Re: [Qemu-arm] [PATCH RFC] fw-cfg: support writeable blobs

[Kevin O'Connor]

On Mon, Feb 22, 2016 at 02:41:38PM +0200, Michael S. Tsirkin wrote:
> Useful to send guest data back to QEMU.
> The write interface is restricted to DMA.
> 
> Suggested-by: Kevin O'Connor <address@hidden>
> Signed-off-by: Michael S. Tsirkin <address@hidden>
> ---
> 
>  hw/lm32/lm32_hwsetup.h    |  2 +-
>  include/hw/loader.h       |  4 ++--
>  include/hw/nvram/fw_cfg.h |  3 ++-
>  hw/arm/virt-acpi-build.c  |  2 +-
>  hw/core/loader.c          | 19 ++++++++++++-------
>  hw/i386/acpi-build.c      |  4 ++--
>  hw/nvram/fw_cfg.c         | 36 ++++++++++++++++++++++++++++--------
>  7 files changed, 48 insertions(+), 22 deletions(-)
> 
> diff --git a/hw/lm32/lm32_hwsetup.h b/hw/lm32/lm32_hwsetup.h
> index 838754d..805b445 100644
> --- a/hw/lm32/lm32_hwsetup.h
> +++ b/hw/lm32/lm32_hwsetup.h
> @@ -74,7 +74,7 @@ static inline void hwsetup_create_rom(HWSetup *hw,
>          hwaddr base)
>  {
>      rom_add_blob("hwsetup", hw->data, TARGET_PAGE_SIZE,
> -                 TARGET_PAGE_SIZE, base, NULL, NULL, NULL);
> +                 TARGET_PAGE_SIZE, base, NULL, NULL, NULL, true);
>  }

Instead of supporting "writable blobs", I wonder if it would be
simpler for both firmware and qemu to implement an "ioctl" like
interface.  That is, just directly invoke a new fw_cfg callback from
fw_cfg_dma_transfer() with the data obtained via dma_memory_read().  I
think that may be more flexible because then the QEMU code doesn't
have to check for changes to a blob - it would instead be immediately
invoked upon a change.

-Kevin