Subject: Re: [PATCH] KVM: arm64: Flush HCR_EL2.VSE to deliver SErrors to pKVM guests

From: Oliver Upton <[email-masked]>
Date: Mon, 1 Jun 2026 12:16:16 -0700

On Sun, May 31, 2026 at 04:45:48PM +0100, [email-masked] wrote:
> With pKVM enabled, the host injects a virtual SError by setting
> HCR_EL2.VSE on its vCPU copy, but flush_hyp_vcpu() only flows TWI/TWE
> into the hyp vCPU that runs, so VSE never reaches it and a deferred
> (masked) SError is never delivered. VSE is a host-owned injection
> control, not a trap-configuration bit, so restricting the host's
> trap-register values should not have dropped it. Flow it on entry;
> sync_hyp_vcpu() already copies hcr_el2 back, so delivery is reflected
> to the host.

Might be worth mentioning that flush_hyp_vcpu() also forwards VSESR_EL2
from the host since that bit is _just_ out of context of this diff :)

> Fixes: b56680de9c648 ("KVM: arm64: Initialize trap register values in hyp in pKVM")
> Reported-by: Sashiko (local):gemini-3.1-pro
> Signed-off-by: Fuad Tabba <[email-masked]>

Reviewed-by: Oliver Upton <[email-masked]>

Thanks,
Oliver

> ---
> Not an urgent fix. I should stop running Sashiko...
> ---
>  arch/arm64/kvm/hyp/nvhe/hyp-main.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kvm/hyp/nvhe/hyp-main.c b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> index 06db299c37a8..9e4a20df6409 100644
> --- a/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> +++ b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> @@ -129,9 +129,14 @@ static void flush_hyp_vcpu(struct pkvm_hyp_vcpu *hyp_vcpu)
>  	hyp_vcpu->vcpu.arch.ctxt	= host_vcpu->arch.ctxt;
>  
>  	hyp_vcpu->vcpu.arch.mdcr_el2	= host_vcpu->arch.mdcr_el2;
> -	hyp_vcpu->vcpu.arch.hcr_el2 &= ~(HCR_TWI | HCR_TWE);
> +	/*
> +	 * HCR_EL2.VSE is host-owned (a pending virtual SError to inject), not a
> +	 * trap-control bit, so it must flow to the hyp vCPU alongside TWI/TWE
> +	 * for the vSError to be delivered. sync_hyp_vcpu() reflects it back.
> +	 */
> +	hyp_vcpu->vcpu.arch.hcr_el2 &= ~(HCR_TWI | HCR_TWE | HCR_VSE);
>  	hyp_vcpu->vcpu.arch.hcr_el2 |= READ_ONCE(host_vcpu->arch.hcr_el2) &
> -						 (HCR_TWI | HCR_TWE);
> +						 (HCR_TWI | HCR_TWE | HCR_VSE);
>  
>  	hyp_vcpu->vcpu.arch.iflags	= host_vcpu->arch.iflags;
>  
> -- 
> 2.54.0.929.g9b7fa37559-goog
>