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

[Igor Mammedov <imammedo@redhat.com>]

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.



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