Re: [Qemu-devel] [PATCH v2 33/42] ivshmem: Implement shm=... with a memory backend

[Markus Armbruster]

Marc-André Lureau <address@hidden> writes:

> Hi
>
> On Mon, Mar 7, 2016 at 8:25 PM, Markus Armbruster <address@hidden> wrote:
>> ivshmem has its very own code to create and map shared memory.
>> Replace that with an implicitly created memory backend.  Reduces the
>> number of ways we create BAR 2 from three to two.
>>
>> Signed-off-by: Markus Armbruster <address@hidden>
>> ---
>>  hw/misc/ivshmem.c | 79 
>> ++++++++++++++++---------------------------------------
>>  1 file changed, 23 insertions(+), 56 deletions(-)
>>
>> diff --git a/hw/misc/ivshmem.c b/hw/misc/ivshmem.c
>> index 1c25621..747f9c3 100644
>> --- a/hw/misc/ivshmem.c
>> +++ b/hw/misc/ivshmem.c
>> @@ -26,6 +26,7 @@
>>  #include "migration/migration.h"
>>  #include "qemu/error-report.h"
>>  #include "qemu/event_notifier.h"
>> +#include "qom/object_interfaces.h"
>>  #include "sysemu/char.h"
>>  #include "sysemu/hostmem.h"
>>  #include "qapi/visitor.h"
>> @@ -369,31 +370,6 @@ static int check_shm_size(IVShmemState *s, int fd, 
>> Error **errp)
>>      }
>>  }
>>
>> -/* create the shared memory BAR when we are not using the server, so we can
>> - * create the BAR and map the memory immediately */
>> -static int create_shared_memory_BAR(IVShmemState *s, int fd, uint8_t attr,
>> -                                    Error **errp)
>> -{
>> -    void * ptr;
>> -
>> -    ptr = mmap(0, s->ivshmem_size, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
>> -    if (ptr == MAP_FAILED) {
>> -        error_setg_errno(errp, errno, "Failed to mmap shared memory");
>> -        return -1;
>> -    }
>> -
>> -    memory_region_init_ram_ptr(&s->ivshmem, OBJECT(s), "ivshmem.bar2",
>> -                               s->ivshmem_size, ptr);
>> -    qemu_set_ram_fd(s->ivshmem.ram_addr, fd);
>> -    vmstate_register_ram(&s->ivshmem, DEVICE(s));
>> -    memory_region_add_subregion(&s->bar, 0, &s->ivshmem);
>> -
>> -    /* region for shared memory */
>> -    pci_register_bar(PCI_DEVICE(s), 2, attr, &s->bar);
>> -
>> -    return 0;
>> -}
>> -
>>  static void ivshmem_add_eventfd(IVShmemState *s, int posn, int i)
>>  {
>>      memory_region_add_eventfd(&s->ivshmem_mmio,
>> @@ -833,6 +809,23 @@ static void ivshmem_write_config(PCIDevice *pdev, 
>> uint32_t address,
>>      }
>>  }
>>
>> +static void desugar_shm(IVShmemState *s)
>> +{
>> +    Object *obj;
>> +    char *path;
>> +
>> +    obj = object_new("memory-backend-file");
>> +    path = g_strdup_printf("/dev/shm/%s", s->shmobj);
>> +    object_property_set_str(obj, path, "mem-path", &error_abort);
>> +    g_free(path);
>> +    object_property_set_int(obj, s->ivshmem_size, "size", &error_abort);
>> +    object_property_set_bool(obj, true, "share", &error_abort);
>> +    object_property_add_child(OBJECT(s), "internal-shm-backend", obj,
>> +                              &error_abort);
>> +    user_creatable_complete(obj, &error_abort);
>> +    s->hostmem = MEMORY_BACKEND(obj);
>> +}
>> +
>>  static void pci_ivshmem_realize(PCIDevice *dev, Error **errp)
>>  {
>>      IVShmemState *s = IVSHMEM(dev);
>> @@ -911,6 +904,10 @@ static void pci_ivshmem_realize(PCIDevice *dev, Error 
>> **errp)
>>          attr |= PCI_BASE_ADDRESS_MEM_TYPE_64;
>>      }
>>
>> +    if (s->shmobj) {
>> +        desugar_shm(s);
>> +    }
>> +
>>      if (s->hostmem != NULL) {
>>          MemoryRegion *mr;
>>
>> @@ -921,7 +918,7 @@ static void pci_ivshmem_realize(PCIDevice *dev, Error 
>> **errp)
>>          vmstate_register_ram(mr, DEVICE(s));
>>          memory_region_add_subregion(&s->bar, 0, mr);
>>          pci_register_bar(PCI_DEVICE(s), 2, attr, &s->bar);
>> -    } else if (s->server_chr != NULL) {
>> +    } else {
>>          IVSHMEM_DPRINTF("using shared memory server (socket = %s)\n",
>>                          s->server_chr->filename);
>>
>> @@ -948,36 +945,6 @@ static void pci_ivshmem_realize(PCIDevice *dev, Error 
>> **errp)
>>              error_setg(errp, "failed to initialize interrupts");
>>              return;
>>          }
>> -    } else {
>> -        /* just map the file immediately, we're not using a server */
>> -        int fd;
>> -
>> -        IVSHMEM_DPRINTF("using shm_open (shm object = %s)\n", s->shmobj);
>> -
>> -        /* try opening with O_EXCL and if it succeeds zero the memory
>> -         * by truncating to 0 */
>> -        if ((fd = shm_open(s->shmobj, O_CREAT|O_RDWR|O_EXCL,
>> -                        S_IRWXU|S_IRWXG|S_IRWXO)) > 0) {
>
> Nice patch, but it's worth pointing out that qemu file_ram_alloc()
> creates file with open O_CREAT. Here you rely on the fact that
> /dev/shm is present and file inside maps to POSIX shared memory object
> names. That's a lot more restrictive than using shm_open().
> Furthermore, the permissions are not the same. The current code uses
> 777 for some reasons, while qemu file_ram_alloc() is 644. I am not
> convinced the cleanup is worth these braking changes, especially the
> restriction to Linux only.

The integrated memory backend has to go.

Yes, my desugaring to memory-backend-file assumes Linux and the
conventional mount point /dev/shm.

For what it's worth, glibc implements shm_open() as a thin wrapper
around open() on all targets.  On Linux, it looks for other mountpoints
when /dev/shm/ isn't there or unsuitable.  On other targets, it always
uses /dev/shm/.  I didn't bother to duplicate glibc's mountpoint search,
because distros converged to /dev/shm/ long ago.

The proper way to support POSIX shared memory objects on systems where
they're not files (and therefore can't be mapped with
memory-backend-file) is to create memory-backend-shm.  If such systems
exist.

I didn't do this now, because one, I'm not aware of a system that needs
it, and two, ivshmem is Linux-specific anyway.  ivshmem-plain could be
made more portable, and once that's done, memory-backend-shm might
become useful.

That leaves permissions.  You're right, the patch changes them from 0777
to 0644.  I'm inclined to call it a bug fix.  I failed to mention it in
my commit message, and I'll fix that.  We may want to mention it in
release notes, too.

>> -           /* truncate file to length PCI device's memory */
>> -            if (ftruncate(fd, s->ivshmem_size) != 0) {
>> -                error_report("could not truncate shared file");
>> -            }
>> -
>> -        } else if ((fd = shm_open(s->shmobj, O_CREAT|O_RDWR,
>> -                        S_IRWXU|S_IRWXG|S_IRWXO)) < 0) {
>> -            error_setg(errp, "could not open shared file");
>> -            return;
>> -        }
>> -
>> -        if (check_shm_size(s, fd, errp) == -1) {
>> -            return;
>> -        }
>> -
>> -        create_shared_memory_BAR(s, fd, attr, &err);
>> -        if (err) {
>> -            error_propagate(errp, err);
>> -            return;
>> -        }
>>      }
>>
>>      if (s->role_val == IVSHMEM_PEER) {
>> --
>> 2.4.3
>>
>>