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

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Message-ID: <91339d4d-38d7-4512-9fda-339bb4c2a3f4@os.amperecomputing.com>
Date: Thu, 10 Apr 2025 18:52:45 +0530
From: Ganapatrao Kulkarni <gankulkarni@...amperecomputing.com>
To: Marc Zyngier <maz@...nel.org>
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 10-04-2025 04:22 pm, Marc Zyngier wrote:
> 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?

smccc_filter.c has 2 tests (test_filter_denied and 
test_filter_fwd_to_user), which runs the vm(guest_code).

I was trying to modify test_filter_denied to run in vEL2, which led to 
this patch/discussion. I agree, it does not makes sense to run this test 
for vEL2.

test_filter_fwd_to_user is not feasible to run in vEL2.
Thanks for the feedback.

BTW, I could add hvc handler and run test_filter_denied in vEL2.

-- 
Thanks,
Ganapat/GK