grained traps to
> nested virtualisation.  Traps are already disabled.

The latter is not true with NV, since the guest is in control of the
FGT registers.

> 
> Signed-off-by: Mark Brown <broonie@xxxxxxxxxx>
> ---
>  arch/arm64/include/asm/kvm_host.h          | 12 ++++++++++++
>  arch/arm64/kvm/emulate-nested.c            |  4 ++++
>  arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h | 17 +++++++++++++++++
>  arch/arm64/kvm/sys_regs.c                  | 22 ++++++++++++++++++++++
>  4 files changed, 55 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 21c57b812569..6c7ea7f9cd92 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -388,6 +388,12 @@ enum vcpu_sysreg {
>  	GCR_EL1,	/* Tag Control Register */
>  	TFSRE0_EL1,	/* Tag Fault Status Register (EL0) */
>  
> +	/* Guarded Control Stack registers */
> +	GCSCRE0_EL1,	/* Guarded Control Stack Control (EL0) */
> +	GCSCR_EL1,	/* Guarded Control Stack Control (EL1) */

This is subjected to VNCR (0x8D0).

> +	GCSPR_EL0,	/* Guarded Control Stack Pointer (EL0) */
> +	GCSPR_EL1,	/* Guarded Control Stack Pointer (EL1) */

So is this one (0x8C0). And how about the *_EL2 versions?

> +
>  	/* 32bit specific registers. */
>  	DACR32_EL2,	/* Domain Access Control Register */
>  	IFSR32_EL2,	/* Instruction Fault Status Register */
> @@ -1221,6 +1227,12 @@ static inline bool __vcpu_has_feature(const struct kvm_arch *ka, int feature)
>  
>  #define vcpu_has_feature(v, f)	__vcpu_has_feature(&(v)->kvm->arch, (f))
>  
> +static inline bool has_gcs(void)
> +{
> +	return IS_ENABLED(CONFIG_ARM64_GCS) &&
> +		cpus_have_final_cap(ARM64_HAS_GCS);
> +}
> +
>  int kvm_trng_call(struct kvm_vcpu *vcpu);
>  #ifdef CONFIG_KVM
>  extern phys_addr_t hyp_mem_base;
> diff --git a/arch/arm64/kvm/emulate-nested.c b/arch/arm64/kvm/emulate-nested.c
> index 431fd429932d..24eb7eccbae4 100644
> --- a/arch/arm64/kvm/emulate-nested.c
> +++ b/arch/arm64/kvm/emulate-nested.c
> @@ -1098,8 +1098,12 @@ static const struct encoding_to_trap_config encoding_to_fgt[] __initconst = {
>  	SR_FGT(SYS_ESR_EL1, 		HFGxTR, ESR_EL1, 1),
>  	SR_FGT(SYS_DCZID_EL0, 		HFGxTR, DCZID_EL0, 1),
>  	SR_FGT(SYS_CTR_EL0, 		HFGxTR, CTR_EL0, 1),
> +	SR_FGT(SYS_GCSPR_EL0,		HFGxTR, nGCS_EL0, 1),
>  	SR_FGT(SYS_CSSELR_EL1, 		HFGxTR, CSSELR_EL1, 1),
>  	SR_FGT(SYS_CPACR_EL1, 		HFGxTR, CPACR_EL1, 1),
> +	SR_FGT(SYS_GCSCR_EL1,		HFGxTR, nGCS_EL1, 1),
> +	SR_FGT(SYS_GCSPR_EL1,		HFGxTR, nGCS_EL1, 1),
> +	SR_FGT(SYS_GCSCRE0_EL1,		HFGxTR, nGCS_EL0, 1),

This is clearly wrong on all 4 counts (the n prefix gives it away...).

>  	SR_FGT(SYS_CONTEXTIDR_EL1, 	HFGxTR, CONTEXTIDR_EL1, 1),
>  	SR_FGT(SYS_CLIDR_EL1, 		HFGxTR, CLIDR_EL1, 1),
>  	SR_FGT(SYS_CCSIDR_EL1, 		HFGxTR, CCSIDR_EL1, 1),
> diff --git a/arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h b/arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h
> index bb6b571ec627..ec34d4a90717 100644
> --- a/arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h
> +++ b/arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h
> @@ -25,6 +25,8 @@ static inline void __sysreg_save_user_state(struct kvm_cpu_context *ctxt)
>  {
>  	ctxt_sys_reg(ctxt, TPIDR_EL0)	= read_sysreg(tpidr_el0);
>  	ctxt_sys_reg(ctxt, TPIDRRO_EL0)	= read_sysreg(tpidrro_el0);
> +	if (has_gcs())
> +		ctxt_sys_reg(ctxt, GCSPR_EL0) = read_sysreg_s(SYS_GCSPR_EL0);

We have had this discussion in the past. This must be based on the
VM's configuration. Guarding the check with the host capability is a
valuable optimisation, but that's nowhere near enough. See the series
that I have posted on this very subject (you're on Cc), but you are
welcome to invent your own mechanism in the meantime.

>  }
>  
>  static inline bool ctxt_has_mte(struct kvm_cpu_context *ctxt)
> @@ -62,6 +64,12 @@ 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 (has_gcs()) {
> +		ctxt_sys_reg(ctxt, GCSPR_EL1)	= read_sysreg_el1(SYS_GCSPR);
> +		ctxt_sys_reg(ctxt, GCSCR_EL1)	= read_sysreg_el1(SYS_GCSCR);
> +		ctxt_sys_reg(ctxt, GCSCRE0_EL1)	= read_sysreg_s(SYS_GCSCRE0_EL1);
> +	}
> +

Same thing.

>  	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);
> @@ -95,6 +103,8 @@ static inline void __sysreg_restore_user_state(struct kvm_cpu_context *ctxt)
>  {
>  	write_sysreg(ctxt_sys_reg(ctxt, TPIDR_EL0),	tpidr_el0);
>  	write_sysreg(ctxt_sys_reg(ctxt, TPIDRRO_EL0),	tpidrro_el0);
> +	if (has_gcs())
> +		write_sysreg_s(ctxt_sys_reg(ctxt, GCSPR_EL0), SYS_GCSPR_EL0);
>  }
>  
>  static inline void __sysreg_restore_el1_state(struct kvm_cpu_context *ctxt)
> @@ -138,6 +148,13 @@ 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 (has_gcs()) {
> +		write_sysreg_el1(ctxt_sys_reg(ctxt, GCSPR_EL1),	SYS_GCSPR);
> +		write_sysreg_el1(ctxt_sys_reg(ctxt, GCSCR_EL1),	SYS_GCSCR);
> +		write_sysreg_s(ctxt_sys_reg(ctxt, GCSCRE0_EL1),
> +			       SYS_GCSCRE0_EL1);
> +	}
> +

For the benefit of the unsuspecting reviewers, and in the absence of a
public specification (which the XML drop isn't), it would be good to
have the commit message explaining the rationale of what gets saved
when.

>  	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);
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 30253bd19917..83ba767e75d2 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -2000,6 +2000,23 @@ static unsigned int mte_visibility(const struct kvm_vcpu *vcpu,
>  	.visibility = mte_visibility,		\
>  }
>  
> +static unsigned int gcs_visibility(const struct kvm_vcpu *vcpu,
> +				   const struct sys_reg_desc *rd)
> +{
> +	if (has_gcs())
> +		return 0;

Yet another case of exposing potentially unwanted state, to the VMM
this time.

> +
> +	return REG_HIDDEN;
> +}
> +
> +#define GCS_REG(name) {				\
> +	SYS_DESC(SYS_##name),			\
> +	.access = undef_access,			\
> +	.reset = reset_unknown,			\
> +	.reg = name,				\
> +	.visibility = gcs_visibility,		\
> +}
> +
>  static unsigned int el2_visibility(const struct kvm_vcpu *vcpu,
>  				   const struct sys_reg_desc *rd)
>  {
> @@ -2376,6 +2393,10 @@ static const struct sys_reg_desc sys_reg_descs[] = {
>  	PTRAUTH_KEY(APDB),
>  	PTRAUTH_KEY(APGA),
>  
> +	GCS_REG(GCSCR_EL1),
> +	GCS_REG(GCSPR_EL1),
> +	GCS_REG(GCSCRE0_EL1),
> +
>  	{ SYS_DESC(SYS_SPSR_EL1), access_spsr},
>  	{ SYS_DESC(SYS_ELR_EL1), access_elr},
>  
> @@ -2462,6 +2483,7 @@ static const struct sys_reg_desc sys_reg_descs[] = {
>  	{ SYS_DESC(SYS_SMIDR_EL1), undef_access },
>  	{ SYS_DESC(SYS_CSSELR_EL1), access_csselr, reset_unknown, CSSELR_EL1 },
>  	{ SYS_DESC(SYS_CTR_EL0), access_ctr },
> +	GCS_REG(GCSPR_EL0),
>  	{ SYS_DESC(SYS_SVCR), undef_access },
>  
>  	{ PMU_SYS_REG(PMCR_EL0), .access = access_pmcr, .reset = reset_pmcr,
> 


Thanks,

	M.

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