linux-kernel - Re: [PATCH] KVM: arm64: nv: Forward hvc traps if originated from nested VM

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Message-ID: <86mscolc0b.wl-maz@kernel.org>
Date: Thu, 10 Apr 2025 08:19:16 +0100
From: Marc Zyngier <maz@...nel.org>
To: Ganapatrao Kulkarni <gankulkarni@...amperecomputing.com>
Cc: linux-arm-kernel@...ts.infradead.org,
	kvmarm@...ts.linux.dev,
	linux-kernel@...r.kernel.org,
	oliver.upton@...ux.dev,
	joey.gouly@....com,
	suzuki.poulose@....com,
	yuzenghui@...wei.com,
	darren@...amperecomputing.com
Subject: Re: [PATCH] KVM: arm64: nv: Forward hvc traps if originated from nested VM

On Thu, 10 Apr 2025 08:07:43 +0100,
Ganapatrao Kulkarni <gankulkarni@...amperecomputing.com> wrote:
> 
> It was discovered while trying selftest(smccc_filter) that the
> hvc trap is getting forwarded to guest hypervisor even if it is
> originated from itself.
> 
> HVC traps from guest hypervisor should be handled by the host
> hypervisor and traps originating from nested VM should be
> forwarded. Adding check to forward only if the hvc is trapped
> from the nested VM.

I disagree. HVC from EL2 must be routed to the same EL2. HVC from EL1
must be routed to the EL2 controlling EL1.

In no circumstances should HVC from a NV guest be directly handled by
the host hypervisor. That's what SMC is for.

Please read the pseudocode for HVC.

	M.

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