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

[Paolo Bonzini]

On 23/08/19 00:32, Kinney, Michael D wrote:
> Paolo,
> 
> It is my understanding that real HW hot plug uses the SDM defined
> methods.  Meaning the initial SMI is to 3000:8000 and they rebase
> to TSEG in the first SMI.  They must have chipset specific methods
> to protect 3000:8000 from DMA.

It would be great if you could check.

> Can we add a chipset feature to prevent DMA to 64KB range from
> 0x30000-0x3FFFF and the UEFI Memory Map and ACPI content can be
> updated so the Guest OS knows to not use that range for DMA?

If real hardware does it at the chipset level, we will probably use
Igor's suggestion of aliasing A-seg to 3000:0000.  Before starting the
new CPU, the SMI handler can prepare the SMBASE relocation trampoline at
A000:8000 and the hot-plugged CPU will find it at 3000:8000 when it
receives the initial SMI.  Because this is backed by RAM at
0xA0000-0xAFFFF, DMA cannot access it and would still go through to RAM
at 0x30000.

Paolo