Re: KVM works on RPi4

From: Marc Zyngier <marc.zyngier@arm.com>
To: Jan Kiszka <jan.kiszka@web.de>
Cc: kvmarm@lists.cs.columbia.edu, kvm <kvm@vger.kernel.org>
Subject: Re: KVM works on RPi4
Date: Sat, 29 Jun 2019 23:42:32 +0100	[thread overview]
Message-ID: <20190629234232.484ca3c0@why> (raw)
In-Reply-To: <1d1198c2-f362-840d-cb14-9a6d74da745c@web.de>

On Sat, 29 Jun 2019 19:09:37 +0200
Jan Kiszka <jan.kiszka@web.de> wrote:

Hi Jan,

> Hi all,
> 
> just got KVM running on the Raspberry Pi4. Seems they now embedded all
> required logic into that new SoC.

Yeah, someone saw the light and decided to enter the 21st century by
attaching a GICv2 to the thing. Who knows, they may plug a GICv3 and a
SMMU in 2050 at that rate! ;-)

> However, as the Raspberry kernel is not yet ready for 64-bit (and
> upstream is not in sight), I had to use legacy 32-bit mode. And there we
> stumble over the core detection. This little patch made it work, though:
> 
> diff --git a/arch/arm/kvm/guest.c b/arch/arm/kvm/guest.c
> index 2b8de885b2bf..01606aad73cc 100644
> --- a/arch/arm/kvm/guest.c
> +++ b/arch/arm/kvm/guest.c
> @@ -290,6 +290,7 @@ int __attribute_const__ kvm_target_cpu(void)
>  	case ARM_CPU_PART_CORTEX_A7:
>  		return KVM_ARM_TARGET_CORTEX_A7;
>  	case ARM_CPU_PART_CORTEX_A15:
> +	case ARM_CPU_PART_CORTEX_A72:
>  		return KVM_ARM_TARGET_CORTEX_A15;
>  	default:
>  		return -EINVAL;
> 
> That raises the question if this is hack or a valid change and if there
> is general interest in mapping 64-bit cores on 32-bit if they happen to
> run in 32-bit mode.

The real thing to do here would be to move to a generic target, much
like we did on the 64bit side. Could you investigate that instead? It
would also allow KVM to be used on other 32bit cores such as
A12/A17/A32.

Although some would argue that the *real* real thing to do would be "rm
-rf arch/arm/kvm" and be done with it, but that's a discussion for next
week... ;-)

> Jan
> 
> PS: The RPi device tree lacks description of the GICH maintenance
> interrupts. Seems KVM is fine without that - because it has the
> information hard-coded or because it can live without that interrupt?

Nah, it really should have an interrupt here. You can end-up in
situation where new virtual interrupts are delayed until the next
natural exit if you don't get a maintenance interrupt. Feels like a bug.

Anyway, if you know of any effort to get a 64bit kernel on that thing,
I'm interested in helping. I bought one on Monday, but didn't get a
change to do any hacking on it just yet...

Thanks,

	M.
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