Re: [PATCH 03/13] ppc/spapr|pnv: Remove SAO from pa-features

[Harsh Prateek Bora]

Hi Nick,

One cosmetic comment, in case you are doing a re-spin:

On 3/12/24 00:21, 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. This is an obscure feature that has been removed from
> POWER10 ISA v3.1, there isn't much concern with removing it.
>
> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> ---
>   hw/ppc/pnv.c   |  2 +-
>   hw/ppc/spapr.c | 14 ++++++++++----
>   2 files changed, 11 insertions(+), 5 deletions(-)
>
> diff --git a/hw/ppc/pnv.c b/hw/ppc/pnv.c
> index 0b47b92baa..aa9786e970 100644
> --- a/hw/ppc/pnv.c
> +++ b/hw/ppc/pnv.c
> @@ -150,7 +150,7 @@ static void pnv_dt_core(PnvChip *chip, PnvCore *pc, void 
> *fdt)
>       uint32_t page_sizes_prop[64];
>       size_t page_sizes_prop_size;
>       const uint8_t pa_features[] = { 24, 0,
> -                                    0xf6, 0x3f, 0xc7, 0xc0, 0x80, 0xf0,
> +                                    0xf6, 0x3f, 0xc7, 0xc0, 0x00, 0xf0,
>                                       0x80, 0x00, 0x00, 0x00, 0x00, 0x00,
>                                       0x00, 0x00, 0x00, 0x00, 0x80, 0x00,
>                                       0x80, 0x00, 0x80, 0x00, 0x80, 0x00 };
> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> index 55263f0815..5099f12cc6 100644
> --- a/hw/ppc/spapr.c
> +++ b/hw/ppc/spapr.c
> @@ -234,16 +234,16 @@ static void spapr_dt_pa_features(SpaprMachineState *spapr,
>                                    void *fdt, int offset)
>   {
>       uint8_t pa_features_206[] = { 6, 0,
> -        0xf6, 0x1f, 0xc7, 0x00, 0x80, 0xc0 };
> +        0xf6, 0x1f, 0xc7, 0x00, 0x00, 0xc0 };
>       uint8_t pa_features_207[] = { 24, 0,
> -        0xf6, 0x1f, 0xc7, 0xc0, 0x80, 0xf0,
> +        0xf6, 0x1f, 0xc7, 0xc0, 0x00, 0xf0,
>           0x80, 0x00, 0x00, 0x00, 0x00, 0x00,
>           0x00, 0x00, 0x00, 0x00, 0x80, 0x00,
>           0x80, 0x00, 0x80, 0x00, 0x00, 0x00 };
>       uint8_t pa_features_300[] = { 66, 0,
>           /* 0: MMU|FPU|SLB|RUN|DABR|NX, 1: fri[nzpm]|DABRX|SPRG3|SLB0|PP110 */
> -        /* 2: VPM|DS205|PPR|DS202|DS206, 3: LSD|URG, SSO, 5: LE|CFAR|EB|LSQ */
> -        0xf6, 0x1f, 0xc7, 0xc0, 0x80, 0xf0, /* 0 - 5 */
> +        /* 2: VPM|DS205|PPR|DS202|DS206, 3: LSD|URG, 5: LE|CFAR|EB|LSQ */

Do we want to mention in comments SSO (disabled), also ..

> +        0xf6, 0x1f, 0xc7, 0xc0, 0x00, 0xf0, /* 0 - 5 */
>           /* 6: DS207 */
>           0x80, 0x00, 0x00, 0x00, 0x00, 0x00, /* 6 - 11 */
>           /* 16: Vector */
> @@ -284,6 +284,12 @@ static void spapr_dt_pa_features(SpaprMachineState *spapr,
>           return;
>       }
>   
> +    /*
> +     * SSO (SAO) ordering is supported on KVM and thread=single hosts,
> +     * but not MTTCG, so disable it. To advertise it, a cap would have
> +     * to be added, or support implemented for MTTCG.
> +     */
> +

This comment could go in the beginning where we are actually disabling it.

Otherwise,

Reviewed-by: Harsh Prateek Bora <harshpb@linux.ibm.com>


>       if (ppc_hash64_has(cpu, PPC_HASH64_CI_LARGEPAGE)) {
>           /*
>            * Note: we keep CI large pages off by default because a 64K capable