Re: [PATCH] hw/block/pflash_cfi01: Restore ROMD mode after migration

On Mon, 3 Aug 2026 at 05:19, Bin Guo <[email protected]> wrote:
>
> pflash_post_load() did not restore the ROMD mode of the memory region.
> Although cmd and wcycle are migrated, the destination retains the
> default ROMD = true from realize.  When the source was in a non-array
> mode (e.g. ID read, cmd = 0x90), reads on the destination bypass
> pflash_read() via the ROM fast path and return raw storage bytes
> instead of the command-specific response.
>
> Derive ROMD from the migrated cmd/wcycle in pflash_post_load.
>
> Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/4042
> Cc: [email protected]
> Signed-off-by: Bin Guo <[email protected]>
> ---
>  hw/block/pflash_cfi01.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> diff --git a/hw/block/pflash_cfi01.c b/hw/block/pflash_cfi01.c
> index 5b9ddb20b1..a13b91967e 100644
> --- a/hw/block/pflash_cfi01.c
> +++ b/hw/block/pflash_cfi01.c
> @@ -1030,6 +1030,16 @@ static int pflash_post_load(void *opaque, int version_id)
>  {
>      PFlashCFI01 *pfl = opaque;
>
> +    /*
> +     * ROMD mode is not in the VMState; derive it from the migrated
> +     * cmd and wcycle.  Only (wcycle == 0, cmd == 0x00) is read-array.
> +     */
> +    if (pfl->wcycle == 0 && pfl->cmd == 0x00) {
> +        memory_region_rom_device_set_romd(&pfl->mem, true);
> +    } else {
> +        memory_region_rom_device_set_romd(&pfl->mem, false);
> +    }

Confirming that this is correct is a bit tricky. It relies on:
 * when we set romd mode to true we also set wcycle = 0, cmd = 0
   (which we do, in reset and in the mode_read_array code)
 * when we set romd mode to false at the top of pflash_write(),
   all paths out of that function either go through the mode_read_array
   path, or else update pfl->cmd to something non-zero
 * nowhere outside pflash_write() udpates cmd or wcycle except
   for the "clear them to 0 and set romd mode" places

This is almost but not quite true. In pflash_read(), the default
case for the pfl->cmd switch sets wcycle = 0 cmd = 0 but doesn't
change the romd state. Luckily the "this should never happen"
comment is true -- there's no way to get a pfl->cmd that falls
into the default (except for being deliberately fed a bogus value
via inbound migration).

It might be worth having that pflash_read() code also set the
romd mode when it clears wcycle and cmd, to make the invariant
more clearly preserved.

But this patch is correct, so:

Reviewed-by: Peter Maydell <[email protected]>

thanks
-- PMM