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

Marc Zyngier
2024-02-05T09:46:20Z

On Sat, 03 Feb 2024 12:25:39 +0000,
Mark Brown <broonie@kernel.org> 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@kernel.org>
> ---
> 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.