Re: [Qemu-devel] [PATCH] x86/acpi: build SRAT when memory hotplug is enabled

[Eduardo Habkost <ehabkost@redhat.com>]

On Fri, Sep 01, 2017 at 12:45:42PM -0300, Thadeu Lima de Souza Cascardo wrote:
> Linux uses SRAT to determine the maximum memory in a system, which is
> used to determine whether to use the swiotlb for IOMMU or not for a
> device that supports only 32 bits of addresses.

Do you have a pointer to the corresponding Linux code, for
reference?  Which SRAT entries Linux uses to make this decision?

> 
> When there is no NUMA configuration, qemu will not build SRAT. And when
> memory hotplug is done, some Linux device drivers start failing.
> 
> Tested by running with -m 512M,slots=8,maxmem=1G, adding the memory,
> putting that online and using the system. Without the patch, swiotlb is
> not used and ATA driver fails. With the patch, swiotlb is used, no
> driver failure is observed.
> 
> Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>

As far as I can see, this will only add APIC entries and a memory
affinity entry for the first 640KB (which would be obviously
wrong) if pcms->numa_nodes is 0.

Once we apply the "Fix SRAT memory building in case of node 0
without RAM" patch from Dou Liyang, no memory affinity entries
will be generated if pcms->numa_nodes is 0.  Would this cause the
problem to happen again?


> ---
>  hw/i386/acpi-build.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/i386/acpi-build.c b/hw/i386/acpi-build.c
> index 98dd424678..fb94249779 100644
> --- a/hw/i386/acpi-build.c
> +++ b/hw/i386/acpi-build.c
> @@ -2645,6 +2645,9 @@ void acpi_build(AcpiBuildTables *tables, MachineState *machine)
>      GArray *tables_blob = tables->table_data;
>      AcpiSlicOem slic_oem = { .id = NULL, .table_id = NULL };
>      Object *vmgenid_dev;
> +    ram_addr_t hotplugabble_address_space_size =
> +        object_property_get_int(OBJECT(pcms), PC_MACHINE_MEMHP_REGION_SIZE,
> +                                NULL);
>  
>      acpi_get_pm_info(&pm);
>      acpi_get_misc_info(&misc);
> @@ -2708,7 +2711,7 @@ void acpi_build(AcpiBuildTables *tables, MachineState *machine)
>              build_tpm2(tables_blob, tables->linker);
>          }
>      }
> -    if (pcms->numa_nodes) {
> +    if (pcms->numa_nodes || hotplugabble_address_space_size) {
>          acpi_add_table(table_offsets, tables_blob);
>          build_srat(tables_blob, tables->linker, machine);
>          if (have_numa_distance) {
> -- 
> 2.11.0
> 

-- 
Eduardo