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physmem: fd-based shared memory

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Create MAP_SHARED RAMBlocks by mmap'ing a file descriptor rather than using
MAP_ANON, so the memory can be accessed in another process by passing and
mmap'ing the fd.  This will allow CPR to support memory-backend-ram and
memory-backend-shm objects, provided the user creates them with share=on.

Use memfd_create if available because it has no constraints.  If not, use
POSIX shm_open.  However, allocation on the opened fd may fail if the shm
mount size is too small, even if the system has free memory, so for backwards
compatibility fall back to qemu_anon_ram_alloc/MAP_ANON on failure.

For backwards compatibility on Windows, always use MAP_ANON.  share=on has
no purpose there, but the syntax is accepted, and must continue to work.

Lastly, quietly fall back to MAP_ANON if the system does not support
qemu_ram_alloc_from_fd.

Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Link: https://lore.kernel.org/r/1736967650-129648-5-git-send-email-steven.sistare@oracle.com
Signed-off-by: Fabiano Rosas <farosas@suse.de>
This commit is contained in:
Steve Sistare 2025-01-15 11:00:30 -08:00 • committed by Fabiano Rosas
parent 3ec0214816
commit 9fb40bb962
3 changed files with 70 additions and 4 deletions
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16
util/memfd.c
View file

@ -194,17 +194,27 @@ bool qemu_memfd_alloc_check(void)
/**
* qemu_memfd_check():
*
* Check if host supports memfd.
* Check if host supports memfd. Cache the answer for the common case flags=0.
*/
bool qemu_memfd_check(unsigned int flags)
{
#ifdef CONFIG_LINUX
int mfd = memfd_create("test", flags | MFD_CLOEXEC);
int mfd;
static int memfd_check = MEMFD_TODO;
if (!flags && memfd_check != MEMFD_TODO) {
return memfd_check;
}
mfd = memfd_create("test", flags | MFD_CLOEXEC);
if (mfd >= 0) {
close(mfd);
return true;
}
if (!flags) {
memfd_check = (mfd >= 0) ? MEMFD_OK : MEMFD_KO;
}
return (mfd >= 0);
#endif
return false;