Re: [PATCH v10 14/40] KVM: arm64: Manage GCS access and registers for guests

Marc Zyngier
2024-08-16T14:15:23Z

On Thu, 01 Aug 2024 13:06:41 +0100,
Mark Brown <broonie@kernel.org> wrote:
>
> GCS introduces a number of system registers for EL1 and EL0, on systems
> with GCS we need to context switch them and expose them to VMMs to allow
> guests to use GCS.
>
> In order to allow guests to use GCS we also need to configure
> HCRX_EL2.GCSEn, if this is not set GCS instructions will be noops and
> CHKFEAT will report GCS as disabled. Also enable fine grained traps for
> access to the GCS registers by guests which do not have the feature
> enabled.
>
> Signed-off-by: Mark Brown <broonie@kernel.org>
> ---
> arch/arm64/include/asm/kvm_host.h | 8 +++++
> arch/arm64/include/asm/vncr_mapping.h | 2 ++
> arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h | 49 ++++++++++++++++++++++++------
> arch/arm64/kvm/sys_regs.c | 12 ++++++++
> 4 files changed, 61 insertions(+), 10 deletions(-)
>
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index a33f5996ca9f..5818e4a1c2d1 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -446,6 +446,10 @@ 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) */
> + GCSPR_EL0, /* Guarded Control Stack Pointer (EL0) */
> +
> /* 32bit specific registers. */
> DACR32_EL2, /* Domain Access Control Register */
> IFSR32_EL2, /* Instruction Fault Status Register */
> @@ -517,6 +521,10 @@ enum vcpu_sysreg {
> VNCR(PIR_EL1), /* Permission Indirection Register 1 (EL1) */
> VNCR(PIRE0_EL1), /* Permission Indirection Register 0 (EL1) */
>
> + /* Guarded Control Stack registers */
> + VNCR(GCSPR_EL1), /* Guarded Control Stack Pointer (EL1) */
> + VNCR(GCSCR_EL1), /* Guarded Control Stack Control (EL1) */
> +
> VNCR(HFGRTR_EL2),
> VNCR(HFGWTR_EL2),
> VNCR(HFGITR_EL2),
> diff --git a/arch/arm64/include/asm/vncr_mapping.h b/arch/arm64/include/asm/vncr_mapping.h
> index df2c47c55972..5e83e6f579fd 100644
> --- a/arch/arm64/include/asm/vncr_mapping.h
> +++ b/arch/arm64/include/asm/vncr_mapping.h
> @@ -88,6 +88,8 @@
> #define VNCR_PMSIRR_EL1 0x840
> #define VNCR_PMSLATFR_EL1 0x848
> #define VNCR_TRFCR_EL1 0x880
> +#define VNCR_GCSPR_EL1 0x8C0
> +#define VNCR_GCSCR_EL1 0x8D0
> #define VNCR_MPAM1_EL1 0x900
> #define VNCR_MPAMHCR_EL2 0x930
> #define VNCR_MPAMVPMV_EL2 0x938
> diff --git a/arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h b/arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h
> index 4c0fdabaf8ae..ac29352e225a 100644
> --- a/arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h
> +++ b/arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h
> @@ -16,6 +16,27 @@
> #include <asm/kvm_hyp.h>
> #include <asm/kvm_mmu.h>
>
> +static inline struct kvm_vcpu *ctxt_to_vcpu(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 vcpu;
> +}
> +
> +static inline bool ctxt_has_gcs(struct kvm_cpu_context *ctxt)
> +{
> + struct kvm_vcpu *vcpu;
> +
> + if (!cpus_have_final_cap(ARM64_HAS_GCS))
> + return false;
> +
> + vcpu = ctxt_to_vcpu(ctxt);
> + return kvm_has_feat(kern_hyp_va(vcpu->kvm), ID_AA64PFR1_EL1, GCS, IMP);
> +}
> +
> static inline void __sysreg_save_common_state(struct kvm_cpu_context *ctxt)
> {
> ctxt_sys_reg(ctxt, MDSCR_EL1) = read_sysreg(mdscr_el1);
> @@ -25,16 +46,10 @@ 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);
> -}
> -
> -static inline struct kvm_vcpu *ctxt_to_vcpu(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 vcpu;
> + if (ctxt_has_gcs(ctxt)) {
> + ctxt_sys_reg(ctxt, GCSPR_EL0) = read_sysreg_s(SYS_GCSPR_EL0);
> + ctxt_sys_reg(ctxt, GCSCRE0_EL1) = read_sysreg_s(SYS_GCSCRE0_EL1);
> + }
> }
>
> static inline bool ctxt_has_mte(struct kvm_cpu_context *ctxt)
> @@ -79,6 +94,10 @@ static inline void __sysreg_save_el1_state(struct kvm_cpu_context *ctxt)
> if (ctxt_has_s1pie(ctxt)) {
> ctxt_sys_reg(ctxt, PIR_EL1) = read_sysreg_el1(SYS_PIR);
> ctxt_sys_reg(ctxt, PIRE0_EL1) = read_sysreg_el1(SYS_PIRE0);
> + if (ctxt_has_gcs(ctxt)) {
> + 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, ESR_EL1) = read_sysreg_el1(SYS_ESR);
> @@ -126,6 +145,11 @@ 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 (ctxt_has_gcs(ctxt)) {
> + write_sysreg_s(ctxt_sys_reg(ctxt, GCSPR_EL0), SYS_GCSPR_EL0);
> + write_sysreg_s(ctxt_sys_reg(ctxt, GCSCRE0_EL1),
> + SYS_GCSCRE0_EL1);
> + }
> }
>
> static inline void __sysreg_restore_el1_state(struct kvm_cpu_context *ctxt)
> @@ -157,6 +181,11 @@ static inline void __sysreg_restore_el1_state(struct kvm_cpu_context *ctxt)
> if (ctxt_has_s1pie(ctxt)) {
> write_sysreg_el1(ctxt_sys_reg(ctxt, PIR_EL1), SYS_PIR);
> write_sysreg_el1(ctxt_sys_reg(ctxt, PIRE0_EL1), SYS_PIRE0);
> +
> + if (ctxt_has_gcs(ctxt)) {
> + write_sysreg_el1(ctxt_sys_reg(ctxt, GCSPR_EL1), SYS_GCSPR);
> + write_sysreg_el1(ctxt_sys_reg(ctxt, GCSCR_EL1), SYS_GCSCR);
> + }
> }
> }
> write_sysreg_el1(ctxt_sys_reg(ctxt, ESR_EL1), SYS_ESR);
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index c90324060436..ac98d3237130 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -2446,6 +2446,10 @@ static const struct sys_reg_desc sys_reg_descs[] = {
> PTRAUTH_KEY(APDB),
> PTRAUTH_KEY(APGA),
>
> + { SYS_DESC(SYS_GCSCR_EL1), NULL, reset_val, GCSCR_EL1, 0 },
> + { SYS_DESC(SYS_GCSPR_EL1), NULL, reset_unknown, GCSPR_EL1 },
> + { SYS_DESC(SYS_GCSCRE0_EL1), NULL, reset_val, GCSCRE0_EL1, 0 },
> +

Global visibility for these registers? Why should we expose them to
userspace if the feature is neither present nor configured?

> { SYS_DESC(SYS_SPSR_EL1), access_spsr},
> { SYS_DESC(SYS_ELR_EL1), access_elr},
>
> @@ -2535,6 +2539,7 @@ static const struct sys_reg_desc sys_reg_descs[] = {
> CTR_EL0_IDC_MASK |
> CTR_EL0_DminLine_MASK |
> CTR_EL0_IminLine_MASK),
> + { SYS_DESC(SYS_GCSPR_EL0), NULL, reset_unknown, GCSPR_EL0 },
> { SYS_DESC(SYS_SVCR), undef_access },
>
> { PMU_SYS_REG(PMCR_EL0), .access = access_pmcr, .reset = reset_pmcr,
> @@ -4560,6 +4565,9 @@ void kvm_calculate_traps(struct kvm_vcpu *vcpu)
>
> if (kvm_has_feat(kvm, ID_AA64MMFR3_EL1, TCRX, IMP))
> vcpu->arch.hcrx_el2 |= HCRX_EL2_TCR2En;
> +
> + if (kvm_has_feat(kvm, ID_AA64PFR1_EL1, GCS, IMP))
> + vcpu->arch.hcrx_el2 |= HCRX_EL2_GCSEn;
> }
>
> if (test_bit(KVM_ARCH_FLAG_FGU_INITIALIZED, &kvm->arch.flags))
> @@ -4604,6 +4612,10 @@ void kvm_calculate_traps(struct kvm_vcpu *vcpu)
> kvm->arch.fgu[HFGxTR_GROUP] |= (HFGxTR_EL2_nPIRE0_EL1 |
> HFGxTR_EL2_nPIR_EL1);
>
> + if (!kvm_has_feat(kvm, ID_AA64PFR1_EL1, GCS, IMP))
> + kvm->arch.fgu[HFGxTR_GROUP] |= (HFGxTR_EL2_nGCS_EL0 |
> + HFGxTR_EL2_nGCS_EL1);

How can this work if you don't handle ID_AA64PFR_EL1 being written to?
You are exposing GCS to all guests without giving the VMM an
opportunity to turn it off. This breaks A->B->A migration, which is
not acceptable.

M.

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