Re: [SeaBIOS] [PATCH 1/2] Add QEMU fw_cfg DMA interface

[Stefan Hajnoczi]

On Thu, Aug 6, 2015 at 12:02 PM, Marc Marí  wrote:
> if (qemu_cfg_dma_enabled()) {
>    QemuCfgDmaAccess access;
>    access.address = (u64)(u32)buf;
>    access.length = len;
>    access.control = QEMU_CFG_DMA_CTL_READ;
>    /*
>     * The out is done before the write of the variables on memory. This
>     * causes misread on the QEMU side.
>     */
>    barrier();
>    outl((u32)&access, PORT_QEMU_CFG_DMA_ADDR);

I thought PORT_QEMU_CFG_DMA_ADDR is a 64-bit register according to the
spec you posted?
>    while(access.length != 0 && !(access.control & QEMU_CFG_DMA_CTL_ERROR));

Either the field accesses need to be marked volatile, or a barrier is
needed to force the compiler to reload these register from memory each
iteration of the loop.

I think the problem is that there are no "sequence points" (according
to the C language specification) in this loop, so the compiler may
assume that access.length and access.control will not change.
https://en.wikipedia.org/wiki/Sequence_point

Stefan