Re: [PATCH v2 3/5] KVM: arm64: Add context-switch of ACCDATA_EL1

[Suzuki K Poulose <suzuki.poulose@arm.com>]

Hi Fuad

On 25/10/2026 07:51, Fuad Tabba wrote:
> From: Marc Zyngier <maz@kernel.org>
> 
> Add ACCDATA_EL1 save/restore as part of the EL0 context. This is
> conditioned on the guest's view of ID_AA64ISAR1_EL1.LS64 reporting
> that this register is supported.
> 
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> Fixes: 58ce78667a641 ("arm64: Add support for FEAT_{LS64, LS64_V}")
> Link: https://lore.kernel.org/r/20240815125959.2097734-5-maz@kernel.org
> Cc: stable@vger.kernel.org
> [Fuad: rebased; switch ACCDATA_EL1 for guests only, as the host never
>   uses it; gate it on a new ARM64_HAS_LS64_ACCDATA capability, since
>   in pKVM the host controls a non-protected guest's ID registers, and
>   an EL2 access to ACCDATA_EL1 on a CPU without the feature is UNDEFINED]
> Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
> ---
>   arch/arm64/kernel/cpufeature.c             |  7 +++++++
>   arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h | 23 ++++++++++++++++++++++
>   arch/arm64/tools/cpucaps                   |  1 +
>   3 files changed, 31 insertions(+)
> 
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 32102c3912fa7..b5720c56eebc9 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -3225,6 +3225,13 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
>   		.cpu_enable = cpu_enable_ls64_v,
>   		ARM64_CPUID_FIELDS(ID_AA64ISAR1_EL1, LS64, LS64_V)
>   	},
> +	{
> +		.desc = "LS64_ACCDATA",
> +		.capability = ARM64_HAS_LS64_ACCDATA,
> +		.type = ARM64_CPUCAP_SYSTEM_FEATURE,
> +		.matches = has_cpuid_feature,
> +		ARM64_CPUID_FIELDS(ID_AA64ISAR1_EL1, LS64, LS64_ACCDATA)
> +	},
>   #ifdef CONFIG_ARM64_LSUI
>   	{
>   		.desc = "Unprivileged Load Store Instructions (LSUI)",
> diff --git a/arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h b/arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h
> index c382848d31947..4a008c8a696ba 100644
> --- a/arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h
> +++ b/arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h
> @@ -17,6 +17,7 @@
>   #include <asm/kvm_mmu.h>
>   
>   static inline bool ctxt_has_s1poe(struct kvm_cpu_context *ctxt);
> +static inline bool ctxt_has_accdata(struct kvm_cpu_context *ctxt);
>   
>   static inline struct kvm_vcpu *ctxt_to_vcpu(struct kvm_cpu_context *ctxt)
>   {
> @@ -67,6 +68,13 @@ 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);
> +
> +	/*
> +	 * Despite the appearances, ACCDATA_EL1 is part of the EL0
> +	 * context, as it can only be used using ST64BV0.
> +	 */
> +	if (ctxt_has_accdata(ctxt))
> +		ctxt_sys_reg(ctxt, ACCDATA_EL1)	= read_sysreg_s(SYS_ACCDATA_EL1);
>   }
>   
>   static inline bool ctxt_has_mte(struct kvm_cpu_context *ctxt)
> @@ -131,6 +139,18 @@ static inline bool ctxt_has_sctlr2(struct kvm_cpu_context *ctxt)
>   	return kvm_has_sctlr2(kern_hyp_va(vcpu->kvm));
>   }
>   
> +static inline bool ctxt_has_accdata(struct kvm_cpu_context *ctxt)
> +{
> +	struct kvm_vcpu *vcpu;
> +
> +	/* The host never uses ACCDATA_EL1 */
> +	if (!cpus_have_final_cap(ARM64_HAS_LS64_ACCDATA) || !ctxt_is_guest(ctxt))
> +		return false;
> +
> +	vcpu = ctxt_to_vcpu(ctxt);
> +	return kvm_has_feat(kern_hyp_va(vcpu->kvm), ID_AA64ISAR1_EL1, LS64, LS64_ACCDATA);

minor nit: Aligning with the other kvm_has_feat() usecsaes, should we 
wrap this under kvm_has_accdata() (with moving the cpus_have_final_cap()
to the wrapper ?)

Otherwise looks good to me.

Suzuki



> +}
> +
>   static inline void __sysreg_save_el1_state(struct kvm_cpu_context *ctxt)
>   {
>   	ctxt_sys_reg(ctxt, SCTLR_EL1)	= read_sysreg_el1(SYS_SCTLR);
> @@ -210,6 +230,9 @@ 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_accdata(ctxt))
> +		write_sysreg_s(ctxt_sys_reg(ctxt, ACCDATA_EL1), SYS_ACCDATA_EL1);
>   }
>   
>   static inline void __sysreg_restore_el1_state(struct kvm_cpu_context *ctxt,
> diff --git a/arch/arm64/tools/cpucaps b/arch/arm64/tools/cpucaps
> index 2775ba3359cfe..f01719bbd6037 100644
> --- a/arch/arm64/tools/cpucaps
> +++ b/arch/arm64/tools/cpucaps
> @@ -49,6 +49,7 @@ HAS_LPA2
>   HAS_LSE_ATOMICS
>   HAS_LS64
>   HAS_LS64_V
> +HAS_LS64_ACCDATA
>   HAS_LSUI
>   HAS_MOPS
>   HAS_NESTED_VIRT