Re: [Qemu-devel] [edk2-rfc] [edk2-devel] CPU hotplug using SMM with QEMU+OVMF

[Laszlo Ersek <lersek@redhat.com>]

On 09/04/19 11:52, Igor Mammedov wrote:

>  it could be stolen RAM + black hole like TSEG, assuming fw can live without RAM(0x30000+128K) range
>   (in this case fwcfg interface would only work for locking down the range)
> 
>  or
> 
>  we can actually have a dedicated SMRAM (like in my earlier RFC),
>  in this case FW can use RAM(0x30000+128K) when SMRAM isn't mapped into RAM address space
>  (in this case fwcfg would be used to temporarily map SMRAM into normal RAM and unmap/lock
>   after SMI relocation handler was initialized).
> 
> If possible I'd prefer a simpler TSEG like variant.

I think TSEG-like behavior is between these two. That is, I believe we
should have explicit open/close/lock operations. And, when the range is
closed (meaning, closed+unlocked, or closed+locked), then the black hole
should take effect for code that's not running in SMM.

Put differently, its like the second choice, except the range never
appears as normal RAM. "When SMRAM isn't mapped into RAM address space",
then the address range shows "nothing" (black hole).


Regarding "fw can live without RAM(0x30000+128K) range" -- do you mean
whether the firmware could use another RAM area for fw_cfg DMA?

If that's the question, then I wouldn't worry about it. I'd remove the
0x30000+128K range from the memory map, so the fw_cfg stuff (or anything
else) would never allocate memory from the range. It's much more
concerning to me however how the SMM infrastructure would deal with a
hole in the memory map right there.

Thanks
Laszlo