linux-kernel - Re: [PATCH] KVM: arm64: Check cpu_has_spe() before initializing PMSCR

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Message-ID: <861pndzn4w.wl-maz@kernel.org>
Date: Wed, 08 Oct 2025 11:46:55 +0100
From: Marc Zyngier <maz@...nel.org>
To:  Mukesh Ojha <mukesh.ojha@....qualcomm.com>, Oliver Upton <oliver.upton@...ux.dev>
Cc:	joey.gouly@....com,
	suzuki.poulose@....com,
	yuzenghui@...wei.com,
	catalin.marinas@....com,
	will@...nel.org,
	alexandru.elisei@....com,
	linux-arm-kernel@...ts.infradead.org,
	kvmarm@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: arm64: Check cpu_has_spe() before initializing PMSCR_EL1 in VHE

On Tue, 07 Oct 2025 19:31:45 +0100,
Oliver Upton <oliver.upton@...ux.dev> wrote:
> 
> Hi Mukesh,
> 
> I find it a bit odd to refer to cpu_has_spe() in the shortlog, which
> doesn't exist prior to this patch.
> 
> On Tue, Oct 07, 2025 at 11:53:56PM +0530, Mukesh Ojha wrote:
> > commit efad60e46057 ("KVM: arm64: Initialize PMSCR_EL1 when in VHE")
> > initializes PMSCR_EL1 to 0 which is making the boot up stuck when KVM
> > runs in VHE mode and reverting the change is fixing the issue.
> > 
> > [    2.967447] RPC: Registered tcp NFSv4.1 backchannel transport module.
> > [    2.974061] PCI: CLS 0 bytes, default 64
> > [    2.978171] Unpacking initramfs...
> > [    2.982889] kvm [1]: nv: 568 coarse grained trap handlers
> > [    2.988573] kvm [1]: IPA Size Limit: 40 bits
> > 
> > Lets guard the change with cpu_has_spe() check so that it only affects
> > the cpu which has SPE feature supported.
> 
> This could benefit from being spelled out a bit more. In both cases we
> check for the presence of FEAT_SPE, however I believe the issue you
> observe is EL3 hasn't delegated ownership of the Profiling Buffer to
> Non-secure nor does it reinject an UNDEF in response to the sysreg trap.
> 
> I agree that the change is correct but the rationale needs to be clear.

To me, this smells a lot more like some sort of papering over a
firmware bug. Why isn't SPE available the first place?

	M.

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