linux-kernel - Re: [PATCH v3 12/47] KVM: arm64: Force guest EL1 to use user-space's partid configuration

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Message-ID: <86ldhzkzzb.wl-maz@kernel.org>
Date: Thu, 15 Jan 2026 09:05:28 +0000
From: Marc Zyngier <maz@...nel.org>
To: Ben Horgan <ben.horgan@....com>
Cc: amitsinght@...vell.com,
	baisheng.gao@...soc.com,
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	kvmarm@...ts.linux.dev
Subject: Re: [PATCH v3 12/47] KVM: arm64: Force guest EL1 to use user-space's partid configuration

On Wed, 14 Jan 2026 14:50:22 +0000,
Ben Horgan <ben.horgan@....com> wrote:
> 
> Hi Marc,
> 
> On 1/14/26 12:06, Marc Zyngier wrote:
> > On Mon, 12 Jan 2026 16:58:39 +0000,
> > Ben Horgan <ben.horgan@....com> wrote:
> >>
> >> From: James Morse <james.morse@....com>
> >>
> >> 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.
> >>
> >> We want to force the guest-EL1 to use KVM's user-space's MPAM
> >> configuration. On nVHE rely on MPAM0_EL1 and MPAM1_EL1 always being
> >> programmed the same and on VHE copy MPAM0_EL1 into the guest's
> >> MPAM1_EL1. There is no need to restore as this is out of context once TGE
> >> is set.
> >>
> >> Signed-off-by: James Morse <james.morse@....com>
> >> Signed-off-by: Ben Horgan <ben.horgan@....com>
> >> ---
> >> Changes since rfc:
> >> Drop the unneeded __mpam_guest_load() in nvhre and the MPAM1_EL1 save restore
> >> Defer EL2 handling until next patch
> >>
> >> Changes since v2:
> >> Use mask (Oliver)
> >> ---
> >>  arch/arm64/kvm/hyp/vhe/sysreg-sr.c | 13 +++++++++++++
> >>  1 file changed, 13 insertions(+)
> >>
> >> diff --git a/arch/arm64/kvm/hyp/vhe/sysreg-sr.c b/arch/arm64/kvm/hyp/vhe/sysreg-sr.c
> >> index f28c6cf4fe1b..9fb8e6628611 100644
> >> --- a/arch/arm64/kvm/hyp/vhe/sysreg-sr.c
> >> +++ b/arch/arm64/kvm/hyp/vhe/sysreg-sr.c
> >> @@ -183,6 +183,18 @@ void sysreg_restore_guest_state_vhe(struct kvm_cpu_context *ctxt)
> >>  }
> >>  NOKPROBE_SYMBOL(sysreg_restore_guest_state_vhe);
> >>  
> >> +/*
> >> + * The _EL0 value was written by the host's context switch and belongs to the
> >> + * VMM. Copy this into the guest's _EL1 register.
> >> + */
> >> +static inline void __mpam_guest_load(void)
> >> +{
> >> +	u64 mask = MPAM0_EL1_PARTID_D | MPAM0_EL1_PARTID_I | MPAM0_EL1_PMG_D | MPAM0_EL1_PMG_I;
> >> +
> >> +	if (system_supports_mpam())
> >> +		write_sysreg_el1(read_sysreg_s(SYS_MPAM0_EL1) & mask, SYS_MPAM1);
> >> +}
> >> +
> >>  /**
> >>   * __vcpu_load_switch_sysregs - Load guest system registers to the physical CPU
> >>   *
> >> @@ -222,6 +234,7 @@ void __vcpu_load_switch_sysregs(struct kvm_vcpu *vcpu)
> >>  	 */
> >>  	__sysreg32_restore_state(vcpu);
> >>  	__sysreg_restore_user_state(guest_ctxt);
> >> +	__mpam_guest_load();
> > 
> > What's the rationale for doing this independently of rest of the MPAM
> > stuff in __activate_traps_mpam()?
> 
> The __activate_traps_mpam() is relevant even for nvhe but
> __mpam_guest_load() is only need in vhe as otherwise we can rely on
> MPAM1_EL1 and MPAM0_EL0 having the same partid/pmg configuration

It is completely unclear to me what enforces this. Please point me to
the code that does that.

> (although this MPAM policy will likely become configurable sometime down
> the line).

Or not. the VM only exists as an extension of userspace, and I don't
see on what grounds it should get its own MPAM configuration.

> Besides that it just makes the naming less exact.

I don't care about the naming. I care about how the configuration flow
is organised. And so far, this seems extremely messy.

Can you please document what gets configured when and in which mode?

Thanks,

	M.

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