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

Marc Zyngier
2026-01-14T17:50:30Z

On Wed, 14 Jan 2026 16:50:50 +0000,
Ben Horgan <ben.horgan@arm.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@arm.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@arm.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.