Re: [Qemu-devel] [PATCH v4] ivshmem: allow the sharing of mmap'd files

[Damien Millescamps]

On 09/20/2013 10:45 PM, Benoît Canet wrote:

Le Friday 20 Sep 2013 à 20:34:47 (+0200), Damien Millescamps a écrit :

This patch permits to share memory areas that do not specifically belong to
/dev/shm. In such case, the file must be already present when launching qemu.
A new parameter 'file' has been added to specify the file to use.

A use case for this patch is sharing huge pages available through a
hugetlbfs mountpoint.

Signed-off-by: Damien Millescamps <address@hidden>
---
 docs/specs/ivshmem_device_spec.txt |    7 +++-
 hw/misc/ivshmem.c                  |   56 +++++++++++++++++++++++------------
 2 files changed, 42 insertions(+), 21 deletions(-)

diff --git a/docs/specs/ivshmem_device_spec.txt b/docs/specs/ivshmem_device_spec.txt
index 667a862..cb7d310 100644
--- a/docs/specs/ivshmem_device_spec.txt
+++ b/docs/specs/ivshmem_device_spec.txt
@@ -4,8 +4,11 @@ Device Specification for Inter-VM shared memory device
 
 The Inter-VM shared memory device is designed to share a region of memory to
 userspace in multiple virtual guests.  The memory region does not belong to any
-guest, but is a POSIX memory object on the host.  Optionally, the device may
-support sending interrupts to other guests sharing the same memory region.
+guest, but is a either a POSIX memory object or a mmap'd file (including
+hugepage-backed file) on the host.
+
+Optionally, the device may support sending interrupts to other guests sharing
+the same memory region.
 
 
 The Inter-VM PCI device
diff --git a/hw/misc/ivshmem.c b/hw/misc/ivshmem.c
index 2838866..5d991cf 100644
--- a/hw/misc/ivshmem.c
+++ b/hw/misc/ivshmem.c
@@ -98,6 +98,7 @@ typedef struct IVShmemState {
     Error *migration_blocker;
 
     char * shmobj;
+    char * fileobj;
     char * sizearg;
     char * role;
     int role_val;   /* scalar to avoid multiple string comparisons */
@@ -715,9 +716,10 @@ static int pci_ivshmem_init(PCIDevice *dev)
         /* if we get a UNIX socket as the parameter we will talk
          * to the ivshmem server to receive the memory region */
 
-        if (s->shmobj != NULL) {
-            fprintf(stderr, "WARNING: do not specify both 'chardev' "
-                                                "and 'shm' with ivshmem\n");
+        if (s->shmobj != NULL || s->fileobj != NULL) {
+            fprintf(stderr, "WARNING: both 'chardev' and '%s' specified.\n"
+                            "Falling back to 'chardev' only.\n",
+                            s->shmobj ? "shm" : "file");
         }
 
         IVSHMEM_DPRINTF("using shared memory server (socket = %s)\n",
@@ -743,28 +745,43 @@ static int pci_ivshmem_init(PCIDevice *dev)
     } else {
         /* just map the file immediately, we're not using a server */
         int fd;
+        int is_shm = !!(s->shmobj != NULL);
+        int is_file = !!(s->fileobj != NULL);

Out of curiosity why doing !! on a boolean which would be converted either to
zero or one by implicit casting ?

 
-        if (s->shmobj == NULL) {
-            fprintf(stderr, "Must specify 'chardev' or 'shm' to ivshmem\n");
+        if (!(is_shm ^ is_file)) {
+            fprintf(stderr, "ERROR: both 'file' and 'shm' specified for the "
+                            "same ivshmem device.\n");
             exit(1);
         }
 
-        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) {
-           /* truncate file to length PCI device's memory */
-            if (ftruncate(fd, s->ivshmem_size) != 0) {
-                fprintf(stderr, "ivshmem: could not truncate shared file\n");
+        if (is_shm) {
+            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) {
+               /* truncate file to length PCI device's memory */
+                if (ftruncate(fd, s->ivshmem_size) != 0) {
+                    fprintf(stderr, "ivshmem: could not truncate"
+                                    " shared file: %m\n");
+                    exit(-1);
+                }
+
+            } else if ((fd = shm_open(s->shmobj, O_RDWR,
+                            S_IRWXU|S_IRWXG|S_IRWXO)) < 0) {
+                fprintf(stderr, "ivshmem: could not open shared file: %m\n");
+                exit(-1);
             }
+        } else {
+            IVSHMEM_DPRINTF("using open (file object = %s)\n", s->fileobj);
 
-        } else if ((fd = shm_open(s->shmobj, O_CREAT|O_RDWR,
-                        S_IRWXU|S_IRWXG|S_IRWXO)) < 0) {
-            fprintf(stderr, "ivshmem: could not open shared file\n");
-            exit(-1);
-
+            /* try opening a mmap's file. This file must have been previously
+             * created on the host */
+            if ((fd = open(s->fileobj, O_RDWR)) < 0) {
+                fprintf(stderr, "ivshmem: could not open mmap'd file: %m\n");
+                exit(-1);
+            }
         }
 
         if (check_shm_size(s, fd) == -1) {
@@ -804,6 +821,7 @@ static Property ivshmem_properties[] = {
     DEFINE_PROP_BIT("ioeventfd", IVShmemState, features, IVSHMEM_IOEVENTFD, false),
     DEFINE_PROP_BIT("msi", IVShmemState, features, IVSHMEM_MSI, true),
     DEFINE_PROP_STRING("shm", IVShmemState, shmobj),
+    DEFINE_PROP_STRING("file", IVShmemState, fileobj),
     DEFINE_PROP_STRING("role", IVShmemState, role),
     DEFINE_PROP_UINT32("use64", IVShmemState, ivshmem_64bit, 1),
     DEFINE_PROP_END_OF_LIST(),
-- 
1.7.2.5

Hi,

I won't be able to help you much on the semantic of the patch.
However you should ./script/checkpatch.pl before posting:

address@hidden:~/qemu (quorum_reboot2)$ ./scripts/checkpatch.pl /tmp/onx 
ERROR: "foo * bar" should be "foo *bar"
#115: FILE: hw/misc/ivshmem.c:101:
+    char * fileobj;

WARNING: suspect code indent for conditional statements (12, 15)
#162: FILE: hw/misc/ivshmem.c:762:
+            if ((fd = shm_open(s->shmobj, O_CREAT|O_RDWR|O_EXCL,
[...]
+               /* truncate file to length PCI device's memory */

ERROR: do not use assignment in if condition
#162: FILE: hw/misc/ivshmem.c:762:
+            if ((fd = shm_open(s->shmobj, O_CREAT|O_RDWR|O_EXCL,

ERROR: do not use assignment in if condition
#171: FILE: hw/misc/ivshmem.c:771:
+            } else if ((fd = shm_open(s->shmobj, O_RDWR,

ERROR: do not use assignment in if condition
#186: FILE: hw/misc/ivshmem.c:781:
+            if ((fd = open(s->fileobj, O_RDWR)) < 0) {

total: 4 errors, 1 warnings, 99 lines checked

/tmp/patch has style problems, please review.  If any of these errors
are false positives report them to the maintainer, see
CHECKPATCH in MAINTAINERS.

Best regards

Benoît

Well, I am actually a bit confused by the rule:
Follow the coding style and run scripts/checkpatch.pl <patchfile> before submitting.
from the submit patch guidelines, since I assume that the coding style is referring to the file coding style ?

The whole file might need a complete clean up, but I understand that it should be in a separate patch from this rule, since the whole file is written like that:
Don't include irrelevant changes. In particular, don't include formatting, coding style or whitespace changes to bits of code that would otherwise not be touched by the patch.

This is a just an indentation modification to put it inside a if/else...

Cheers,
--
Damien