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

[Igor Mammedov <imammedo@redhat.com>]

On Fri, 1 Sep 2017 13:11:18 -0300
Eduardo Habkost <ehabkost@redhat.com> wrote:

> 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?
ec941c5f x86/mm/64: Enable SWIOTLB if system has SRAT memory regions above MAX_DMA32_PFN


> > 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.
I think safest way to achieve what patch needs is to
enable add numa node implicitly if user hasn't done it explicitly on CLI
when memory hotplug is enabled i.e. slots != 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
> >   
>