Re: [PATCH v2 03/10] ppc/spapr|pnv: Remove SAO from pa-features

[David Gibson]

On Tue, Mar 12, 2024 at 11:14:12PM +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. This is an obscure feature that has been removed from
> POWER10 ISA v3.1, there isn't much concern with removing it.
> 
> Reviewed-by: Harsh Prateek Bora <harshpb@linux.ibm.com>
> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>

Usually altering a user visible feature like this without versioning
would be a no-no.  However, I think it's probably ok here: AFAICT the
feature was basically never used, it didn't work in some cases anyway,
and it's now gone away.

> ---
>  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..3108d7c532 100644
> --- a/hw/ppc/spapr.c
> +++ b/hw/ppc/spapr.c
> @@ -233,17 +233,23 @@ static void spapr_dt_pa_features(SpaprMachineState 
> *spapr,
>                                   PowerPCCPU *cpu,
>                                   void *fdt, int offset)
>  {
> +    /*
> +     * 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.
> +     */
> +
>      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 */
> +        0xf6, 0x1f, 0xc7, 0xc0, 0x00, 0xf0, /* 0 - 5 */
>          /* 6: DS207 */
>          0x80, 0x00, 0x00, 0x00, 0x00, 0x00, /* 6 - 11 */
>          /* 16: Vector */

-- 
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

signature.asc
Description: PGP signature