[RFC] [PATCH 0/4] KVM: arm64: emulation for CTR_EL0

From: Sebastian Ott <sebott@redhat.com>
To: linux-arm-kernel@lists.infradead.org
Cc: Marc Zyngier <maz@kernel.org>, Oliver Upton <oliver.upton@linux.dev>
Subject: [RFC] [PATCH 0/4] KVM: arm64: emulation for CTR_EL0
Date: Mon, 18 Mar 2024 12:16:32 +0100	[thread overview]
Message-ID: <20240318111636.10613-1-sebott@redhat.com> (raw)

Hej folks,

I'm looking into supporting migration between 2 Ampere Altra (Max)
machines (using Neoverse-N1). They are almost identical regarding
their feature id register state except for CTR_EL0.DIC which is set
on one machine but not the other.

For a complete picture it's worth noting that the machine with
CTR_EL0.DIC = 1 also suffers from erratum 1542419 meaning for
userspace we trap the access and fake DIC=0. A KVM guest still sees
the original host value for that register (I assume that this is
intentional so that a guest OS can apply its own workaround to the
erratum).

Anyway, CTR_EL0 is currently marked as invariant and migrating a VM
between those 2 machines using qemu fails.

With the patches below guest access to CTR_EL0 is emulated and
CTR_EL0.DIC can be disabled using KVM_SET_ONE_REG. I'm sending this
as an RFC since I likely missed something obvious and there's still
stuff that needs improving - e.g. I didn't look into FGT. The last
patch adds a tool to dump the KVM register state together with the
writable masks - not sure if that's helpful to others but I've used
this to compare the register state between different machines
(abusing kvm selftests for this is probably not the best idea - I
just wanted to use the neat helpers they provide).

Thanks,
Sebastian

Sebastian Ott (4):
  KVM: arm64: add emulation for CTR_EL0 register
  KVM: arm64: ensure guest access to CTR_EL0 is trapped
  KVM: arm64: show writable masks for feature registers
  KVM: selftests: aarch64: add tool to dump registers

 arch/arm64/include/asm/kvm_emulate.h          |  7 +-
 arch/arm64/kvm/sys_regs.c                     | 63 ++++++++++------
 tools/testing/selftests/kvm/Makefile          |  1 +
 .../testing/selftests/kvm/aarch64/dump_regs.c | 72 +++++++++++++++++++
 4 files changed, 114 insertions(+), 29 deletions(-)
 create mode 100644 tools/testing/selftests/kvm/aarch64/dump_regs.c

-- 
2.42.0

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