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

[Kevin O'Connor]

On Thu, Aug 06, 2015 at 01:35:27PM +0100, Stefan Hajnoczi wrote:
> 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.

In seabios, one would use the yield() call.  For example:

while (access.control & QEMU_CFG_DMA_CTL_READ)
        yield();

-Kevin