Re: [Qemu-devel] [PATCH v3 0/5] fw_cfg DMA interface

[Kevin O'Connor]

On Fri, Sep 18, 2015 at 11:47:52PM +0100, Peter Maydell wrote:
> On 18 September 2015 at 19:25, Kevin O'Connor <address@hidden> wrote:
> > +Additionaly, if the DMA interface is available then a read to the DMA
> > +Address will return 0x51454d5520434647 ("QEMU CFG" in big-endian
> > +format).
> > +
> 
> I don't think I understand this. If you know the DMA Address
> port or register exists, then you know (by definition) that
> the DMA interface is available. If you don't know that the
> DMA interface is available then you can't read from the DMA
> Address port or register because it might not exist and could
> therefore cause you to blow up.
> 
> If you want to be able to tell without doing the "use the
> old-style interface to query the version" thing, then you
> need to look in the ACPI or device tree tables (and those
> tables need to be such that you can tell the difference,
> which is the case for at least device tree; haven't checked
> ACPI.)

Hi Peter,

On x86 the firmware can't use acpi (nor device tree) to find fw_cfg
because fw_cfg is what is used to transfer acpi to the firmware.  So,
the firmware just hard codes the address.  As a "sanity check", the
firmware currently checks for a signature before using fw_cfg to
verify everything is working correctly (outw(0x0000, 0x510);
inb(0x511) == 'Q'; inb(0x511) == 'E'; ...).  A check for the new dma
interface involves an additional query (outw(0x0001, 0x510);
inb(0x511) == 3; ...).

I'm proposing that a future firmware (that didn't need to support old
versions of QEMU) could use a simpler sanity check instead (inl(0x514)
== "QEMU"; inl(0x518) == " CFG").

Granted, both the old check and the new proposed check would not be
needed on platforms that have a device tree transmitted separately
from fw_cfg.  Though, even on those platforms, there is no harm in
defining what happens on a read event.

-Kevin