Re: [PATCH 1/3] memory: add readonly support to memory_region_init_ram_from_file()

[Philippe Mathieu-Daudé]

Hi Stefan,

On 8/4/20 12:12 PM, Stefan Hajnoczi wrote:
> There is currently no way to open(O_RDONLY) and mmap(PROT_READ) when
> creating a memory region from a file. This functionality is needed since
> the underlying host file may not allow writing.
> 
> Add a bool readonly argument to memory_region_init_ram_from_file() and
> the APIs it calls.
> 
> Extend memory_region_init_ram_from_file() rather than introducing a
> memory_region_init_rom_from_file() API so that callers can easily make a
> choice between read/write and read-only at runtime without calling
> different APIs.

What happens if we call:

 memory_region_init_ram_from_file(mr, ..., readonly=false, ...);
 memory_region_set_readonly(mr, false);

?

> 
> No new RAMBlock flag is introduced for read-only because it's unclear
> whether RAMBlocks need to know that they are read-only. Pass a bool
> readonly argument instead.
> 
> Both of these design decisions can be changed in the future. It just
> seemed like the simplest approach to me.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  include/exec/memory.h     |  2 ++
>  include/exec/ram_addr.h   |  5 +++--
>  include/qemu/mmap-alloc.h |  2 ++
>  backends/hostmem-file.c   |  2 +-
>  exec.c                    | 18 +++++++++++-------
>  softmmu/memory.c          |  7 +++++--
>  util/mmap-alloc.c         | 10 ++++++----
>  util/oslib-posix.c        |  2 +-
>  8 files changed, 31 insertions(+), 17 deletions(-)
> 
> diff --git a/include/exec/memory.h b/include/exec/memory.h
> index 307e527835..1ae7b31e3a 100644
> --- a/include/exec/memory.h
> +++ b/include/exec/memory.h
> @@ -884,6 +884,7 @@ void memory_region_init_resizeable_ram(MemoryRegion *mr,
>   *             - RAM_PMEM: the memory is persistent memory
>   *             Other bits are ignored now.
>   * @path: the path in which to allocate the RAM.
> + * @readonly: true to open @path for reading, false for read/write.
>   * @errp: pointer to Error*, to store an error if it happens.
>   *
>   * Note that this function does not do anything to cause the data in the
> @@ -896,6 +897,7 @@ void memory_region_init_ram_from_file(MemoryRegion *mr,
>                                        uint64_t align,
>                                        uint32_t ram_flags,
>                                        const char *path,
> +                                      bool readonly,
>                                        Error **errp);
>  
[...]
> diff --git a/softmmu/memory.c b/softmmu/memory.c
> index af25987518..d228635bb3 100644
> --- a/softmmu/memory.c
> +++ b/softmmu/memory.c
> @@ -1553,15 +1553,18 @@ void memory_region_init_ram_from_file(MemoryRegion 
> *mr,
>                                        uint64_t align,
>                                        uint32_t ram_flags,
>                                        const char *path,
> +                                      bool readonly,
>                                        Error **errp)
>  {
>      Error *err = NULL;
>      memory_region_init(mr, owner, name, size);
>      mr->ram = true;
> +    mr->readonly = readonly;
>      mr->terminates = true;
>      mr->destructor = memory_region_destructor_ram;
>      mr->align = align;
> -    mr->ram_block = qemu_ram_alloc_from_file(size, mr, ram_flags, path, 
> &err);
> +    mr->ram_block = qemu_ram_alloc_from_file(size, mr, ram_flags, path,
> +                                             readonly, &err);
>      mr->dirty_log_mask = tcg_enabled() ? (1 << DIRTY_MEMORY_CODE) : 0;
>      if (err) {
>          mr->size = int128_zero();
> @@ -1585,7 +1588,7 @@ void memory_region_init_ram_from_fd(MemoryRegion *mr,
>      mr->destructor = memory_region_destructor_ram;
>      mr->ram_block = qemu_ram_alloc_from_fd(size, mr,
>                                             share ? RAM_SHARED : 0,
> -                                           fd, &err);
> +                                           fd, false, &err);
>      mr->dirty_log_mask = tcg_enabled() ? (1 << DIRTY_MEMORY_CODE) : 0;
>      if (err) {
>          mr->size = int128_zero();
> diff --git a/util/mmap-alloc.c b/util/mmap-alloc.c
> index 27dcccd8ec..890fda6a35 100644
> --- a/util/mmap-alloc.c
> +++ b/util/mmap-alloc.c
> @@ -85,9 +85,11 @@ size_t qemu_mempath_getpagesize(const char *mem_path)
>  void *qemu_ram_mmap(int fd,
>                      size_t size,
>                      size_t align,
> +                    bool readonly,
>                      bool shared,
>                      bool is_pmem)
>  {
> +    int prot;
>      int flags;
>      int map_sync_flags = 0;
>      int guardfd;
> @@ -146,8 +148,9 @@ void *qemu_ram_mmap(int fd,
>  
>      offset = QEMU_ALIGN_UP((uintptr_t)guardptr, align) - (uintptr_t)guardptr;
>  
> -    ptr = mmap(guardptr + offset, size, PROT_READ | PROT_WRITE,
> -               flags | map_sync_flags, fd, 0);
> +    prot = PROT_READ | (readonly ? 0 : PROT_WRITE);
> +
> +    ptr = mmap(guardptr + offset, size, prot, flags | map_sync_flags, fd, 0);
>  
>      if (ptr == MAP_FAILED && map_sync_flags) {
>          if (errno == ENOTSUP) {
> @@ -171,8 +174,7 @@ void *qemu_ram_mmap(int fd,
>           * if map failed with MAP_SHARED_VALIDATE | MAP_SYNC,
>           * we will remove these flags to handle compatibility.
>           */
> -        ptr = mmap(guardptr + offset, size, PROT_READ | PROT_WRITE,
> -                   flags, fd, 0);
> +        ptr = mmap(guardptr + offset, size, prot, flags, fd, 0);
>      }
>  
>      if (ptr == MAP_FAILED) {
[...]