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

[Igor Mammedov]

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?
> 
> 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?

(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)


> 
> 
> >      "                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  
> 
>