Re: [Qemu-arm] [Qemu-devel] [PATCH] arm: virt-acpi: each MADT.GICC entry as enabled unconditionally

[Igor Mammedov <imammedo@redhat.com>]

On Fri, 29 Jan 2016 22:59:32 +0800
Shannon Zhao <shannon.zhao@linaro.org> wrote:

> On 2016/1/29 22:24, Igor Mammedov wrote:
> > in current impl. condition
> >
> > build_madt() {
> >    ...
> >    if (test_bit(i, cpuinfo->found_cpus))
> >
> > is always true since loop handles only present CPUs
> > in range [0..smp_cpus).
> > But to fill usless cpuinfo->found_cpus we do unnecessary
> > scan over QOM tree to find the same CPUs.
> > So mark GICC as present always and drop not needed
> > code that fills cpuinfo->found_cpus.
> >
> > Signed-off-by: Igor Mammedov<imammedo@redhat.com>
> > ---
> > It's just simple cleanup but I'm trying to generalize
> > a bit CPU related ACPI tables and as part of it get rid
> > of found_cpus bitmap and if possible cpu_index usage
> > in ACPI parts of code.
> > ---
> >   hw/arm/virt-acpi-build.c | 26 +++-----------------------
> >   1 file changed, 3 insertions(+), 23 deletions(-)
> >
> > diff --git a/hw/arm/virt-acpi-build.c b/hw/arm/virt-acpi-build.c
> > index 87fbe7c..3ed39fc 100644
> > --- a/hw/arm/virt-acpi-build.c
> > +++ b/hw/arm/virt-acpi-build.c
> > @@ -46,20 +46,6 @@
> >   #define ARM_SPI_BASE 32
> >   #define ACPI_POWER_BUTTON_DEVICE "PWRB"
> >
> > -typedef struct VirtAcpiCpuInfo {
> > -    DECLARE_BITMAP(found_cpus, VIRT_ACPI_CPU_ID_LIMIT);
> > -} VirtAcpiCpuInfo;
> > -
> > -static void virt_acpi_get_cpu_info(VirtAcpiCpuInfo *cpuinfo)
> > -{
> > -    CPUState *cpu;
> > -
> > -    memset(cpuinfo->found_cpus, 0, sizeof cpuinfo->found_cpus);
> > -    CPU_FOREACH(cpu) {
> > -        set_bit(cpu->cpu_index, cpuinfo->found_cpus);
> > -    }
> > -}
> > -
> >   static void acpi_dsdt_add_cpus(Aml *scope, int smp_cpus)
> >   {
> >       uint16_t i;
> > @@ -458,8 +444,7 @@ build_gtdt(GArray *table_data, GArray *linker)
> >
> >   /* MADT */
> >   static void
> > -build_madt(GArray *table_data, GArray *linker, VirtGuestInfo *guest_info,
> > -           VirtAcpiCpuInfo *cpuinfo)
> > +build_madt(GArray *table_data, GArray *linker, VirtGuestInfo *guest_info)
> >   {
> >       int madt_start = table_data->len;
> >       const MemMapEntry *memmap = guest_info->memmap;
> > @@ -489,9 +474,7 @@ build_madt(GArray *table_data, GArray *linker, VirtGuestInfo *guest_info,
> >           gicc->cpu_interface_number = i;
> >           gicc->arm_mpidr = armcpu->mp_affinity;
> >           gicc->uid = i;
> > -        if (test_bit(i, cpuinfo->found_cpus)) {
> > -            gicc->flags = cpu_to_le32(ACPI_GICC_ENABLED);
> > -        }
> > +        gicc->flags = cpu_to_le32(ACPI_GICC_ENABLED);
> >       }  
> Ah, yes, it uses smp_cpus not max_cpus. But we still needs to support 
> max_cpus usage even though it doesn't support vcpu hotplug currently. So
> we may need to introduce guest_info->max_cpus and use it here.

We should support max_cpus but only when hotplug is supported.
Problem with hotplug is that currently it's assumed that
cpu_index is in range [0..max_cpus) and that works for now but
with a large number of CPUs that won't scale, that's a problem
we are facing now in x86.
I'm trying to re-factor CPU related ACPI parts to use CPU id
used in hardware (APIC ID for x86) and while it make it reusable
for ARM as well where as such ID we could use 'mpidr'.

So this clean up beside of removing not needed code also
reduces ACPI dependency on cpu_index.

> And below check in virt.c is not right while it should compare the 
> global max_cpus with the max_cpus GIC supports.
> 
>      if (smp_cpus > max_cpus) {
>          error_report("Number of SMP CPUs requested (%d) exceeds max CPUs "
>                       "supported by machine 'mach-virt' (%d)",
>                       smp_cpus, max_cpus);
>          exit(1);
>      }
> 
> Thanks,