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

[Marc Zyngier <maz@kernel.org>]

On Sun, 15 Dec 2024 09:22:53 +0000,
Greg KH <gregkh@linuxfoundation.org> wrote:
> 
> 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?

Posted as [1].

	M.

[1] https://lore.kernel.org/r/20241216085002.334880-1-maz@kernel.org

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