Re: [Qemu-devel] [RFC] q35/mch: implement extended TSEG sizes

[Laszlo Ersek]

On 06/07/17 07:52, Gerd Hoffmann wrote:
>   Hi,
> 
> Patch looks sane overall.
> 
>> Invent a new, QEMU-specific register in the config space of the DRAM
>> Controller, at offset 0x50, in order to allow guest firmware to query
>> the
>> TSEG (SMRAM) size.
> 
> Hmm, 0x50 appears to be the only unused config space register in the
> specs.

I did find more holes, in "Table 5-1. DRAM Controller Register Address
Map (D0:F0)", in "Document Number: 316966-002". The hole at 0x50 is just
the one with the lowest config space offset (after the standard PCI
device header). The others are:

- 0x58-0x59 (word)
- 0x9c (byte)
- 0x9f (byte)
- 0xb2-0xc7 (eight words)
- 0xce-0xdb (seven words)
- 0xeb-0xff (fifteen bytes, but this range appears to come after the
             capability list, so I felt it would be unsafe to use)

> I suspect in reality it isn't unused but undocumented.  I don't
> have a better idea though, and in practice it probably isn't much of a
> problem.

Thanks for confirming. Then I'll set out to write the OVMF-side patches
for this. Once I have it all working, I'm going to send out the QEMU
change as a real patch.

Thanks!
Laszlo