Re: [PATCH v2 6/7] hostmem: Handle remapping of RAM

[William Roche <william.roche@oracle.com>]

On 11/12/24 14:45, David Hildenbrand wrote:
> On 07.11.24 11:21, “William Roche wrote:
>> From: David Hildenbrand <david@redhat.com>
>>
>> Let's register a RAM block notifier and react on remap notifications.
>> Simply re-apply the settings. Warn only when something goes wrong.
>>
>> Note: qemu_ram_remap() will not remap when RAM_PREALLOC is set. Could be
>> that hostmem is still missing to update that flag ...
>>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> Signed-off-by: William Roche <william.roche@oracle.com>
>> ---
>>   backends/hostmem.c       | 29 +++++++++++++++++++++++++++++
>>   include/sysemu/hostmem.h |  1 +
>>   2 files changed, 30 insertions(+)
>>
>> diff --git a/backends/hostmem.c b/backends/hostmem.c
>> index bf85d716e5..fbd8708664 100644
>> --- a/backends/hostmem.c
>> +++ b/backends/hostmem.c
>> @@ -361,11 +361,32 @@ static void 
>> host_memory_backend_set_prealloc_threads(Object *obj, Visitor *v,
>>       backend->prealloc_threads = value;
>>   }
>> +static void host_memory_backend_ram_remapped(RAMBlockNotifier *n, 
>> void *host,
>> +                                             size_t offset, size_t size)
>> +{
>> +    HostMemoryBackend *backend = container_of(n, HostMemoryBackend,
>> +                                              ram_notifier);
>> +    Error *err = NULL;
>> +
>> +    if (!host_memory_backend_mr_inited(backend) ||
>> +        memory_region_get_ram_ptr(&backend->mr) != host) {
>> +        return;
>> +    }
>> +
>> +    host_memory_backend_apply_settings(backend, host + offset, size, 
>> &err);
>> +    if (err) {
>> +        warn_report_err(err);
> 
> I wonder if we want to fail hard instead, or have a way to tell the 
> notifier that something wen wrong.
> 

It depends on what the caller would do with this information. Is there a 
way to workaround the problem ? (I don't think so)
Can the VM continue to run without doing anything about it ? (Maybe?)

Currently all numa notifiers don't return errors.

This function is only called from ram_block_notify_remap() in 
qemu_ram_remap(), I would vote for a "fail hard" in case where the 
settings are mandatory to continue.

HTH.