Re: [Qemu-devel] [PATCH v4 12/14] memory-device: factor out pre-plug into hotplug handler

[David Hildenbrand]

On 01.06.2018 13:17, Igor Mammedov wrote:
> On Thu, 17 May 2018 10:15:25 +0200
> David Hildenbrand <address@hidden> wrote:
> 
>> Let's move all pre-plug checks we can do without the device being
>> realized into the applicable hotplug handler for pc and spapr.
>>
>> Signed-off-by: David Hildenbrand <address@hidden>
>> ---
>>  hw/i386/pc.c                   | 11 +++++++
>>  hw/mem/memory-device.c         | 72 
>> +++++++++++++++++++-----------------------
>>  hw/ppc/spapr.c                 | 11 +++++++
>>  include/hw/mem/memory-device.h |  2 ++
>>  4 files changed, 57 insertions(+), 39 deletions(-)
>>
>> diff --git a/hw/i386/pc.c b/hw/i386/pc.c
>> index 8bc41ef24b..61f1537e14 100644
>> --- a/hw/i386/pc.c
>> +++ b/hw/i386/pc.c
>> @@ -2010,6 +2010,16 @@ static void 
>> pc_machine_device_pre_plug_cb(HotplugHandler *hotplug_dev,
>>  {
>>      Error *local_err = NULL;
>>  
>> +    /* first stage hotplug handler */
>> +    if (object_dynamic_cast(OBJECT(dev), TYPE_MEMORY_DEVICE)) {
>> +        memory_device_pre_plug(MACHINE(hotplug_dev), MEMORY_DEVICE(dev),
>> +                               &local_err);
>> +    }
>> +
>> +    if (local_err) {
>> +        goto out;
>> +    }
>> +
>>      /* final stage hotplug handler */
>>      if (object_dynamic_cast(OBJECT(dev), TYPE_CPU)) {
>>          pc_cpu_pre_plug(hotplug_dev, dev, &local_err);
>> @@ -2017,6 +2027,7 @@ static void 
>> pc_machine_device_pre_plug_cb(HotplugHandler *hotplug_dev,
>>          hotplug_handler_pre_plug(dev->parent_bus->hotplug_handler, dev,
>>                                   &local_err);
>>      }
>> +out:
>>      error_propagate(errp, local_err);
>>  }
>>  
>> diff --git a/hw/mem/memory-device.c b/hw/mem/memory-device.c
>> index 361d38bfc5..d22c91993f 100644
>> --- a/hw/mem/memory-device.c
>> +++ b/hw/mem/memory-device.c
>> @@ -68,58 +68,26 @@ static int memory_device_used_region_size(Object *obj, 
>> void *opaque)
>>      return 0;
>>  }
>>  
>> -static void memory_device_check_addable(MachineState *ms, uint64_t size,
>> -                                        Error **errp)
>> -{
>> -    uint64_t used_region_size = 0;
>> -
>> -    /* we will need a new memory slot for kvm and vhost */
>> -    if (kvm_enabled() && !kvm_has_free_slot(ms)) {
>> -        error_setg(errp, "hypervisor has no free memory slots left");
>> -        return;
>> -    }
>> -    if (!vhost_has_free_slot()) {
>> -        error_setg(errp, "a used vhost backend has no free memory slots 
>> left");
>> -        return;
>> -    }
>> -
>> -    /* will we exceed the total amount of memory specified */
>> -    memory_device_used_region_size(OBJECT(ms), &used_region_size);
>> -    if (used_region_size + size > ms->maxram_size - ms->ram_size) {
>> -        error_setg(errp, "not enough space, currently 0x%" PRIx64
>> -                   " in use of total hot pluggable 0x" RAM_ADDR_FMT,
>> -                   used_region_size, ms->maxram_size - ms->ram_size);
>> -        return;
>> -    }
>> -
>> -}
>> -
>>  uint64_t memory_device_get_free_addr(MachineState *ms, const uint64_t *hint,
>>                                       uint64_t align, uint64_t size,
>>                                       Error **errp)
>>  {
>>      uint64_t address_space_start, address_space_end;
>> +    uint64_t used_region_size = 0;
>>      GSList *list = NULL, *item;
>>      uint64_t new_addr = 0;
>>  
>> -    if (!ms->device_memory) {
>> -        error_setg(errp, "memory devices (e.g. for memory hotplug) are not "
>> -                         "supported by the machine");
>> -        return 0;
>> -    }
>> -
>> -    if (!memory_region_size(&ms->device_memory->mr)) {
>> -        error_setg(errp, "memory devices (e.g. for memory hotplug) are not "
>> -                         "enabled, please specify the maxmem option");
>> -        return 0;
>> -    }
>>      address_space_start = ms->device_memory->base;
>>      address_space_end = address_space_start +
>>                          memory_region_size(&ms->device_memory->mr);
>>      g_assert(address_space_end >= address_space_start);
>>  
>> -    memory_device_check_addable(ms, size, errp);
>> -    if (*errp) {
>> +    /* will we exceed the total amount of memory specified */
>> +    memory_device_used_region_size(OBJECT(ms), &used_region_size);
>> +    if (used_region_size + size > ms->maxram_size - ms->ram_size) {
>> +        error_setg(errp, "not enough space, currently 0x%" PRIx64
>> +                   " in use of total hot pluggable 0x" RAM_ADDR_FMT,
>> +                   used_region_size, ms->maxram_size - ms->ram_size);
>>          return 0;
>>      }
>>  
>> @@ -242,6 +210,32 @@ uint64_t get_plugged_memory_size(void)
>>      return size;
>>  }
>>  
>> +void memory_device_pre_plug(MachineState *ms, const MemoryDeviceState *md,
>> +                            Error **errp)
>> +{
>> +    if (!ms->device_memory) {
>> +        error_setg(errp, "memory devices (e.g. for memory hotplug) are not "
>> +                         "supported by the machine");
>> +        return;
>> +    }
>> +
>> +    if (!memory_region_size(&ms->device_memory->mr)) {
>> +        error_setg(errp, "memory devices (e.g. for memory hotplug) are not "
>> +                         "enabled, please specify the maxmem option");
>> +        return;
>> +    }
>> +
>> +    /* we will need a new memory slot for kvm and vhost */
>> +    if (kvm_enabled() && !kvm_has_free_slot(ms)) {
>> +        error_setg(errp, "hypervisor has no free memory slots left");
>> +        return;
>> +    }
>> +    if (!vhost_has_free_slot()) {
>> +        error_setg(errp, "a used vhost backend has no free memory slots 
>> left");
>> +        return;
>> +    }
> thanks for extracting preparations steps into the right callback.
> 
> on top of this _preplug() handler should also set being plugged
> device properties if they weren't set by user.
>  memory_device_get_free_addr() should be here to.
> 
> general rule for _preplug() would be to check and prepare device
> for being plugged but not touch anything beside the device (it's _plug 
> handler job)

I disagree. Or at least I disagree as part of this patch series because
it over-complicates things :)

preplug() can do basic checks but I don't think it should be used to
change actual properties. And I give you the main reason for this:

We have to do quite some amount of unnecessary error handling (please
remember the pc_dimm plug code madness before I refactored it - 80%
consisted of error handling) if we want to work on device properties
before a device is realized. And we even have duplicate checks both in
the realize() and the preplug() code then (again, what we had in the
pc_dimm plug code - do we have a memdev already or not).

Right now, I assume, that all MemoryDevice functions can be safely
called after the device has been realized without error handling. This
is nice. It e.g. guarantees that we have a memdev assigned. Otherwise,
every time we access some MemoryDevice property (e.g. region size), we
have to handle possible uninitialized properties (e.g. memdev) -
something I don't like.

So I want to avoid this by any means. And I don't really see a benefit
for this additional error handling that will be necessary: We don't care
about the performance in case something went wrong.

-- 

Thanks,

David / dhildenb