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

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Message-ID: <86jz7sl25r.wl-maz@kernel.org>
Date: Thu, 10 Apr 2025 11:52:00 +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 11:20:24 +0100,
Ganapatrao Kulkarni <gankulkarni@...amperecomputing.com> wrote:
> 
> 
> 
> On 10-04-2025 12:49 pm, Marc Zyngier wrote:
> > 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.
> 
> Thanks, Understood, In NV case, hvc has to be forwarded to L1
> irrespective of it origin (L1 or L2). Need to add hvc handler in the
> smccc_filter.c for the vm (when run as L1), so that it is handled and
> returns with required args set.

Why? This test checks under which conditions an HVC/SMC gets routed to
userspace. What does it even mean to test HVC if it doesn't make it
outside of the guest itself?

	M.

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