Re: [PATCH v2 0/7] KVM: arm64: Use visibility hook to treat ID regs as RAZ

From: Oliver Upton <oliver.upton@linux.dev>
To: Marc Zyngier <maz@kernel.org>, James Morse <james.morse@arm.com>,
	Alexandru Elisei <alexandru.elisei@arm.com>
Cc: linux-arm-kernel@lists.infradead.org,
	kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org,
	Reiji Watanabe <reijiw@google.com>
Subject: Re: [PATCH v2 0/7] KVM: arm64: Use visibility hook to treat ID regs as RAZ
Date: Fri, 2 Sep 2022 15:52:48 +0000	[thread overview]
Message-ID: <YxIm0GE7LA6cvB12@google.com> (raw)
In-Reply-To: <20220902154804.1939819-1-oliver.upton@linux.dev>

Lol, mess up my own copypasta:

On Fri, Sep 02, 2022 at 03:47:56PM +0000, Oliver Upton wrote:
> For reasons unknown, the Arm architecture defines the 64-bit views of
> the 32-bit ID registers as UNKNOWN [1]. This combines poorly with the
					^ on AArch64-only systems.

> fact that KVM unconditionally exposes these registers to userspace,
> which could throw a wrench in migration between 64-bit only systems.
> 
> This series reworks KVM's definition of these registers to RAZ/WI with
> the goal of providing consistent register values across 64-bit machines.
> 
> Patches 1-3 clean up the ID register accessors, taking advantage of the
> fact that the generic accessors know how to handle RAZ.
> 
> Patches 4-6 start switch the handling of potentially nonzero AArch32 ID
> registers to RAZ/WI. RAZ covers up the architecturally UNKNOWN values,
> and WI allows for migration off of kernels that may provide garbage.
> Note that hidden AArch32 ID registers continue to have RAZ behavior with
> the additional expectation of invariance.
> 
> Lastly, patch 7 includes a small test for the issue.
> 
> Applies to 6.0-rc3. Tested with KVM selftests under the fast model w/
> asymmetric 32 bit support and no 32 bit support whatsoever.

[1]: DDI0487H.a Table D12-2 'Instruction encodings for non-Debug System Register accesses'

v1: https://lore.kernel.org/kvmarm/20220817214818.3243383-1-oliver.upton@linux.dev/

--
Thanks,
Oliver

> v1 -> v2:
>  - Collect Reiji's r-b tags (thanks!)
>  - Call sysreg_visible_as_raz() from read_id_reg() (Reiji)
>  - Hoist sysreg_user_write_ignore() into kvm_sys_reg_set_user() (Reiji)
> 
> Oliver Upton (7):
>   KVM: arm64: Use visibility hook to treat ID regs as RAZ
>   KVM: arm64: Remove internal accessor helpers for id regs
>   KVM: arm64: Drop raz parameter from read_id_reg()
>   KVM: arm64: Spin off helper for calling visibility hook
>   KVM: arm64: Add a visibility bit to ignore user writes
>   KVM: arm64: Treat 32bit ID registers as RAZ/WI on 64bit-only system
>   KVM: selftests: Add test for RAZ/WI AArch32 ID registers
> 
>  arch/arm64/kvm/sys_regs.c                     | 150 +++++++++---------
>  arch/arm64/kvm/sys_regs.h                     |  24 ++-
>  tools/testing/selftests/kvm/.gitignore        |   1 +
>  tools/testing/selftests/kvm/Makefile          |   1 +
>  .../kvm/aarch64/aarch64_only_id_regs.c        | 135 ++++++++++++++++
>  5 files changed, 225 insertions(+), 86 deletions(-)
>  create mode 100644 tools/testing/selftests/kvm/aarch64/aarch64_only_id_regs.c
> 
> 
> base-commit: b90cb1053190353cc30f0fef0ef1f378ccc063c5
> -- 
> 2.37.2.789.g6183377224-goog
> 

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