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From: Marc Zyngier <maz@kernel.org>
To: Kristina Martsenko <kristina.martsenko@arm.com>
Cc: linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Oliver Upton <oliver.upton@linux.dev>,
	James Morse <james.morse@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Zenghui Yu <yuzenghui@huawei.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Mark Brown <broonie@kernel.org>,
	Luis Machado <luis.machado@arm.com>,
	Vladimir Murzin <vladimir.murzin@arm.com>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 03/10] KVM: arm64: switch HCRX_EL2 between host and guest
Date: Thu, 16 Feb 2023 16:35:49 +0000	[thread overview]
Message-ID: <86o7pty3dm.wl-maz@kernel.org> (raw)
In-Reply-To: <20230216160012.272345-4-kristina.martsenko@arm.com>

On Thu, 16 Feb 2023 16:00:05 +0000,
Kristina Martsenko <kristina.martsenko@arm.com> wrote:
> 
> Switch the HCRX_EL2 register between host and guest configurations, in
> order to enable different features in the host and guest.
> 
> Note that the guest flags are only set if all CPUs have HCRX_EL2.
> Asymmetric systems where only some CPUs have HCRX_EL2 are not supported
> and will result in guests running with the host flags set (and a "SANITY
> CHECK" warning printed for the host).
> 
> After this change, SMPME is no longer set for guests, which should have
> no effect as SME is currently disabled for guests.

Why not preserve the behaviour by propagating the flag into the guest
setup?

> 
> Signed-off-by: Kristina Martsenko <kristina.martsenko@arm.com>
> ---
> 
> I wasn't sure what to do about asymmetric systems. It seems a bit
> fragile, maybe someone has a better idea?

I would simply prevent these CPUs from booting if they come after a
primary CPU that has the feature. These hypothetical asymmetric setups
put a huge complexity on the kernel, and I'm worried that we're just
giving implementers too much freedom.

If someone comes up with that sort of stuff, they can write the
patches themselves... Or do you know of any braindead setup involving
an asymmetric FEAT_HCX implementation?

Thanks,

	M.

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

  parent reply	other threads:[~2023-02-16 16:35 UTC|newest]

Thread overview: 28+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-02-16 16:00 [PATCH 00/10] arm64: support Armv8.8 memcpy instructions in userspace Kristina Martsenko
2023-02-16 16:00 ` [PATCH 01/10] KVM: arm64: initialize HCRX_EL2 Kristina Martsenko
2023-03-17 14:25   ` Catalin Marinas
2023-02-16 16:00 ` [PATCH 02/10] arm64: cpufeature: detect FEAT_HCX Kristina Martsenko
2023-03-17 14:25   ` Catalin Marinas
2023-02-16 16:00 ` [PATCH 03/10] KVM: arm64: switch HCRX_EL2 between host and guest Kristina Martsenko
2023-02-16 16:20   ` Mark Brown
2023-02-22 18:36     ` Kristina Martsenko
2023-02-16 16:35   ` Marc Zyngier [this message]
2023-02-22 18:42     ` Kristina Martsenko
2023-02-16 16:00 ` [PATCH 04/10] arm64: mops: document boot requirements for MOPS Kristina Martsenko
2023-03-17 15:07   ` Catalin Marinas
2023-03-24  1:00     ` Kristina Martsenko
2023-04-04 10:50       ` Catalin Marinas
2023-04-11 16:57         ` Kristina Martsenko
2023-02-16 16:00 ` [PATCH 05/10] arm64: mops: don't disable host MOPS instructions from EL2 Kristina Martsenko
2023-03-17 15:07   ` Catalin Marinas
2023-02-16 16:00 ` [PATCH 06/10] KVM: arm64: hide MOPS from guests Kristina Martsenko
2023-03-17 15:09   ` Catalin Marinas
2023-02-16 16:00 ` [PATCH 07/10] arm64: mops: handle MOPS exceptions Kristina Martsenko
2023-03-17 15:45   ` Catalin Marinas
2023-02-16 16:00 ` [PATCH 08/10] arm64: mops: handle single stepping after MOPS exception Kristina Martsenko
2023-03-17 16:02   ` Catalin Marinas
2023-02-16 16:00 ` [PATCH 09/10] arm64: mops: detect and enable FEAT_MOPS Kristina Martsenko
2023-02-16 16:22   ` Mark Brown
2023-03-17 16:03   ` Catalin Marinas
2023-02-16 16:00 ` [PATCH 10/10] arm64: mops: allow disabling MOPS from the kernel command line Kristina Martsenko
2023-03-17 16:04   ` Catalin Marinas

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