linux-kernel - Re: [RFC PATCH 06/38] KVM: arm64: Force guest EL1 to use user-space's partid configuration

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Message-ID: <f7011785-90df-4355-bc4c-f4be96799015@arm.com>
Date: Tue, 9 Dec 2025 15:32:00 +0000
From: Ben Horgan <ben.horgan@....com>
To: James Morse <james.morse@....com>, linux-kernel@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org
Cc: D Scott Phillips OS <scott@...amperecomputing.com>,
 carl@...amperecomputing.com, lcherian@...vell.com,
 bobo.shaobowang@...wei.com, tan.shaopeng@...itsu.com,
 baolin.wang@...ux.alibaba.com, Jamie Iles <quic_jiles@...cinc.com>,
 Xin Hao <xhao@...ux.alibaba.com>, peternewman@...gle.com,
 dfustini@...libre.com, amitsinght@...vell.com,
 David Hildenbrand <david@...nel.org>, Dave Martin <dave.martin@....com>,
 Koba Ko <kobak@...dia.com>, Shanker Donthineni <sdonthineni@...dia.com>,
 fenghuay@...dia.com, baisheng.gao@...soc.com,
 Jonathan Cameron <jonathan.cameron@...wei.com>, Gavin Shan
 <gshan@...hat.com>, rohit.mathew@....com, reinette.chatre@...el.com,
 Punit Agrawal <punit.agrawal@....qualcomm.com>
Subject: Re: [RFC PATCH 06/38] KVM: arm64: Force guest EL1 to use user-space's
 partid configuration

Hi James,

On 12/5/25 21:58, James Morse wrote:
> While we trap the guest's attempts to read/write the MPAM control
> registers, the hardware continues to use them. Guest-EL0 uses KVM's
> user-space's configuration, as the value is left in the register, and
> guest-EL1 uses either the host kernel's configuration, or in the case of
> VHE, the UNKNOWN reset value of MPAM1_EL1.
> 
> On nVHE systems, EL2 continues to use partid-0 for world-switch, even
> when the host may have configured its kernel threads to use a different
> partid. 0 may have been assigned to another task.
> > We want to force the guest-EL1 to use KVM's user-space's MPAM
> configuration, and EL2s to match the host's EL1 config.
> 
> On a nVHE system, copy the EL1 MPAM register to EL2. This ensures
> world-switch uses the same partid as the kernel thread does on the host.
> 
> When loading the guest's EL1 registers, copy the VMM's EL0 partid to
> the EL1 register.
> 
> For VHE systems, we can skip restoring the EL1 register for the host,
> as it is out-of-context once HCR_EL2.TGE is set.
> 
> This is done outside the usual sysreg save/restore as the values can
> change behind KVMs back, so should not be stored in the guest context.
> 
> Signed-off-by: James Morse <james.morse@....com>

One thing that is needed in kvm now that we are changing MPAM for EL1 is
an update to the __*activate_traps_mpam() to not assume partids and pmgs
are 0. I have an existing patch to deal with this.

Thanks,

Ben