linux-kernel - Re: [PATCH v3 00/10] arm64: ptdump: View the second stage page-tables

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Message-ID: <ZV6MVYI8J6NFS9Wl@linux.dev>
Date:   Wed, 22 Nov 2023 23:18:45 +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 v3 00/10] arm64: ptdump: View the second stage page-tables

Hi Seb,

On Wed, Nov 15, 2023 at 05:16:30PM +0000, Sebastian Ene wrote:
> Hi,
> 
> This can be used as a debugging tool for dumping the second stage
> page-tables.
> 
> When CONFIG_PTDUMP_STAGE2_DEBUGFS is enabled, ptdump registers 
> '/sys/debug/kvm/<guest_id>/stage2_page_tables' entry with debugfs
> upon guest creation. This allows userspace tools (eg. cat) to dump the
> stage-2 pagetables by reading the registered file.
> 
> Reading the debugfs file shows stage-2 memory ranges in following format:
> <IPA range> <size> <descriptor type> <access permissions> <mem_attributes>
> 
> Under pKVM configuration(kvm-arm.mode=protected) ptdump registers an entry
> for the host stage-2 pagetables in the following path:
> /sys/debug/kvm/host_stage2_page_tables/
> 
> The tool interprets the pKVM ownership annotation stored in the invalid
> entries and dumps to the console the ownership information. To be able
> to access the host stage-2 page-tables from the kernel, a new hypervisor
> call was introduced which allows us to snapshot the page-tables in a host
> provided buffer. The hypervisor call is hidden behind CONFIG_NVHE_EL2_DEBUG
> as this should be used under debugging environment.

While I think the value of the feature you're proposing is great, I'm
not a fan of the current shape of this series.

Reusing note_page() for the stage-2 dump is somewhat convenient, but the
series pulls a **massive** amount of KVM details outside of KVM:

 - Open-coding the whole snapshotting interface with EL2 outside of KVM.
   This is a complete non-starter for me; the kernel<->EL2 interface
   needs to be owned by the EL1 portions of KVM.

 - Building page-table walkers using the KVM pgtable library outside of
   KVM.

 - Copying (rather than directly calling) the logic responsible for
   things like FWB and PGD concatenation.

 - Hoisting the definition of _software bits_ outside of KVM. I'm less
   concerned about hardware bits since they have an unambiguous meaning.

I think exporting the necessary stuff from ptdump into KVM will lead to
a much cleaner implementation.

-- 
Thanks,
Oliver