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.sist...@oracle.com>
Reviewed-by: Peter Xu <pet...@redhat.com>
---
system/physmem.c | 57 ++++++++++++++++++++++++++++++++++++++++++++++++++++-
system/trace-events | 1 +
util/memfd.c | 16 ++++++++++++---
3 files changed, 70 insertions(+), 4 deletions(-)
diff --git a/system/physmem.c b/system/physmem.c
index 4d13761..e435564 100644
--- a/system/physmem.c
+++ b/system/physmem.c
@@ -48,6 +48,7 @@
#include "qemu/qemu-print.h"
#include "qemu/log.h"
#include "qemu/memalign.h"
+#include "qemu/memfd.h"
#include "exec/memory.h"
#include "exec/ioport.h"
#include "system/dma.h"
@@ -1948,6 +1949,7 @@ RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size,
ram_addr_t max_size,
bool grow,
Error **errp)
{
+ ERRP_GUARD();
RAMBlock *new_block;
Error *local_err = NULL;
int64_t file_size, file_align;
@@ -2068,6 +2070,25 @@ RAMBlock *qemu_ram_alloc_from_file(ram_addr_t size,
MemoryRegion *mr,
}
#endif
+#ifdef CONFIG_POSIX
+/*
+ * Create MAP_SHARED RAMBlocks by mmap'ing a file descriptor, so it can be
+ * shared with another process if CPR is being used. Use memfd if available
+ * because it has no size limits, else use POSIX shm.
+ */
+static int qemu_ram_get_shared_fd(const char *name, Error **errp)
+{
+ int fd;
+
+ if (qemu_memfd_check(0)) {
+ fd = qemu_memfd_create(name, 0, 0, 0, 0, errp);
+ } else {
+ fd = qemu_shm_alloc(0, errp);
+ }
+ return fd;
+}
+#endif
+
static
RAMBlock *qemu_ram_alloc_internal(ram_addr_t size, ram_addr_t max_size,
qemu_ram_resize_cb resized,
@@ -2081,6 +2102,41 @@ RAMBlock *qemu_ram_alloc_internal(ram_addr_t size,
ram_addr_t max_size,
assert((ram_flags & ~(RAM_SHARED | RAM_RESIZEABLE | RAM_PREALLOC |
RAM_NORESERVE | RAM_GUEST_MEMFD)) == 0);
assert(!host ^ (ram_flags & RAM_PREALLOC));
+ assert(max_size >= size);
+
+#ifdef CONFIG_POSIX /* ignore RAM_SHARED for Windows */
+ if (!host) {
+ if (ram_flags & RAM_SHARED) {
+ const char *name = memory_region_name(mr);
+ int fd = qemu_ram_get_shared_fd(name, errp);
+
+ if (fd < 0) {
+ return NULL;
+ }
+
+ /* Use same alignment as qemu_anon_ram_alloc */
+ mr->align = QEMU_VMALLOC_ALIGN;
+
+ /*
+ * This can fail if the shm mount size is too small, or alloc from
+ * fd is not supported, but previous QEMU versions that called
+ * qemu_anon_ram_alloc for anonymous shared memory could have
+ * succeeded. Quietly fail and fall back.
+ */
+ new_block = qemu_ram_alloc_from_fd(size, max_size, resized, mr,
+ ram_flags, fd, 0, false, NULL);
+ if (new_block) {
+ trace_qemu_ram_alloc_shared(name, new_block->used_length,
+ new_block->max_length, fd,
+ new_block->host);
+ return new_block;
+ }
+
+ close(fd);
+ /* fall back to anon allocation */
+ }
+ }
+#endif
align = qemu_real_host_page_size();
align = MAX(align, TARGET_PAGE_SIZE);
@@ -2092,7 +2148,6 @@ RAMBlock *qemu_ram_alloc_internal(ram_addr_t size,
ram_addr_t max_size,
new_block->resized = resized;
new_block->used_length = size;
new_block->max_length = max_size;
- assert(max_size >= size);
new_block->fd = -1;
new_block->guest_memfd = -1;
new_block->page_size = qemu_real_host_page_size();
diff --git a/system/trace-events b/system/trace-events
index 5bbc3fb..be12ebf 100644
--- a/system/trace-events
+++ b/system/trace-events
@@ -33,6 +33,7 @@ address_space_map(void *as, uint64_t addr, uint64_t len, bool
is_write, uint32_t
find_ram_offset(uint64_t size, uint64_t offset) "size: 0x%" PRIx64 " @ 0x%"
PRIx64
find_ram_offset_loop(uint64_t size, uint64_t candidate, uint64_t offset,
uint64_t next, uint64_t mingap) "trying size: 0x%" PRIx64 " @ 0x%" PRIx64 ",
offset: 0x%" PRIx64" next: 0x%" PRIx64 " mingap: 0x%" PRIx64
ram_block_discard_range(const char *rbname, void *hva, size_t length, bool
need_madvise, bool need_fallocate, int ret) "%s@%p + 0x%zx: madvise: %d
fallocate: %d ret: %d"
+qemu_ram_alloc_shared(const char *name, size_t size, size_t max_size, int fd,
void *host) "%s size %zu max_size %zu fd %d host %p"
# cpus.c
vm_stop_flush_all(int ret) "ret %d"
diff --git a/util/memfd.c b/util/memfd.c
index 8a2e906..07beab1 100644
--- a/util/memfd.c
+++ b/util/memfd.c
@@ -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;
--
1.8.3.1
