Re: [PATCH v6 0/7] KVM: arm64: Hide unsupported MPAM from the guest

[Marc Zyngier <maz@kernel.org>]

On Wed, 30 Oct 2024 16:03:10 +0000,
Joey Gouly <joey.gouly@arm.com> wrote:
> 
> Hi,
> 
> changes since v5 [1]:
> 	- Removed Kconfig option, the KVM traps shouldn't rely on host kernel support
> 	- Renamed cpus_support_* to system_supports_* since that matches other functions
> 	- Replace static branch arm64_mpam_has_hcr with a normal cpufeature capability
> 	- Add MPAM*_EL2 regs to KVM (undef_access)
> 	- Use constants in the test, instead of hardcoded values
> 	- Added R-b and T-b tags
> 	- Rebased on v6.12-rc5
> 
> James wrote:
> 	This series fixes up a long standing bug where MPAM was accidentally exposed
> 	to a guest, but the feature was not otherwise trapped or context switched.
> 	This could result in KVM warning about unexpected traps, and injecting an
> 	undef into the guest contradicting the ID registers.
> 	This would prevent an MPAM aware kernel from booting - fortunately, there
> 	aren't any of those.
> 
> 	Ideally, we'd take the MPAM feature away from the ID registers, but that
> 	would leave existing guests unable to migrate to a newer kernel. Instead,
> 	just ignore that field when it matches the hardware. KVM wasn't going to
> 	expose MPAM anyway. The guest will not see MPAM in the id registers.
> 
> 	This series includes the head.S and KVM changes to enable/disable traps. If
> 	MPAM is neither enabled nor emulated by EL3 firmware, these system register
> 	accesses will trap to EL3.
> 	If your kernel doesn't boot, and the problem bisects here - please update
> 	your firmware. MPAM has been supported by trusted firmware since v1.6 in
> 	2018. (also noted on patch 3).
> 
> Thanks,
> Joey
> 
> [1] https://lore.kernel.org/kvmarm/20241015133923.3910916-1-joey.gouly@arm.com/
> 
> James Morse (7):
>   arm64/sysreg: Convert existing MPAM sysregs and add the remaining
>     entries
>   arm64: head.S: Initialise MPAM EL2 registers and disable traps
>   arm64: cpufeature: discover CPU support for MPAM
>   KVM: arm64: Fix missing traps of guest accesses to the MPAM registers
>   KVM: arm64: Add a macro for creating filtered sys_reg_descs entries
>   KVM: arm64: Disable MPAM visibility by default and ignore VMM writes
>   KVM: arm64: selftests: Test ID_AA64PFR0.MPAM isn't completely ignored
> 
>  .../arch/arm64/cpu-feature-registers.rst      |   2 +
>  arch/arm64/include/asm/cpu.h                  |   1 +
>  arch/arm64/include/asm/cpucaps.h              |   5 +
>  arch/arm64/include/asm/cpufeature.h           |  17 ++
>  arch/arm64/include/asm/el2_setup.h            |  14 ++
>  arch/arm64/include/asm/kvm_arm.h              |   1 +
>  arch/arm64/include/asm/sysreg.h               |  12 --
>  arch/arm64/kernel/cpufeature.c                |  96 +++++++++++
>  arch/arm64/kernel/cpuinfo.c                   |   3 +
>  arch/arm64/kvm/hyp/include/hyp/switch.h       |  31 ++++
>  arch/arm64/kvm/sys_regs.c                     | 121 +++++++++----
>  arch/arm64/tools/cpucaps                      |   2 +
>  arch/arm64/tools/sysreg                       | 161 ++++++++++++++++++
>  .../selftests/kvm/aarch64/set_id_regs.c       |  99 ++++++++++-
>  14 files changed, 523 insertions(+), 42 deletions(-)

For the series:

Reviewed-by: Marc Zyngier <maz@kernel.org>

	M.

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