Re: [RFC PATCH v2 4/7] migration: Make qemu_get_buffer_at() thread-safe

[Peter Xu <peterx@redhat.com>]

On Tue, Jun 30, 2026 at 01:49:37PM +0530, Aadeshveer Singh wrote:
> Remove the internal QEMUFile error state modification from
> qemu_get_buffer_at(). This function is called by two functions, both of
> which already check for unexpected return values and handle their own
> error reporting.
> 
> Removing this shared state modification makes qemu_get_buffer_at()
> strictly thread-safe for concurrent disk reads, serving as a
> preparatory change for the upcoming fast snapshot load feature.
> 
> Signed-off-by: Aadeshveer Singh <aadeshveer07@gmail.com>
> ---
>  migration/qemu-file.c | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/migration/qemu-file.c b/migration/qemu-file.c
> index d5a48115bd..c73f8178d7 100644
> --- a/migration/qemu-file.c
> +++ b/migration/qemu-file.c
> @@ -553,14 +553,11 @@ void qemu_put_buffer_at(QEMUFile *f, const uint8_t *buf, size_t buflen,
>  size_t qemu_get_buffer_at(QEMUFile *f, uint8_t *buf, size_t buflen,
>                            off_t pos)
>  {
> -    Error *err = NULL;
> -
>      if (f->last_error) {
>          return 0;
>      }
>  
> -    if (qio_channel_pread_all(f->ioc, buf, buflen, pos, &err) < 0) {
> -        qemu_file_set_error_obj(f, -EIO, err);
> +    if (qio_channel_pread_all(f->ioc, buf, buflen, pos, NULL) < 0) {
>          return 0;
>      }

This is better from thread safe pov by removing the function that isn't
thread safe, but it goes backward too by dropping the &err to capture the
verbose error.

I think a better way to do this is having Error** passed into
qemu_get_buffer_at(), to get both thread safety and verbose errors.

I should have suggested that last time, sorry.

>  
> -- 
> 2.54.0
> 

-- 
Peter Xu