Re: [RFC] [PATCH 0/4] KVM: arm64: emulation for CTR_EL0

From: Marc Zyngier <maz@kernel.org>
To: Sebastian Ott <sebott@redhat.com>
Cc: linux-arm-kernel@lists.infradead.org,
	Oliver Upton <oliver.upton@linux.dev>
Subject: Re: [RFC] [PATCH 0/4] KVM: arm64: emulation for CTR_EL0
Date: Mon, 18 Mar 2024 15:24:44 +0000	[thread overview]
Message-ID: <86y1afzhoj.wl-maz@kernel.org> (raw)
In-Reply-To: <20240318111636.10613-1-sebott@redhat.com>

On Mon, 18 Mar 2024 11:16:32 +0000,
Sebastian Ott <sebott@redhat.com> wrote:
> 
> Hej folks,
> 
> I'm looking into supporting migration between 2 Ampere Altra (Max)
> machines (using Neoverse-N1). They are almost identical regarding
> their feature id register state except for CTR_EL0.DIC which is set
> on one machine but not the other.
> 
> For a complete picture it's worth noting that the machine with
> CTR_EL0.DIC = 1 also suffers from erratum 1542419 meaning for
> userspace we trap the access and fake DIC=0. A KVM guest still sees
> the original host value for that register (I assume that this is
> intentional so that a guest OS can apply its own workaround to the
> erratum).

Indeed.

The intention is that the EL1 guest will hide DIC for EL0, while EL3
will trap IC instructions from EL0 and replace them with a TLBI.
That's of course assuming that the machine has received an updated
firmware, something that cannot be probed AFAICT.

	M.

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

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