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softmmu/physmem: Hint that "readonly=on,rom=off" exists when opening file R/W for private mapping fails

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It's easy to miss that memory-backend-file with "share=off" (default)
will always try opening the file R/W as default, and fail if we don't
have write permissions to the file.

In that case, the user has to explicit specify "readonly=on,rom=off" to
get usable RAM, for example, for VM templating.

Let's hint that '-object memory-backend-file,readonly=on,rom=off,...'
exists to consume R/O files in a private mapping to create writable RAM,
but only if we have permissions to open the file read-only.

Message-ID: <20230906120503.359863-11-david@redhat.com>
Suggested-by: ThinerLogoer <logoerthiner1@163.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
This commit is contained in:
David Hildenbrand 2023-09-06 14:05:02 +02:00
parent 9cd9313fc3
commit 6da4b1c25d
1 changed files with 19 additions and 0 deletions
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19
softmmu/physmem.c
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@ -1969,6 +1969,25 @@ RAMBlock *qemu_ram_alloc_from_file(ram_addr_t size, MemoryRegion *mr,
if (fd < 0) {
error_setg_errno(errp, -fd, "can't open backing store %s for guest RAM",
mem_path);
if (!(ram_flags & RAM_READONLY_FD) && !(ram_flags & RAM_SHARED) &&
fd == -EACCES) {
/*
* If we can open the file R/O (note: will never create a new file)
* and we are dealing with a private mapping, there are still ways
* to consume such files and get RAM instead of ROM.
*/
fd = file_ram_open(mem_path, memory_region_name(mr), true,
&created);
if (fd < 0) {
return NULL;
}
assert(!created);
close(fd);
error_append_hint(errp, "Consider opening the backing store"
" read-only but still creating writable RAM using"
" '-object memory-backend-file,readonly=on,rom=off...'"
" (see \"VM templating\" documentation)\n");
}
return NULL;
}