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

Paolo Bonzini <pbonzini@...>

On 23/08/19 00:32, Kinney, Michael D wrote:

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.


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