Re: [PATCH V7 10/29] machine: memfd-alloc option

[Steven Sistare]

On 3/10/2022 12:28 PM, Steven Sistare wrote:
> On 3/10/2022 11:00 AM, Igor Mammedov wrote:
>> On Thu, 10 Mar 2022 10:36:08 -0500
>> Steven Sistare <steven.sistare@oracle.com> wrote:
>>
>>> On 3/8/2022 2:20 AM, Igor Mammedov wrote:
>>>> On Tue, 8 Mar 2022 01:50:11 -0500
>>>> "Michael S. Tsirkin" <mst@redhat.com> wrote:
>>>>   
>>>>> On Mon, Mar 07, 2022 at 09:41:44AM -0500, Steven Sistare wrote:  
>>>>>> On 3/4/2022 5:41 AM, Igor Mammedov wrote:    
>>>>>>> On Thu, 3 Mar 2022 12:21:15 -0500
>>>>>>> "Michael S. Tsirkin" <mst@redhat.com> wrote:
>>>>>>>     
>>>>>>>> On Wed, Dec 22, 2021 at 11:05:15AM -0800, Steve Sistare wrote:    
>>>>>>>>> Allocate anonymous memory using memfd_create if the memfd-alloc 
>>>>>>>>> machine
>>>>>>>>> option is set.
>>>>>>>>>
>>>>>>>>> Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
>>>>>>>>> ---
>>>>>>>>>  hw/core/machine.c   | 19 +++++++++++++++++++
>>>>>>>>>  include/hw/boards.h |  1 +
>>>>>>>>>  qemu-options.hx     |  6 ++++++
>>>>>>>>>  softmmu/physmem.c   | 47 
>>>>>>>>> ++++++++++++++++++++++++++++++++++++++---------
>>>>>>>>>  softmmu/vl.c        |  1 +
>>>>>>>>>  trace-events        |  1 +
>>>>>>>>>  util/qemu-config.c  |  4 ++++
>>>>>>>>>  7 files changed, 70 insertions(+), 9 deletions(-)
>>>>>>>>>
>>>>>>>>> diff --git a/hw/core/machine.c b/hw/core/machine.c
>>>>>>>>> index 53a99ab..7739d88 100644
>>>>>>>>> --- a/hw/core/machine.c
>>>>>>>>> +++ b/hw/core/machine.c
>>>>>>>>> @@ -392,6 +392,20 @@ static void machine_set_mem_merge(Object *obj, 
>>>>>>>>> bool value, Error **errp)
>>>>>>>>>      ms->mem_merge = value;
>>>>>>>>>  }
>>>>>>>>>  
>>>>>>>>> +static bool machine_get_memfd_alloc(Object *obj, Error **errp)
>>>>>>>>> +{
>>>>>>>>> +    MachineState *ms = MACHINE(obj);
>>>>>>>>> +
>>>>>>>>> +    return ms->memfd_alloc;
>>>>>>>>> +}
>>>>>>>>> +
>>>>>>>>> +static void machine_set_memfd_alloc(Object *obj, bool value, Error 
>>>>>>>>> **errp)
>>>>>>>>> +{
>>>>>>>>> +    MachineState *ms = MACHINE(obj);
>>>>>>>>> +
>>>>>>>>> +    ms->memfd_alloc = value;
>>>>>>>>> +}
>>>>>>>>> +
>>>>>>>>>  static bool machine_get_usb(Object *obj, Error **errp)
>>>>>>>>>  {
>>>>>>>>>      MachineState *ms = MACHINE(obj);
>>>>>>>>> @@ -829,6 +843,11 @@ static void machine_class_init(ObjectClass *oc, 
>>>>>>>>> void *data)
>>>>>>>>>      object_class_property_set_description(oc, "mem-merge",
>>>>>>>>>          "Enable/disable memory merge support");
>>>>>>>>>  
>>>>>>>>> +    object_class_property_add_bool(oc, "memfd-alloc",
>>>>>>>>> +        machine_get_memfd_alloc, machine_set_memfd_alloc);
>>>>>>>>> +    object_class_property_set_description(oc, "memfd-alloc",
>>>>>>>>> +        "Enable/disable allocating anonymous memory using 
>>>>>>>>> memfd_create");
>>>>>>>>> +
>>>>>>>>>      object_class_property_add_bool(oc, "usb",
>>>>>>>>>          machine_get_usb, machine_set_usb);
>>>>>>>>>      object_class_property_set_description(oc, "usb",
>>>>>>>>> diff --git a/include/hw/boards.h b/include/hw/boards.h
>>>>>>>>> index 9c1c190..a57d7a0 100644
>>>>>>>>> --- a/include/hw/boards.h
>>>>>>>>> +++ b/include/hw/boards.h
>>>>>>>>> @@ -327,6 +327,7 @@ struct MachineState {
>>>>>>>>>      char *dt_compatible;
>>>>>>>>>      bool dump_guest_core;
>>>>>>>>>      bool mem_merge;
>>>>>>>>> +    bool memfd_alloc;
>>>>>>>>>      bool usb;
>>>>>>>>>      bool usb_disabled;
>>>>>>>>>      char *firmware;
>>>>>>>>> diff --git a/qemu-options.hx b/qemu-options.hx
>>>>>>>>> index 7d47510..33c8173 100644
>>>>>>>>> --- a/qemu-options.hx
>>>>>>>>> +++ b/qemu-options.hx
>>>>>>>>> @@ -30,6 +30,7 @@ DEF("machine", HAS_ARG, QEMU_OPTION_machine, \
>>>>>>>>>      "                vmport=on|off|auto controls emulation of vmport 
>>>>>>>>> (default: auto)\n"
>>>>>>>>>      "                dump-guest-core=on|off include guest memory in 
>>>>>>>>> a core dump (default=on)\n"
>>>>>>>>>      "                mem-merge=on|off controls memory merge support 
>>>>>>>>> (default: on)\n"
>>>>>>>>> +    "                memfd-alloc=on|off controls allocating 
>>>>>>>>> anonymous guest RAM using memfd_create (default: off)\n"      
>>>>>>>>
>>>>>>>> Question: are there any disadvantages associated with using
>>>>>>>> memfd_create? I guess we are using up an fd, but that seems minor.  Any
>>>>>>>> reason not to set to on by default? maybe with a fallback option to
>>>>>>>> disable that?    
>>>>>>
>>>>>> Old Linux host kernels, circa 4.1, do not support huge pages for shared 
>>>>>> memory.
>>>>>> Also, the tunable to enable huge pages for share memory is different 
>>>>>> than for
>>>>>> anon memory, so there could be performance loss if it is not set 
>>>>>> correctly.
>>>>>>     /sys/kernel/mm/transparent_hugepage/enabled
>>>>>>     vs
>>>>>>     /sys/kernel/mm/transparent_hugepage/shmem_enabled    
>>>>>
>>>>> I guess we can test this when launching the VM, and select
>>>>> a good default.
>>>>>  
>>>>>> It might make sense to use memfd_create by default for the secondary 
>>>>>> segments.    
>>>>>
>>>>> Well there's also KSM now you mention it.  
>>>>
>>>> then another quest, is there downside to always using memfd_create
>>>> without any knobs being involved?  
>>>
>>> Lower performance if small pages are used (but Michael suggests qemu could 
>>> automatically check the tunable and use anon memory instead)
>>>
>>> KSM (same page merging) is not supported for shared memory, so 
>>> ram_block_add ->
>>> memory_try_enable_merging will not enable it.
>>>
>>> In both cases, I expect the degradation would be negligible if memfd_create 
>>> is
>>> only automatically applied to the secondary segments, which are typically 
>>> small.
>>> But, someone's secondary segment could be larger, and it is time consuming 
>>> to
>>> prove innocence when someone claims your change caused their performance 
>>> regression.
>>
>> Adding David as memory subsystem maintainer, maybe he will a better
>> idea instead of introducing global knob that would also magically alter 
>> backends' behavior despite of its their configured settings.
> 
> OK, in ram_block_add I can set the RAM_SHARED flag based on the 
> memory-backend object's
> shared flag.  I already set the latter in create_default_memdev when 
> memfd-alloc is
> specified.  With that change, we do not override configured settings.  Users 
> can no longer
> use memory-backend-ram for CPR, and must change all memory-backend-ram to 
> memory-backend-memfd
> in the command-line arguments.  That is fine.
> 
> With that change, are you OK with this patch?

Sorry, I mis-read my own code in ram_block_add.  The existing code is correct 
and does 
not alter any backend's behavior.   It only sets the shared flag when the ram 
is *not* 
being allocated for a backend:

                if (!object_dynamic_cast(parent, TYPE_MEMORY_BACKEND)) {
                    new_block->flags |= RAM_SHARED;
                }

- Steve

>>>>>>>> I am concerned that it's actually a kind of memory backend, this flag
>>>>>>>> seems to instead be closer to the deprecated mem-prealloc. E.g.
>>>>>>>> it does not work with a mem path, does it?    
>>>>>>
>>>>>> One can still define a memory backend with mempath to create the main 
>>>>>> ram segment,
>>>>>> though it must be some form of shared to work with live update.  Indeed, 
>>>>>> I would 
>>>>>> expect most users to specify an explicit memory backend for it.  The 
>>>>>> secondary
>>>>>> segments would still use memfd_create.
>>>>>>     
>>>>>>> (mem path and mem-prealloc are transparently aliased to used memory 
>>>>>>> backend
>>>>>>> if I recall it right.)
>>>>>>>
>>>>>>> Steve,
>>>>>>>
>>>>>>> For allocating guest RAM, we switched exclusively to using 
>>>>>>> memory-backends
>>>>>>> including initial guest RAM (-m size option) and we have hostmem-memfd
>>>>>>> that uses memfd_create() and I'd rather avoid adding random knobs to 
>>>>>>> machine
>>>>>>> for tweaking how RAM should be allocated, we have memory backends for 
>>>>>>> this,
>>>>>>> so this patch begs the question: why hostmem-memfd is not sufficient?
>>>>>>> (patch description is rather lacking on rationale behind the patch)    
>>>>>>
>>>>>> There is currently no way to specify memory backends for the secondary 
>>>>>> memory
>>>>>> segments (vram, roms, etc), and IMO it would be onerous to specify a 
>>>>>> backend for
>>>>>> each of them.  On x86_64, these include pc.bios, vga.vram, pc.rom, 
>>>>>> vga.rom,
>>>>>> /rom@etc/acpi/tables, /rom@etc/table-loader, /rom@etc/acpi/rsdp.
>>
>> MemoryRegion is not the only place where state is stored.
>> If we only talk about fwcfg entries state, it can also reference
>> plain malloced memory allocated elsewhere or make a deep copy internally.
>> Similarly devices also may store state outside of RamBlock framework.
>>
>> How are you dealing with that?
>>
>>>>>>
>>>>>> - Steve
>>>>>>     
>>>>>>>>>      "                aes-key-wrap=on|off controls support for AES 
>>>>>>>>> key wrapping (default=on)\n"
>>>>>>>>>      "                dea-key-wrap=on|off controls support for DEA 
>>>>>>>>> key wrapping (default=on)\n"
>>>>>>>>>      "                suppress-vmdesc=on|off disables self-describing 
>>>>>>>>> migration (default=off)\n"
>>>>>>>>> @@ -76,6 +77,11 @@ SRST
>>>>>>>>>          supported by the host, de-duplicates identical memory pages
>>>>>>>>>          among VMs instances (enabled by default).
>>>>>>>>>  
>>>>>>>>> +    ``memfd-alloc=on|off``
>>>>>>>>> +        Enables or disables allocation of anonymous guest RAM using
>>>>>>>>> +        memfd_create.  Any associated memory-backend objects are 
>>>>>>>>> created with
>>>>>>>>> +        share=on.  The memfd-alloc default is off.
>>>>>>>>> +
>>>>>>>>>      ``aes-key-wrap=on|off``
>>>>>>>>>          Enables or disables AES key wrapping support on s390-ccw 
>>>>>>>>> hosts.
>>>>>>>>>          This feature controls whether AES wrapping keys will be 
>>>>>>>>> created
>>>>>>>>> diff --git a/softmmu/physmem.c b/softmmu/physmem.c
>>>>>>>>> index 3524c04..95e2b49 100644
>>>>>>>>> --- a/softmmu/physmem.c
>>>>>>>>> +++ b/softmmu/physmem.c
>>>>>>>>> @@ -41,6 +41,7 @@
>>>>>>>>>  #include "qemu/config-file.h"
>>>>>>>>>  #include "qemu/error-report.h"
>>>>>>>>>  #include "qemu/qemu-print.h"
>>>>>>>>> +#include "qemu/memfd.h"
>>>>>>>>>  #include "exec/memory.h"
>>>>>>>>>  #include "exec/ioport.h"
>>>>>>>>>  #include "sysemu/dma.h"
>>>>>>>>> @@ -1964,35 +1965,63 @@ static void ram_block_add(RAMBlock 
>>>>>>>>> *new_block, Error **errp)
>>>>>>>>>      const bool shared = qemu_ram_is_shared(new_block);
>>>>>>>>>      RAMBlock *block;
>>>>>>>>>      RAMBlock *last_block = NULL;
>>>>>>>>> +    struct MemoryRegion *mr = new_block->mr;
>>>>>>>>>      ram_addr_t old_ram_size, new_ram_size;
>>>>>>>>>      Error *err = NULL;
>>>>>>>>> +    const char *name;
>>>>>>>>> +    void *addr = 0;
>>>>>>>>> +    size_t maxlen;
>>>>>>>>> +    MachineState *ms = MACHINE(qdev_get_machine());
>>>>>>>>>  
>>>>>>>>>      old_ram_size = last_ram_page();
>>>>>>>>>  
>>>>>>>>>      qemu_mutex_lock_ramlist();
>>>>>>>>> -    new_block->offset = find_ram_offset(new_block->max_length);
>>>>>>>>> +    maxlen = new_block->max_length;
>>>>>>>>> +    new_block->offset = find_ram_offset(maxlen);
>>>>>>>>>  
>>>>>>>>>      if (!new_block->host) {
>>>>>>>>>          if (xen_enabled()) {
>>>>>>>>> -            xen_ram_alloc(new_block->offset, new_block->max_length,
>>>>>>>>> -                          new_block->mr, &err);
>>>>>>>>> +            xen_ram_alloc(new_block->offset, maxlen, new_block->mr, 
>>>>>>>>> &err);
>>>>>>>>>              if (err) {
>>>>>>>>>                  error_propagate(errp, err);
>>>>>>>>>                  qemu_mutex_unlock_ramlist();
>>>>>>>>>                  return;
>>>>>>>>>              }
>>>>>>>>>          } else {
>>>>>>>>> -            new_block->host = 
>>>>>>>>> qemu_anon_ram_alloc(new_block->max_length,
>>>>>>>>> -                                                  
>>>>>>>>> &new_block->mr->align,
>>>>>>>>> -                                                  shared, noreserve);
>>>>>>>>> -            if (!new_block->host) {
>>>>>>>>> +            name = memory_region_name(mr);
>>>>>>>>> +            if (ms->memfd_alloc) {
>>>>>>>>> +                Object *parent = &mr->parent_obj;
>>>>>>>>> +                int mfd = -1;          /* placeholder until next 
>>>>>>>>> patch */
>>>>>>>>> +                mr->align = QEMU_VMALLOC_ALIGN;
>>>>>>>>> +                if (mfd < 0) {
>>>>>>>>> +                    mfd = qemu_memfd_create(name, maxlen + mr->align,
>>>>>>>>> +                                            0, 0, 0, &err);
>>>>>>>>> +                    if (mfd < 0) {
>>>>>>>>> +                        return;
>>>>>>>>> +                    }
>>>>>>>>> +                }
>>>>>>>>> +                qemu_set_cloexec(mfd);
>>>>>>>>> +                /* The memory backend already set its desired flags. 
>>>>>>>>> */
>>>>>>>>> +                if (!object_dynamic_cast(parent, 
>>>>>>>>> TYPE_MEMORY_BACKEND)) {
>>>>>>>>> +                    new_block->flags |= RAM_SHARED;
>>>>>>>>> +                }
>>>>>>>>> +                addr = file_ram_alloc(new_block, maxlen, mfd,
>>>>>>>>> +                                      false, false, 0, errp);
>>>>>>>>> +                trace_anon_memfd_alloc(name, maxlen, addr, mfd);
>>>>>>>>> +            } else {
>>>>>>>>> +                addr = qemu_anon_ram_alloc(maxlen, &mr->align,
>>>>>>>>> +                                           shared, noreserve);
>>>>>>>>> +            }
>>>>>>>>> +
>>>>>>>>> +            if (!addr) {
>>>>>>>>>                  error_setg_errno(errp, errno,
>>>>>>>>>                                   "cannot set up guest memory '%s'",
>>>>>>>>> -                                 memory_region_name(new_block->mr));
>>>>>>>>> +                                 name);
>>>>>>>>>                  qemu_mutex_unlock_ramlist();
>>>>>>>>>                  return;
>>>>>>>>>              }
>>>>>>>>> -            memory_try_enable_merging(new_block->host, 
>>>>>>>>> new_block->max_length);
>>>>>>>>> +            memory_try_enable_merging(addr, maxlen);
>>>>>>>>> +            new_block->host = addr;
>>>>>>>>>          }
>>>>>>>>>      }
>>>>>>>>>  
>>>>>>>>> diff --git a/softmmu/vl.c b/softmmu/vl.c
>>>>>>>>> index 620a1f1..ab3648a 100644
>>>>>>>>> --- a/softmmu/vl.c
>>>>>>>>> +++ b/softmmu/vl.c
>>>>>>>>> @@ -2440,6 +2440,7 @@ static void create_default_memdev(MachineState 
>>>>>>>>> *ms, const char *path)
>>>>>>>>>          object_property_set_str(obj, "mem-path", path, &error_fatal);
>>>>>>>>>      }
>>>>>>>>>      object_property_set_int(obj, "size", ms->ram_size, &error_fatal);
>>>>>>>>> +    object_property_set_bool(obj, "share", ms->memfd_alloc, 
>>>>>>>>> &error_fatal);
>>>>>>>>>      object_property_add_child(object_get_objects_root(), 
>>>>>>>>> mc->default_ram_id,
>>>>>>>>>                                obj);
>>>>>>>>>      /* Ensure backend's memory region name is equal to 
>>>>>>>>> mc->default_ram_id */
>>>>>>>>> diff --git a/trace-events b/trace-events
>>>>>>>>> index a637a61..770a9ac 100644
>>>>>>>>> --- a/trace-events
>>>>>>>>> +++ b/trace-events
>>>>>>>>> @@ -45,6 +45,7 @@ ram_block_discard_range(const char *rbname, void 
>>>>>>>>> *hva, size_t length, bool need_
>>>>>>>>>  # accel/tcg/cputlb.c
>>>>>>>>>  memory_notdirty_write_access(uint64_t vaddr, uint64_t ram_addr, 
>>>>>>>>> unsigned size) "0x%" PRIx64 " ram_addr 0x%" PRIx64 " size %u"
>>>>>>>>>  memory_notdirty_set_dirty(uint64_t vaddr) "0x%" PRIx64
>>>>>>>>> +anon_memfd_alloc(const char *name, size_t size, void *ptr, int fd) 
>>>>>>>>> "%s size %zu ptr %p fd %d"
>>>>>>>>>  
>>>>>>>>>  # gdbstub.c
>>>>>>>>>  gdbstub_op_start(const char *device) "Starting gdbstub using device 
>>>>>>>>> %s"
>>>>>>>>> diff --git a/util/qemu-config.c b/util/qemu-config.c
>>>>>>>>> index 436ab63..3606e5c 100644
>>>>>>>>> --- a/util/qemu-config.c
>>>>>>>>> +++ b/util/qemu-config.c
>>>>>>>>> @@ -207,6 +207,10 @@ static QemuOptsList machine_opts = {
>>>>>>>>>              .type = QEMU_OPT_BOOL,
>>>>>>>>>              .help = "enable/disable memory merge support",
>>>>>>>>>          },{
>>>>>>>>> +            .name = "memfd-alloc",
>>>>>>>>> +            .type = QEMU_OPT_BOOL,
>>>>>>>>> +            .help = "enable/disable memfd_create for anonymous 
>>>>>>>>> memory",
>>>>>>>>> +        },{
>>>>>>>>>              .name = "usb",
>>>>>>>>>              .type = QEMU_OPT_BOOL,
>>>>>>>>>              .help = "Set on/off to enable/disable usb",
>>>>>>>>> -- 
>>>>>>>>> 1.8.3.1      
>>>>>>>>
>>>>>>>>    
>>>>>>>     
>>>>>  
>>>>   
>>>
>>