Re: [PATCH 6.6 v1] KVM: arm64: Disable MPAM visibility by default and ignore VMM writes

[Greg KH <gregkh@linuxfoundation.org>]

On Thu, Dec 12, 2024 at 03:14:06PM +0000, Joey Gouly wrote:
> From: James Morse <james.morse@arm.com>
> 
> commit 6685f5d572c22e1003e7c0d089afe1c64340ab1f upstream.
> 
> commit 011e5f5bf529f ("arm64/cpufeature: Add remaining feature bits in
> ID_AA64PFR0 register") exposed the MPAM field of AA64PFR0_EL1 to guests,
> but didn't add trap handling. A previous patch supplied the missing trap
> handling.
> 
> Existing VMs that have the MPAM field of ID_AA64PFR0_EL1 set need to
> be migratable, but there is little point enabling the MPAM CPU
> interface on new VMs until there is something a guest can do with it.
> 
> Clear the MPAM field from the guest's ID_AA64PFR0_EL1 and on hardware
> that supports MPAM, politely ignore the VMMs attempts to set this bit.
> 
> Guests exposed to this bug have the sanitised value of the MPAM field,
> so only the correct value needs to be ignored. This means the field
> can continue to be used to block migration to incompatible hardware
> (between MPAM=1 and MPAM=5), and the VMM can't rely on the field
> being ignored.
> 
> Signed-off-by: James Morse <james.morse@arm.com>
> Co-developed-by: Joey Gouly <joey.gouly@arm.com>
> Signed-off-by: Joey Gouly <joey.gouly@arm.com>
> Reviewed-by: Gavin Shan <gshan@redhat.com>
> Tested-by: Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>
> Reviewed-by: Marc Zyngier <maz@kernel.org>
> Link: https://lore.kernel.org/r/20241030160317.2528209-7-joey.gouly@arm.com
> Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
> [ joey: fixed up merge conflict, no ID_FILTERED macro in 6.6 ]
> Signed-off-by: Joey Gouly <joey.gouly@arm.com>
> Cc: stable@vger.kernel.org # 6.6.x
> Cc: Vitaly Chikunov <vt@altlinux.org>
> Link: https://lore.kernel.org/linux-arm-kernel/20241202045830.e4yy3nkvxtzaybxk@altlinux.org/
> ---
> 
> This fixes an issue seen when using KVM with a 6.6 host kernel, and
> newer (6.13+) kernels in the guest.
> 
> Tested with a stripped down version of set_id_regs from the original
> patch series.

What about 6.12.y?  You can't just skip a stable tree, otherwise you
will get a regression when you upgrade to 6.12.y, right?

thanks,

greg k-h