Re: [PATCH v3 1/1] vl/s390x: fixup ram sizes for compat machines

[Igor Mammedov <imammedo@redhat.com>]

On Wed,  1 Apr 2020 08:37:54 -0400
Christian Borntraeger <borntraeger@de.ibm.com> wrote:

> Older QEMU versions did fixup the ram size to match what can be reported
> via sclp. We need to mimic this behaviour for machine types 4.2 and
> older to not fail on inbound migration for memory sizes that do not fit.
> Old machines with proper aligned memory sizes are not affected.
> 
> Alignment table:
>  VM size (<=) | Alignment
> --------------------------
>       1020M   |     1M
>       2040M   |     2M
>       4080M   |     4M
>       8160M   |     8M
>      16320M   |    16M
>      32640M   |    32M
>      65280M   |    64M
>     130560M   |   128M
>     261120M   |   256M
>     522240M   |   512M
>    1044480M   |     1G
>    2088960M   |     2G
>    4177920M   |     4G
>    8355840M   |     8G
> 
> Suggested action is to replace unaligned -m value with a suitable
> aligned one or if a change to a newer machine type is possible, use a
> machine version >= 5.0.
> 
> A future versions might remove the compatibility handling.
> 
> For machine types >= 5.0 we can simply use an increment size of 1M and
> use the full range of increment number which allows for all possible
> memory sizes. The old limitation of having a maximum of 1020 increments
> was added for standby memory, which we no longer support. With that we
> can now support even weird memory sizes like 10001234 MB.
> 
> As we no longer fixup maxram_size as well, make other users use ram_size
> instead. Keep using maxram_size when setting the maximum ram size in KVM,
> as that will come in handy in the future when supporting memory hotplug
> (in contrast, storage keys and storage attributes for hotplugged memory
> will have to be migrated per RAM block in the future).
> 
> Fixes: 3a12fc61af5c ("390x/s390-virtio-ccw: use memdev for RAM")
> Reported-by: Lukáš Doktor <ldoktor@redhat.com>
> Cc: Igor Mammedov <imammedo@redhat.com>
> Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>

Acked-by: Igor Mammedov <imammedo@redhat.com>

minor nit below if you have to respin

> ---
>  hw/s390x/s390-skeys.c        |  2 +-
>  hw/s390x/s390-stattrib-kvm.c |  4 ++--
>  hw/s390x/s390-virtio-ccw.c   | 21 +++++++++++++++++++++
>  hw/s390x/sclp.c              | 17 +++++------------
>  include/hw/boards.h          |  7 +++++++
>  softmmu/vl.c                 |  3 +++
>  6 files changed, 39 insertions(+), 15 deletions(-)
> 
> diff --git a/hw/s390x/s390-skeys.c b/hw/s390x/s390-skeys.c
> index 5da6e5292f..a9a4ae7b39 100644
> --- a/hw/s390x/s390-skeys.c
> +++ b/hw/s390x/s390-skeys.c
> @@ -176,7 +176,7 @@ static void qemu_s390_skeys_init(Object *obj)
>      QEMUS390SKeysState *skeys = QEMU_S390_SKEYS(obj);
>      MachineState *machine = MACHINE(qdev_get_machine());
>  
> -    skeys->key_count = machine->maxram_size / TARGET_PAGE_SIZE;
> +    skeys->key_count = machine->ram_size / TARGET_PAGE_SIZE;
>      skeys->keydata = g_malloc0(skeys->key_count);
>  }
>  
> diff --git a/hw/s390x/s390-stattrib-kvm.c b/hw/s390x/s390-stattrib-kvm.c
> index c7e1f35524..f89d8d9d16 100644
> --- a/hw/s390x/s390-stattrib-kvm.c
> +++ b/hw/s390x/s390-stattrib-kvm.c
> @@ -85,7 +85,7 @@ static int kvm_s390_stattrib_set_stattr(S390StAttribState *sa,
>  {
>      KVMS390StAttribState *sas = KVM_S390_STATTRIB(sa);
>      MachineState *machine = MACHINE(qdev_get_machine());
> -    unsigned long max = machine->maxram_size / TARGET_PAGE_SIZE;
> +    unsigned long max = machine->ram_size / TARGET_PAGE_SIZE;
>  
>      if (start_gfn + count > max) {
>          error_report("Out of memory bounds when setting storage attributes");
> @@ -104,7 +104,7 @@ static void kvm_s390_stattrib_synchronize(S390StAttribState *sa)
>  {
>      KVMS390StAttribState *sas = KVM_S390_STATTRIB(sa);
>      MachineState *machine = MACHINE(qdev_get_machine());
> -    unsigned long max = machine->maxram_size / TARGET_PAGE_SIZE;
> +    unsigned long max = machine->ram_size / TARGET_PAGE_SIZE;
>      /* We do not need to reach the maximum buffer size allowed */
>      unsigned long cx, len = KVM_S390_SKEYS_MAX / 2;
>      int r;
> diff --git a/hw/s390x/s390-virtio-ccw.c b/hw/s390x/s390-virtio-ccw.c
> index 3cf19c99f3..61a8a0e693 100644
> --- a/hw/s390x/s390-virtio-ccw.c
> +++ b/hw/s390x/s390-virtio-ccw.c
> @@ -27,6 +27,7 @@
>  #include "qemu/ctype.h"
>  #include "qemu/error-report.h"
>  #include "qemu/option.h"
> +#include "qemu/qemu-print.h"
>  #include "s390-pci-bus.h"
>  #include "sysemu/reset.h"
>  #include "hw/s390x/storage-keys.h"
> @@ -579,6 +580,25 @@ static void s390_nmi(NMIState *n, int cpu_index, Error **errp)
>      s390_cpu_restart(S390_CPU(cs));
>  }
>  
> +static ram_addr_t s390_fixup_ram_size(ram_addr_t sz)
> +{
> +    /* same logic as in sclp.c */
> +    int increment_size = 20;
> +    ram_addr_t newsz;
> +
> +    while ((sz >> increment_size) > MAX_STORAGE_INCREMENTS) {
> +        increment_size++;
> +    }
> +    newsz = sz >> increment_size << increment_size;
> +
> +    if (sz != newsz) {
> +        qemu_printf("Ram size %" PRIu64 "MB was fixed up to %" PRIu64
                                                   ^^^^^^^^

for unaware  user it could be confusing as it could be read as 'value was increased'
s/fixed up/amended/ might be better

> +                    "MB to match machine restrictions. Consider updating "
> +                    "the guest definition.i\n", sz / MiB, newsz / MiB);

also it might be better to use size_to_str() to format numbers

> +    }
> +    return newsz;
> +}
> +
>  static void ccw_machine_class_init(ObjectClass *oc, void *data)
>  {
>      MachineClass *mc = MACHINE_CLASS(oc);
> @@ -808,6 +828,7 @@ static void ccw_machine_4_2_instance_options(MachineState *machine)
>  static void ccw_machine_4_2_class_options(MachineClass *mc)
>  {
>      ccw_machine_5_0_class_options(mc);
> +    mc->fixup_ram_size = s390_fixup_ram_size;
>      compat_props_add(mc->compat_props, hw_compat_4_2, hw_compat_4_2_len);
>  }
>  DEFINE_CCW_MACHINE(4_2, "4.2", false);
> diff --git a/hw/s390x/sclp.c b/hw/s390x/sclp.c
> index d8ae207731..ede056b3ef 100644
> --- a/hw/s390x/sclp.c
> +++ b/hw/s390x/sclp.c
> @@ -361,27 +361,20 @@ out:
>  static void sclp_memory_init(SCLPDevice *sclp)
>  {
>      MachineState *machine = MACHINE(qdev_get_machine());
> +    MachineClass *machine_class = MACHINE_GET_CLASS(qdev_get_machine());
>      ram_addr_t initial_mem = machine->ram_size;
>      int increment_size = 20;
>  
>      /* The storage increment size is a multiple of 1M and is a power of 2.
> -     * The number of storage increments must be MAX_STORAGE_INCREMENTS or fewer.
> +     * For some machine types, the number of storage increments must be
> +     * MAX_STORAGE_INCREMENTS or fewer.
>       * The variable 'increment_size' is an exponent of 2 that can be
>       * used to calculate the size (in bytes) of an increment. */
> -    while ((initial_mem >> increment_size) > MAX_STORAGE_INCREMENTS) {
> +    while (machine_class->fixup_ram_size != NULL &&
> +           (initial_mem >> increment_size) > MAX_STORAGE_INCREMENTS) {
>          increment_size++;
>      }
>      sclp->increment_size = increment_size;
> -
> -    /* The core memory area needs to be aligned with the increment size.
> -     * In effect, this can cause the user-specified memory size to be rounded
> -     * down to align with the nearest increment boundary. */
> -    initial_mem = initial_mem >> increment_size << increment_size;
> -
> -    machine->ram_size = initial_mem;
> -    machine->maxram_size = initial_mem;
> -    /* let's propagate the changed ram size into the global variable. */
> -    ram_size = initial_mem;
>  }
>  
>  static void sclp_init(Object *obj)
> diff --git a/include/hw/boards.h b/include/hw/boards.h
> index 236d239c19..fd4d62b501 100644
> --- a/include/hw/boards.h
> +++ b/include/hw/boards.h
> @@ -152,6 +152,12 @@ typedef struct {
>   *    It also will be used as a way to optin into "-m" option support.
>   *    If it's not set by board, '-m' will be ignored and generic code will
>   *    not create default RAM MemoryRegion.
> + * @fixup_ram_size:
> + *    Amends user provided ram size (with -m option) using machine
> + *    specific algorithm. To be used by old machine types for compat
> + *    purposes only.
> + *    Applies only to default memory backend, i.e., explicit memory backend
> + *    wasn't used.
>   */
>  struct MachineClass {
>      /*< private >*/
> @@ -218,6 +224,7 @@ struct MachineClass {
>                                                           unsigned cpu_index);
>      const CPUArchIdList *(*possible_cpu_arch_ids)(MachineState *machine);
>      int64_t (*get_default_cpu_node_id)(const MachineState *ms, int idx);
> +    ram_addr_t (*fixup_ram_size)(ram_addr_t size);
>  };
>  
>  /**
> diff --git a/softmmu/vl.c b/softmmu/vl.c
> index 1d33a28340..09f8a1b0a7 100644
> --- a/softmmu/vl.c
> +++ b/softmmu/vl.c
> @@ -2601,6 +2601,9 @@ static bool set_memory_options(uint64_t *ram_slots, ram_addr_t *maxram_size,
>      }
>  
>      sz = QEMU_ALIGN_UP(sz, 8192);
> +    if (mc->fixup_ram_size) {
> +        sz = mc->fixup_ram_size(sz);
> +    }
>      ram_size = sz;
>      if (ram_size != sz) {
>          error_report("ram size too large");