Re: [PATCH v2 2/2] KVM: arm64: Trap access to SMPRI_EL1 and TPIDR2 in VHE mode

[Marc Zyngier <maz@kernel.org>]

On Tue, 01 Nov 2022 11:27:15 +0000,
Mark Brown <broonie@kernel.org> wrote:
> 
> On systems with SME access to the SMPRI_EL1 priority management register is
> controlled by the nSMPRI_EL1 fine grained trap and TPIDR2_EL0 is controlled
> by nTPIDR2_EL0. We manage these traps in nVHE mode but do not do so when in
> VHE mode, add the required management.
> 
> Without this these registers could be used as side channels where implemented.
> 
> Fixes: 861262ab8627 ("KVM: arm64: Handle SME host state when running guests")
> Signed-off-by: Mark Brown <broonie@kernel.org>
> Cc: stable@vger.kernel.org
> ---
>  arch/arm64/kvm/hyp/vhe/switch.c | 26 ++++++++++++++++++++++++--
>  1 file changed, 24 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kvm/hyp/vhe/switch.c b/arch/arm64/kvm/hyp/vhe/switch.c
> index 7acb87eaa092..9dac3a1a85f7 100644
> --- a/arch/arm64/kvm/hyp/vhe/switch.c
> +++ b/arch/arm64/kvm/hyp/vhe/switch.c
> @@ -63,10 +63,20 @@ static void __activate_traps(struct kvm_vcpu *vcpu)
>  		__activate_traps_fpsimd32(vcpu);
>  	}
>  
> -	if (cpus_have_final_cap(ARM64_SME))
> +	if (cpus_have_final_cap(ARM64_SME)) {
>  		write_sysreg(read_sysreg(sctlr_el2) & ~SCTLR_ELx_ENTP2,
>  			     sctlr_el2);

I still question this. As far as I can tell, it only affects the host
context (HCR_EL2.{E2H,TGE}={1,1}).

This is outlined in the description of the HFGWTR_EL2.nTPIDR2_EL0 bit:

<quote>
	If EL2 is implemented and enabled in the current Security
	state, HCR_EL2.{E2H, TGE} != {1, 1}, and either EL3 is
	not implemented or SCR_EL3.FGTEn == 1, then MSR writes
	of TPIDR2_EL0 at EL1 and EL0 using AArch64 are
	trapped to EL2 and reported with EC syndrome value 0x18,
	unless the write generates a higher priority exception.
</quote>

Similar language is there for the read counterpart.

So I can only conclude that messing with SCTLR_EL2 is superfluous and
doesn't affect the execution in a guest context.

>  
> +		sysreg_clear_set_s(SYS_HFGRTR_EL2,
> +				   HFGxTR_EL2_nSMPRI_EL1_MASK |
> +				   HFGxTR_EL2_nTPIDR2_EL0_MASK,
> +				   0);
> +		sysreg_clear_set_s(SYS_HFGWTR_EL2,
> +				   HFGxTR_EL2_nSMPRI_EL1_MASK |
> +				   HFGxTR_EL2_nTPIDR2_EL0_MASK,
> +				   0);
> +	}
> +
>  	write_sysreg(val, cpacr_el1);
>  
>  	write_sysreg(__this_cpu_read(kvm_hyp_vector), vbar_el1);
> @@ -88,9 +98,21 @@ static void __deactivate_traps(struct kvm_vcpu *vcpu)
>  	 */
>  	asm(ALTERNATIVE("nop", "isb", ARM64_WORKAROUND_SPECULATIVE_AT));
>  
> -	if (cpus_have_final_cap(ARM64_SME))
> +	if (cpus_have_final_cap(ARM64_SME)) {
> +		/*
> +		 * Enable access to SMPRI_EL1 - we don't need to
> +		 * control nTPIDR2_EL0 in VHE mode.
> +		 */

This comment is factually wrong.

> +		sysreg_clear_set_s(SYS_HFGRTR_EL2, 0,
> +				   HFGxTR_EL2_nSMPRI_EL1_MASK |
> +				   HFGxTR_EL2_nTPIDR2_EL0_MASK);
> +		sysreg_clear_set_s(SYS_HFGWTR_EL2, 0,
> +				   HFGxTR_EL2_nSMPRI_EL1_MASK |
> +				   HFGxTR_EL2_nTPIDR2_EL0_MASK);
> +
>  		write_sysreg(read_sysreg(sctlr_el2) | SCTLR_ELx_ENTP2,
>  			     sctlr_el2);

Same as above. It serves no purpose AFAICT.

> +	}
>  
>  	write_sysreg(CPACR_EL1_DEFAULT, cpacr_el1);
>  

And then you realise that the VHE and nVHE behaviours are strictly
identical and that they should simply be merged. Or rather, that we
should make use of the existing infrastructure to enable/disable the
trapping of the SME ops when it actually matters: at load/put time for
VHE, at enter/exit for nVHE.

I came up with the following patch, compile-tested only.

Thanks,

	M.

From 5c0913a6de47e32b071ea5534319015e0a61db71 Mon Sep 17 00:00:00 2001
From: Marc Zyngier <maz@kernel.org>
Date: Tue, 1 Nov 2022 12:19:51 +0000
Subject: [PATCH] KVM: arm64: Fix SMPRI_EL1/TPIDR2_EL0 trapping on VHE

The trapping of SMPRI_EL1 and TPIDR2_EL0 currently only really
work on nVHE, as only this mode uses the fine-grained trapping
that controls these two registers.

Move the trapping enable/disable code into
__{de,}activate_traps_common(), allowing it to be called when it
actually matters on VHE, and remove the flipping of EL2 control
for TPIDR2_EL0, which only affects the host access of this
register.

Fixes: 861262ab8627 ("KVM: arm64: Handle SME host state when running guests")
Signed-off-by: Marc Zyngier <maz@kernel.org>
Cc: stable@vger.kernel.org
---
 arch/arm64/kvm/hyp/include/hyp/switch.h | 20 +++++++++++++++++++
 arch/arm64/kvm/hyp/nvhe/switch.c        | 26 -------------------------
 arch/arm64/kvm/hyp/vhe/switch.c         |  8 --------
 3 files changed, 20 insertions(+), 34 deletions(-)

diff --git a/arch/arm64/kvm/hyp/include/hyp/switch.h b/arch/arm64/kvm/hyp/include/hyp/switch.h
index 6cbbb6c02f66..3330d1b76bdd 100644
--- a/arch/arm64/kvm/hyp/include/hyp/switch.h
+++ b/arch/arm64/kvm/hyp/include/hyp/switch.h
@@ -87,6 +87,17 @@ static inline void __activate_traps_common(struct kvm_vcpu *vcpu)
 
 	vcpu->arch.mdcr_el2_host = read_sysreg(mdcr_el2);
 	write_sysreg(vcpu->arch.mdcr_el2, mdcr_el2);
+
+	if (cpus_have_final_cap(ARM64_SME)) {
+		sysreg_clear_set_s(SYS_HFGRTR_EL2,
+				   HFGxTR_EL2_nSMPRI_EL1_MASK |
+				   HFGxTR_EL2_nTPIDR2_EL0_MASK,
+				   0);
+		sysreg_clear_set_s(SYS_HFGWTR_EL2,
+				   HFGxTR_EL2_nSMPRI_EL1_MASK |
+				   HFGxTR_EL2_nTPIDR2_EL0_MASK,
+				   0);
+	}
 }
 
 static inline void __deactivate_traps_common(struct kvm_vcpu *vcpu)
@@ -96,6 +107,15 @@ static inline void __deactivate_traps_common(struct kvm_vcpu *vcpu)
 	write_sysreg(0, hstr_el2);
 	if (kvm_arm_support_pmu_v3())
 		write_sysreg(0, pmuserenr_el0);
+
+	if (cpus_have_final_cap(ARM64_SME)) {
+		sysreg_clear_set_s(SYS_HFGRTR_EL2, 0,
+				   HFGxTR_EL2_nSMPRI_EL1_MASK |
+				   HFGxTR_EL2_nTPIDR2_EL0_MASK);
+		sysreg_clear_set_s(SYS_HFGWTR_EL2, 0,
+				   HFGxTR_EL2_nSMPRI_EL1_MASK |
+				   HFGxTR_EL2_nTPIDR2_EL0_MASK);
+	}
 }
 
 static inline void ___activate_traps(struct kvm_vcpu *vcpu)
diff --git a/arch/arm64/kvm/hyp/nvhe/switch.c b/arch/arm64/kvm/hyp/nvhe/switch.c
index 8e9d49a964be..c2cb46ca4fb6 100644
--- a/arch/arm64/kvm/hyp/nvhe/switch.c
+++ b/arch/arm64/kvm/hyp/nvhe/switch.c
@@ -55,18 +55,6 @@ static void __activate_traps(struct kvm_vcpu *vcpu)
 	write_sysreg(val, cptr_el2);
 	write_sysreg(__this_cpu_read(kvm_hyp_vector), vbar_el2);
 
-	if (cpus_have_final_cap(ARM64_SME)) {
-		val = read_sysreg_s(SYS_HFGRTR_EL2);
-		val &= ~(HFGxTR_EL2_nTPIDR2_EL0_MASK |
-			 HFGxTR_EL2_nSMPRI_EL1_MASK);
-		write_sysreg_s(val, SYS_HFGRTR_EL2);
-
-		val = read_sysreg_s(SYS_HFGWTR_EL2);
-		val &= ~(HFGxTR_EL2_nTPIDR2_EL0_MASK |
-			 HFGxTR_EL2_nSMPRI_EL1_MASK);
-		write_sysreg_s(val, SYS_HFGWTR_EL2);
-	}
-
 	if (cpus_have_final_cap(ARM64_WORKAROUND_SPECULATIVE_AT)) {
 		struct kvm_cpu_context *ctxt = &vcpu->arch.ctxt;
 
@@ -110,20 +98,6 @@ static void __deactivate_traps(struct kvm_vcpu *vcpu)
 
 	write_sysreg(this_cpu_ptr(&kvm_init_params)->hcr_el2, hcr_el2);
 
-	if (cpus_have_final_cap(ARM64_SME)) {
-		u64 val;
-
-		val = read_sysreg_s(SYS_HFGRTR_EL2);
-		val |= HFGxTR_EL2_nTPIDR2_EL0_MASK |
-			HFGxTR_EL2_nSMPRI_EL1_MASK;
-		write_sysreg_s(val, SYS_HFGRTR_EL2);
-
-		val = read_sysreg_s(SYS_HFGWTR_EL2);
-		val |= HFGxTR_EL2_nTPIDR2_EL0_MASK |
-			HFGxTR_EL2_nSMPRI_EL1_MASK;
-		write_sysreg_s(val, SYS_HFGWTR_EL2);
-	}
-
 	cptr = CPTR_EL2_DEFAULT;
 	if (vcpu_has_sve(vcpu) && (vcpu->arch.fp_state == FP_STATE_GUEST_OWNED))
 		cptr |= CPTR_EL2_TZ;
diff --git a/arch/arm64/kvm/hyp/vhe/switch.c b/arch/arm64/kvm/hyp/vhe/switch.c
index 7acb87eaa092..1a97391fedd2 100644
--- a/arch/arm64/kvm/hyp/vhe/switch.c
+++ b/arch/arm64/kvm/hyp/vhe/switch.c
@@ -63,10 +63,6 @@ static void __activate_traps(struct kvm_vcpu *vcpu)
 		__activate_traps_fpsimd32(vcpu);
 	}
 
-	if (cpus_have_final_cap(ARM64_SME))
-		write_sysreg(read_sysreg(sctlr_el2) & ~SCTLR_ELx_ENTP2,
-			     sctlr_el2);
-
 	write_sysreg(val, cpacr_el1);
 
 	write_sysreg(__this_cpu_read(kvm_hyp_vector), vbar_el1);
@@ -88,10 +84,6 @@ static void __deactivate_traps(struct kvm_vcpu *vcpu)
 	 */
 	asm(ALTERNATIVE("nop", "isb", ARM64_WORKAROUND_SPECULATIVE_AT));
 
-	if (cpus_have_final_cap(ARM64_SME))
-		write_sysreg(read_sysreg(sctlr_el2) | SCTLR_ELx_ENTP2,
-			     sctlr_el2);
-
 	write_sysreg(CPACR_EL1_DEFAULT, cpacr_el1);
 
 	if (!arm64_kernel_unmapped_at_el0())
-- 
2.34.1


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

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