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

[Michael S. Tsirkin]

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.

> >> 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  
> >>
> >>
> >