Re: [PATCH] KVM: arm64: Cap default IPA size to the host's own size

From: Andrew Jones <drjones@redhat.com>
To: Marc Zyngier <maz@kernel.org>
Cc: kvmarm@lists.cs.columbia.edu,
	linux-arm-kernel@lists.infradead.org, kvm@vger.kernel.org,
	James Morse <james.morse@arm.com>,
	Julien Thierry <julien.thierry.kdev@gmail.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Alexandru Elisei <alexandru.elisei@arm.com>,
	kernel-team@android.com
Subject: Re: [PATCH] KVM: arm64: Cap default IPA size to the host's own size
Date: Tue, 9 Mar 2021 14:20:21 +0100	[thread overview]
Message-ID: <20210309132021.7vuuf73joybhlhg3@kamzik.brq.redhat.com> (raw)
In-Reply-To: <20210308174643.761100-1-maz@kernel.org>

Hi Marc,

On Mon, Mar 08, 2021 at 05:46:43PM +0000, Marc Zyngier wrote:
> KVM/arm64 has forever used a 40bit default IPA space, partially
> due to its 32bit heritage (where the only choice is 40bit).
> 
> However, there are implementations in the wild that have a *cough*
> much smaller *cough* IPA space, which leads to a misprogramming of
> VTCR_EL2, and a guest that is stuck on its first memory access
> if userspace dares to ask for the default IPA setting (which most
> VMMs do).
> 
> Instead, cap the default IPA size to what the host can actually
> do, and spit out a one-off message on the console. The boot warning
> is turned into a more meaningfull message, and the new behaviour
> is also documented.
> 
> Although this is a userspace ABI change, it doesn't really change
> much for userspace:
> 
> - the guest couldn't run before this change, while it now has
>   a chance to if the memory range fits the reduced IPA space
> 
> - a memory slot that was accepted because it did fit the default
>   IPA space but didn't fit the HW constraints is now properly
>   rejected

I'm not sure deferring the misconfiguration error until memslot
request time is better than just failing to create a VM. If
userspace doesn't use KVM_CAP_ARM_VM_IPA_SIZE to determine the
limit (which it hasn't been obliged to do) and it is able to
successfully create a VM, then it will assume up to 40-bit IPAs
are supported. Later, when it tries to add memslots and fails
it may be confused, especially if that later is much, much later
with memory hotplug.

> 
> The other thing that's left doing is to convince userspace to
> actually use the IPA space setting instead of relying on the
> antiquated default.

Failing to create any VM which hasn't selected a valid IPA limit
should be pretty convincing :-)

Thanks,
drew

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