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

[Igor Mammedov]

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?

> 
> > >> 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.
> > 
> > - 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    
> > >>
> > >>  
> > >   
>