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

[Kevin O'Connor]

On Thu, Aug 06, 2015 at 06:15:57PM +0200, Laszlo Ersek wrote:
> On 08/06/15 14:35, Stefan Hajnoczi wrote:
> > On Thu, Aug 6, 2015 at 12:02 PM, Marc Marí <markmb at redhat.com> 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.
> 
> Better yet, I would recommend declaring (and here, also defining)
> "access" as "volatile".
> 
>   volatile QemuCfgDmaAccess access;

I'd prefer to avoid volatile for the reasons described at:

  https://www.kernel.org/doc/Documentation/volatile-considered-harmful.txt

I think barrier() and yield() are what we want - in particular, we
really do want to yield() while busy so that interrupts and other code
can continue to run.

-Kevin