Re: [edk2-devel] [Qemu-devel] [PATCH 1/2] q35: implement 128K SMRAM at default SMBASE address

Laszlo Ersek

On 10/04/19 13:31, Igor Mammedov wrote:
On Tue, 1 Oct 2019 20:03:20 +0200
"Laszlo Ersek" <lersek@...> wrote:
(1) What values to use.
SeaBIOS writes 0x00 into command port, but it seems that's taken by
EFI_SMM_COMMUNICATION_PROTOCOL. So we can use the next unused value
(lets say 0x4). We probably don't have to use status port or
EFI_SMM_COMMUNICATION_PROTOCOL, since the value of written into 0xB2
is sufficient to distinguish hotplug event.
Thanks. Can you please write a QEMU patch for the ACPI generator such
that hotplugging a VCPU writes value 4 to IO port 0xB2?

That will allow me to experiment with OVMF.

(I can experiment with some other parts in edk2 even before that.)

(2) How the parameters are passed.

(2a) For the new CPU, the SMI remains pending, until it gets an
INIT-SIPI-SIPI from one of the previously plugged CPUs (most likely, the
BSP). At that point, the new CPU will execute the "initial SMI handler
for hotplugged CPUs", at the default SMBASE.

That's a routine we'll have to write in assembly, from zero. In this
routine, we can read back IO ports 0xB2 and 0xB3. And QEMU will be happy
to provide the values last written (see apm_ioport_readb() in
"hw/isa/apm.c"). So we can receive the values in this routine. Alright.
Potentially we can can avoid writing custom SMI handler,
what do you think about following workflow:

on system boot after initial CPUs relocation, firmware set NOP SMI handler
at default SMBASE.
Then as reaction to GPE triggered SMI (on cpu hotplug), after SMI rendezvous,
a host cpu reads IO port 0xB2 and does hotplugged CPUs enumeration.

a) assuming we allow hotplug only in case of negotiated SMI broadcast
host CPU shoots down all in-flight INIT/SIPI/SIPI for hotpugged CPUs
to avoid race within relocation handler.
How is that "shootdown" possible?

After that host CPU in loop

b) it prepares/initializes necessary CPU structures for a hotplugged
CPU if necessary and replaces NOP SMI handler with the relocation
SMI handler that is used during system boot.

c) a host CPU sends NOP INIT/SIPI/SIPI to the hotplugged CPU

d) the woken up hotplugged CPU, jumps to default SMBASE and
executes hotplug relocation handler.

e) after the hotplugged CPU is relocated and if there are more
hotplugged CPUs, a host CPU repeats b-d steps for the next
hotplugged CPU.

f) after all CPUs are relocated, restore NOP SMI handler at default

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