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


Laszlo Ersek
 

On 08/21/19 19:05, Paolo Bonzini wrote:
On 21/08/19 17:48, Kinney, Michael D wrote:
Perhaps there is a way to avoid the 3000:8000 startup
vector.

If a CPU is added after a cold reset, it is already in a
different state because one of the active CPUs needs to
release it by interacting with the hot plug controller.

Can the SMRR for CPUs in that state be pre-programmed to
match the SMRR in the rest of the active CPUs?

For OVMF we expect all the active CPUs to use the same
SMRR value, so a check can be made to verify that all
the active CPUs have the same SMRR value. If they do,
then any CPU released through the hot plug controller
can have its SMRR pre-programmed and the initial SMI
will start within TSEG.

We just need to decide what to do in the unexpected
case where all the active CPUs do not have the same
SMRR value.

This should also reduce the total number of steps.
The problem is not the SMRR but the SMBASE. If the SMBASE area is
outside TSEG, it is vulnerable to DMA attacks independent of the SMRR.
SMBASE is also different for all CPUs, so it cannot be preprogrammed.
The firmware and QEMU could agree on a formula, which would compute the
CPU-specific SMBASE from a value pre-programmed by the firmware, and the
initial APIC ID of the hot-added CPU.

Yes, it would duplicate code -- the calculation -- between QEMU and
edk2. While that's not optimal, it wouldn't be a first.

Thanks
Laszlo

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