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

[Steven Sistare <steven.sistare@oracle.com>]

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?

- 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      
>>>>>>>
>>>>>>>    
>>>>>>     
>>>>  
>>>   
>>
>