linux-kernel - Re: [PATCH v4 02/10] KVM: arm64: Add ptdump registration with debugfs for the stage-2 pagetables

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Date: Mon, 5 Feb 2024 13:14:27 +0000
From: Oliver Upton <oliver.upton@...ux.dev>
To: Sebastian Ene <sebastianene@...gle.com>
Cc: will@...nel.org, James Morse <james.morse@....com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Zenghui Yu <yuzenghui@...wei.com>, catalin.marinas@....com,
	mark.rutland@....com, akpm@...ux-foundation.org, maz@...nel.org,
	kvmarm@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org, kernel-team@...roid.com,
	vdonnefort@...gle.com, qperret@...gle.com, smostafa@...gle.com
Subject: Re: [PATCH v4 02/10] KVM: arm64: Add ptdump registration with
 debugfs for the stage-2 pagetables

On Thu, Feb 01, 2024 at 11:20:53AM +0000, Sebastian Ene wrote:
> On Thu, Dec 21, 2023 at 06:14:20PM +0000, Oliver Upton wrote:
> 
> Hi Oliver,
> 
> I am planning to split the series based on your suggestion and I
> wanted to make sure that I understand your feedback.
> 
> > On Mon, Dec 18, 2023 at 01:58:52PM +0000, Sebastian Ene wrote:
> > > +config PTDUMP_STAGE2_DEBUGFS
> > > +       bool "Present the stage-2 pagetables to debugfs"
> > > +       depends on PTDUMP_DEBUGFS && KVM
> > > +       default n
> > > +       help
> > > +         Say Y here if you want to show the stage-2 kernel pagetables
> > > +         layout in a debugfs file. This information is only useful for kernel developers
> > > +         who are working in architecture specific areas of the kernel.
> > > +         It is probably not a good idea to enable this feature in a production
> > > +         kernel.
> > 
> > It isn't really a good idea to mount debugfs at all in a production
> > system. There are already plenty worse interfaces lurking in that
> > filesystem. The pKVM portions already depend on CONFIG_NVHE_EL2_DEBUG,
> > so I don't see a need for this Kconfig option.
> > 
> 
> I created a separate option because I wanted to re-use the parsing
> functionality from the already existing ptdump code for EL1. This option
> is turned off in production and only enabled for debug.
> 
> I was thinking to make use of the `CONFIG_NVHE_EL2_DEBUG` but then I abandoned 
> this ideea as one can use ptdump for vHE as well.

Fair enough. I was going to say we could just have KVM follow
CONFIG_PTDUMP_DEBUGFS, but it doesn't matter either way.

> > > +void kvm_ptdump_register_host(void)
> > > +{
> > > +	if (!is_protected_kvm_enabled())
> > > +		return;
> > > +
> > > +	kvm_ptdump_debugfs_register(&host_reg, "host_page_tables",
> > > +				    kvm_debugfs_dir);
> > > +}
> > > +
> > > +static int __init kvm_host_ptdump_init(void)
> > > +{
> > > +	host_reg.priv = (void *)host_s2_pgtable_pages();
> > > +	return 0;
> > > +}
> > > +
> > > +device_initcall(kvm_host_ptdump_init);
> > 
> > Why can't all of this be called from finalize_pkvm()?
> > 
> 
> I guess it can be called from finalize_pkvm before the is_protected_kvm_enabled
> check. This should work for nvhe & vhe as well.

What does nvhe and vhe modes have to do with it? I thought this was for
hooking up the host's S2, which does not exist outside protected mode.

-- 
Thanks,
Oliver