Re: [Qemu-devel] [RFC 2/4] spapr: Adjust placement of PCI host bridge to allow > 1TiB RAM

[Laurent Vivier]

On 06/10/2016 05:03, David Gibson wrote:
> Currently the default PCI host bridge for the 'pseries' machine type is
> constructed with its IO windows in the 1TiB..(1TiB + 64GiB) range in
> guest memory space.  This means that if > 1TiB of guest RAM is specified,
> the RAM will collide with the PCI IO windows, causing serious problems.
> 
> Problems won't be obvious until guest RAM goes a bit beyond 1TiB, because
> there's a little unused space at the bottom of the area reserved for PCI,
> but essentially this means that > 1TiB of RAM has never worked with the
> pseries machine type.
> 
> This patch fixes this by altering the placement of PHBs on large-RAM VMs.
> Instead of always placing the first PHB at 1TiB, it is placed at the next
> 1 TiB boundary after the maximum RAM address.
> 
> Technically, this changes behaviour in a migration-breaking way for
> existing machines with > 1TiB maximum memory, but since having > 1 TiB
> memory was broken anyway, this seems like a reasonable trade-off.
> 
> Signed-off-by: David Gibson <address@hidden>

Reviewed-by: Laurent Vivier <address@hidden>

> ---
>  hw/ppc/spapr.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> index f6e9c2a..9f3e004 100644
> --- a/hw/ppc/spapr.c
> +++ b/hw/ppc/spapr.c
> @@ -2376,12 +2376,15 @@ static void spapr_phb_placement(sPAPRMachineState 
> *spapr, uint32_t index,
>                                  unsigned n_dma, uint32_t *liobns, Error 
> **errp)
>  {
>      const uint64_t base_buid = 0x800000020000000ULL;
> -    const hwaddr phb0_base = 0x10000000000ULL; /* 1 TiB */
>      const hwaddr phb_spacing = 0x1000000000ULL; /* 64 GiB */
>      const hwaddr mmio_offset = 0xa0000000; /* 2 GiB + 512 MiB */
>      const hwaddr pio_offset = 0x80000000; /* 2 GiB */
>      const uint32_t max_index = 255;
> +    const hwaddr phb0_alignment = 0x10000000000ULL; /* 1 TiB */
>  
> +    uint64_t max_hotplug_addr = spapr->hotplug_memory.base +
> +        memory_region_size(&spapr->hotplug_memory.mr);
> +    hwaddr phb0_base = QEMU_ALIGN_UP(max_hotplug_addr, phb0_alignment);
>      hwaddr phb_base;
>      int i;
>  
>