Re: [PATCH v7 15/17] spapr: Don't clamp RMA to 16GiB on new machine types

[Greg Kurz <groug@kaod.org>]

On Tue,  3 Mar 2020 14:43:49 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> In spapr_machine_init() we clamp the size of the RMA to 16GiB and the
> comment saying why doesn't make a whole lot of sense.  In fact, this was
> done because the real mode handling code elsewhere limited the RMA in TCG
> mode to the maximum value configurable in LPCR[RMLS], 16GiB.
> 
> But,
>  * Actually LPCR[RMLS] has been able to encode a 256GiB size for a very
>    long time, we just didn't implement it properly in the softmmu
>  * LPCR[RMLS] shouldn't really be relevant anyway, it only was because we
>    used to abuse the RMOR based translation mode in order to handle the
>    fact that we're not modelling the hypervisor parts of the cpu
> 
> We've now removed those limitations in the modelling so the 16GiB clamp no
> longer serves a function.  However, we can't just remove the limit
> universally: that would break migration to earlier qemu versions, where
> the 16GiB RMLS limit still applies, no matter how bad the reasons for it
> are.
> 
> So, we replace the 16GiB clamp, with a clamp to a limit defined in the
> machine type class.  We set it to 16 GiB for machine types 4.2 and earlier,
> but set it to 0 meaning unlimited for the new 5.0 machine type.
> 
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---

Reviewed-by: Greg Kurz <groug@kaod.org>

>  hw/ppc/spapr.c         | 13 ++++++++-----
>  include/hw/ppc/spapr.h |  1 +
>  2 files changed, 9 insertions(+), 5 deletions(-)
> 
> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> index ef7667455c..95bda4a615 100644
> --- a/hw/ppc/spapr.c
> +++ b/hw/ppc/spapr.c
> @@ -2701,12 +2701,14 @@ static void spapr_machine_init(MachineState *machine)
>  
>      spapr->rma_size = node0_size;
>  
> -    /* Actually we don't support unbounded RMA anymore since we added
> -     * proper emulation of HV mode. The max we can get is 16G which
> -     * also happens to be what we configure for PAPR mode so make sure
> -     * we don't do anything bigger than that
> +    /*
> +     * Clamp the RMA size based on machine type.  This is for
> +     * migration compatibility with older qemu versions, which limited
> +     * the RMA size for complicated and mostly bad reasons.
>       */
> -    spapr->rma_size = MIN(spapr->rma_size, 0x400000000ull);
> +    if (smc->rma_limit) {
> +        spapr->rma_size = MIN(spapr->rma_size, smc->rma_limit);
> +    }
>  
>      if (spapr->rma_size > node0_size) {
>          error_report("Numa node 0 has to span the RMA (%#08"HWADDR_PRIx")",
> @@ -4598,6 +4600,7 @@ static void spapr_machine_4_2_class_options(MachineClass *mc)
>      compat_props_add(mc->compat_props, hw_compat_4_2, hw_compat_4_2_len);
>      smc->default_caps.caps[SPAPR_CAP_CCF_ASSIST] = SPAPR_CAP_OFF;
>      smc->default_caps.caps[SPAPR_CAP_FWNMI_MCE] = SPAPR_CAP_OFF;
> +    smc->rma_limit = 16 * GiB;
>      mc->nvdimm_supported = false;
>  }
>  
> diff --git a/include/hw/ppc/spapr.h b/include/hw/ppc/spapr.h
> index 90dbc55931..2015e37ac5 100644
> --- a/include/hw/ppc/spapr.h
> +++ b/include/hw/ppc/spapr.h
> @@ -126,6 +126,7 @@ struct SpaprMachineClass {
>      bool pre_4_1_migration; /* don't migrate hpt-max-page-size */
>      bool linux_pci_probe;
>      bool smp_threads_vsmt; /* set VSMT to smp_threads by default */
> +    hwaddr rma_limit;          /* clamp the RMA to this size */
>  
>      void (*phb_placement)(SpaprMachineState *spapr, uint32_t index,
>                            uint64_t *buid, hwaddr *pio,