Re: [PATCH v2 09/11] KVM: arm64: Trap host SVE accesses when the FPSIMD state is dirty

[Marc Zyngier <maz@kernel.org>]

On Thu, 18 Mar 2021 12:25:30 +0000,
Marc Zyngier <maz@kernel.org> wrote:
> 
> ZCR_EL2 controls the upper bound for ZCR_EL1, and is set to
> a potentially lower limit when the guest uses SVE. In order
> to restore the SVE state on the EL1 host, we must first
> reset ZCR_EL2 to its original value.
> 
> To make it as lazy as possible on the EL1 host side, set
> the SVE trapping in place when returning exiting from
> the guest. On the first EL1 access to SVE, ZCR_EL2 will
> be restored to its full glory.
> 
> Suggested-by: Andrew Scull <ascull@google.com>
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> ---
>  arch/arm64/kvm/hyp/nvhe/hyp-main.c | 4 ++++
>  arch/arm64/kvm/hyp/nvhe/switch.c   | 9 +++++++--
>  2 files changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kvm/hyp/nvhe/hyp-main.c b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> index f012f8665ecc..8d04d69edd15 100644
> --- a/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> +++ b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> @@ -177,6 +177,10 @@ void handle_trap(struct kvm_cpu_context *host_ctxt)
>  	case ESR_ELx_EC_SMC64:
>  		handle_host_smc(host_ctxt);
>  		break;
> +	case ESR_ELx_EC_SVE:
> +		sve_cond_update_zcr_vq(ZCR_ELx_LEN_MASK, SYS_ZCR_EL2);
> +		sysreg_clear_set(cptr_el2, CPTR_EL2_TZ, 0);
> +		break;

It turns out that my last test-run was flawed, as my model was stuck
with VHE, meaning this snippet was never run. If it ran, I would have
noticed that CPTR_EL2.TZ being set results in the ZCR_EL2 access to
trap at EL2, meaning the above explodes very quickly.

I've queued the below patch on top of the existing series, which cures
the issue and let the tests run for real this time.

Thanks to Will for the timely report, and apologies for the lousy
testing...

	M.

From 5b08709313718e95ba06ef49aa82f964a605bd9c Mon Sep 17 00:00:00 2001
From: Marc Zyngier <maz@kernel.org>
Date: Thu, 18 Mar 2021 18:30:26 +0000
Subject: [PATCH] KVM: arm64: Fix host's ZCR_EL2 restore on nVHE

We re-enter the EL1 host with CPTR_EL2.TZ set in order to
be able to lazily restore ZCR_EL2 when required.

However, the same CPTR_EL2 configuration also leads to trapping
when ZCR_EL2 is accessed from EL2. Duh!

Clear CPTR_EL2.TZ *before* writing to ZCR_EL2.

Fixes: beed09067b42 ("KVM: arm64: Trap host SVE accesses when the FPSIMD state is dirty")
Reported-by: Will Deacon <will@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
---
 arch/arm64/kvm/hyp/nvhe/hyp-main.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/hyp/nvhe/hyp-main.c b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
index 8d04d69edd15..84a702dc4a92 100644
--- a/arch/arm64/kvm/hyp/nvhe/hyp-main.c
+++ b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
@@ -178,8 +178,9 @@ void handle_trap(struct kvm_cpu_context *host_ctxt)
 		handle_host_smc(host_ctxt);
 		break;
 	case ESR_ELx_EC_SVE:
-		sve_cond_update_zcr_vq(ZCR_ELx_LEN_MASK, SYS_ZCR_EL2);
 		sysreg_clear_set(cptr_el2, CPTR_EL2_TZ, 0);
+		isb();
+		sve_cond_update_zcr_vq(ZCR_ELx_LEN_MASK, SYS_ZCR_EL2);
 		break;
 	default:
 		hyp_panic();
-- 
2.29.2


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

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel