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

[Kinney, Michael D]

Paolo,

The SMBASE register is internal and cannot be directly accessed 
by any CPU.  There is an SMBASE field that is member of the SMM Save
State area and can only be modified from SMM and requires the
execution of an RSM instruction from SMM for the SMBASE register to
be updated from the current SMBASE field value.  The new SMBASE
register value is only used on the next SMI.

https://software.intel.com/sites/default/files/managed/39/c5/325462-sdm-vol-1-2abcd-3abcd.pdf

Vol 3C - Section 34.11

  The default base address for the SMRAM is 30000H. This value is contained in 
an internal processor register called
  the SMBASE register. The operating system or executive can relocate the SMRAM 
by setting the SMBASE field in the
  saved state map (at offset 7EF8H) to a new value (see Figure 34-4). The RSM 
instruction reloads the internal
  SMBASE register with the value in the SMBASE field each time it exits SMM. 
All subsequent SMI requests will use
  the new SMBASE value to find the starting address for the SMI handler (at 
SMBASE + 8000H) and the SMRAM state
  save area (from SMBASE + FE00H to SMBASE + FFFFH). (The processor resets the 
value in its internal SMBASE
  register to 30000H on a RESET, but does not change it on an INIT.)

One idea to work around these issues is to startup OVMF with the maximum number 
of
CPUs.  All the CPUs will be assigned an SMBASE address and at a safe time to 
assign
the SMBASE values using the initial 3000:8000 SMI vector because there is a 
guarantee
of no DMA at that point in the FW init.

Once all the CPUs have been initialized for SMM, the CPUs that are not needed
can be hot removed.  As noted above, the SMBASE value does not change on
an INIT.  So as long as the hot add operation does not do a RESET, the
SMBASE value must be preserved.

Of course, this is not a good idea from a boot performance perspective, 
especially if the max CPUs is a large value.

Another idea is to emulate this behavior.  If the hot plug controller
provide registers (only accessible from SMM) to assign the SMBASE address
for every CPU.  When a CPU is hot added, QEMU can set the internal SMBASE
register value from the hot plug controller register value.  If the SMM
Monarch sends an INIT or an SMI from the Local APIC to the hot added CPU,
then the SMBASE register should not be modified and the CPU starts execution
within TSEG the first time it receives an SMI.

Jiewen and I can collect specific questions on this topic and continue
the discussion here.  For example, I do not think there is any method
other than what I referenced above to program the SMBASE register, but
I can ask if there are any other methods.

Thanks,

Mike

> -----Original Message-----
> From: Paolo Bonzini [mailto:address@hidden]
> Sent: Thursday, August 22, 2019 11:43 AM
> To: Laszlo Ersek <address@hidden>; Kinney, Michael D
> <address@hidden>; address@hidden; Yao,
> Jiewen <address@hidden>
> Cc: Alex Williamson <address@hidden>;
> address@hidden; qemu devel list <qemu-
> address@hidden>; Igor Mammedov <address@hidden>;
> Chen, Yingwen <address@hidden>; Nakajima, Jun
> <address@hidden>; Boris Ostrovsky
> <address@hidden>; Joao Marcal Lemos Martins
> <address@hidden>; Phillip Goerl
> <address@hidden>
> Subject: Re: [edk2-rfc] [edk2-devel] CPU hotplug using
> SMM with QEMU+OVMF
> 
> On 22/08/19 19:59, Laszlo Ersek wrote:
> > 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.
> 
> No, that would be unmaintainable.  The best solution to
> me seems to be to make SMBASE programmable from non-SMM
> code if some special conditions hold.  Michael, would it
> be possible to get in contact with the Intel architects?
> 
> Paolo