On Fri, Jan 19, 2024 at 12:09:36AM +1000, Nicholas Piggin wrote: > SAO is a page table attribute that strengthens the memory ordering of > accesses. QEMU with MTTCG does not implement this, so clear it in > ibm,pa-features. There is a complication with spapr migration that is > addressed with comments, it is not a new problem here. > > Signed-off-by: Nicholas Piggin <npig...@gmail.com> > --- > hw/ppc/pnv.c | 5 +++++ > hw/ppc/spapr.c | 15 +++++++++++++++ > 2 files changed, 20 insertions(+) > > diff --git a/hw/ppc/pnv.c b/hw/ppc/pnv.c > index b949398689..4969fbdb05 100644 > --- a/hw/ppc/pnv.c > +++ b/hw/ppc/pnv.c > @@ -158,6 +158,11 @@ static void pnv_dt_core(PnvChip *chip, PnvCore *pc, void > *fdt) > char *nodename; > int cpus_offset = get_cpus_node(fdt); > > + if (qemu_tcg_mttcg_enabled()) { > + /* SSO (SAO) ordering is not supported under MTTCG. */ > + pa_features[4 + 2] &= ~0x80; > + } > + > nodename = g_strdup_printf("%s@%x", dc->fw_name, pc->pir); > offset = fdt_add_subnode(fdt, cpus_offset, nodename); > _FDT(offset); > diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c > index 021b1a00e1..1c79d5670d 100644 > --- a/hw/ppc/spapr.c > +++ b/hw/ppc/spapr.c > @@ -284,6 +284,21 @@ static void spapr_dt_pa_features(SpaprMachineState > *spapr, > return; > } > > + if (qemu_tcg_mttcg_enabled()) { > + /* > + * SSO (SAO) ordering is not supported under MTTCG, so disable it. > + * There is no cap for this, so there is a migration bug here. > + * However don't disable it entirely, to allow it to be used under > + * KVM. This is a minor concern because: > + * - SAO is an obscure an rarely (if ever) used feature. > + * - SAO is removed from POWER10 / v3.1, so there is already a > + * migration problem today. > + * - Linux does not test this pa-features bit today anyway, so it's > + * academic. > + */ > + pa_features[4 + 2] &= ~0x80;
Oof.. I see the reasoning but modifying guest visible parameters based on host capabilities without a cap really worries me nonetheless. > + } > + > if (ppc_hash64_has(cpu, PPC_HASH64_CI_LARGEPAGE)) { > /* > * Note: we keep CI large pages off by default because a 64K capable -- David Gibson | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_ | _way_ _around_! http://www.ozlabs.org/~dgibson
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