linux-kernel - Re: [PATCH v3 13/47] KVM: arm64: Use kernel-space partid configuration for hypercalls

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Message-ID: <86ms2gkrrx.wl-maz@kernel.org>
Date: Wed, 14 Jan 2026 17:50:26 +0000
From: Marc Zyngier <maz@...nel.org>
To: Ben Horgan <ben.horgan@....com>
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Subject: Re: [PATCH v3 13/47] KVM: arm64: Use kernel-space partid configuration for hypercalls

On Wed, 14 Jan 2026 16:50:50 +0000,
Ben Horgan <ben.horgan@....com> wrote:
> 
> Hi Marc,
> 
> On 1/14/26 14:39, Ben Horgan wrote:
> > Hi Marc,
> > 
> > On 1/14/26 12:09, Marc Zyngier wrote:
> >> On Mon, 12 Jan 2026 16:58:40 +0000,
> >> Ben Horgan <ben.horgan@....com> wrote:
> >>>
> >>> On nVHE systems whether or not MPAM is enabled, EL2 continues to use
> >>> partid-0 for hypercalls, even when the host may have configured its kernel
> >>> threads to use a different partid. 0 may have been assigned to another
> >>> task. Copy the EL1 MPAM register to EL2. This ensures hypercalls use the
> >>> same partid as the kernel thread does on the host.
> >>>
> >>> Signed-off-by: Ben Horgan <ben.horgan@....com>
> >>> ---
> >>> Changes since v2:
> >>> Use mask
> >>> Use read_sysreg_el1 to cope with hvhe
> >>> ---
> >>>  arch/arm64/kvm/hyp/nvhe/hyp-main.c | 8 ++++++++
> >>>  1 file changed, 8 insertions(+)
> >>>
> >>> diff --git a/arch/arm64/kvm/hyp/nvhe/hyp-main.c b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> >>> index a7c689152f68..ad99d8a73a9e 100644
> >>> --- a/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> >>> +++ b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> >>> @@ -635,6 +635,14 @@ static void handle_host_hcall(struct kvm_cpu_context *host_ctxt)
> >>>  	unsigned long hcall_min = 0;
> >>>  	hcall_t hfn;
> >>>  
> >>> +	if (system_supports_mpam()) {
> >>> +		u64 mask = MPAM1_EL1_PARTID_D | MPAM1_EL1_PARTID_I |
> >>> +			MPAM1_EL1_PMG_D | MPAM1_EL1_PMG_I;
> >>> +
> >>> +		write_sysreg_s(read_sysreg_el1(SYS_MPAM1) & mask, SYS_MPAM2_EL2);
> >>> +		isb();
> >>> +	}
> >>
> >> Is it really OK to not preserve the rest of MPAM2_EL2? This explicitly
> >> clears MPAM2_EL2.MPAMEN, which feels counter-productive.
> >>
> >> 	M.
> >>
> > 
> > There are 3 things to consider:
> > 1. traps - these are only relevant when we leave EL2 and are dealt with
> > in __activate_traps_mpam(). (This also covers EnMPAMSM which is a
> > not-trap bit.)
> > 2. MPAM2_EL2.MPAMEN - this is read only as long as we have an EL3 and if
> > we don't have EL3 will be 0 anyway from el2_setup.h and MPAM won't be
> > considered supported in the kernel.
> > 3. The alternate partid space fields which are kept as zero and relate
> > to FEAT_RME.
> > 
> > So, safe. Ok with you or would you rather I make it more obviously safe?
> 
> As discussed offline, to avoid having to reason about MPAM2_EL2.MPAMEN
> I'll set this bit to 1 in this write as we are already assuming mpam is
> enabled and we want to keep it enabled.

Sounds good, thanks.

	M.

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