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

[Paolo Bonzini]

On 22/08/19 22:06, Kinney, Michael D wrote:
> 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.

Actually there is also an SMBASE MSR, even though in current silicon
it's read-only and its use is theoretically limited to SMM-transfer
monitors.  If that MSR could be made accessible somehow outside SMM,
that would be great.

> 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.

IIRC, hot-remove + hot-add will unplugs/plugs a completely different CPU.

> 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.

Yes, this would work.  But again---if the issue is real on current
hardware too, I'd rather have a matching solution for virtual platforms.

If the current hardware for example remembers INIT-preserved across
hot-remove/hot-add, we could emulate that.

I guess the fundamental question is: how do bare metal platforms avoid
this issue, or plan to avoid this issue?  Once we know that, we can use
that information to find a way to implement it in KVM.  Only if it is
impossible we'll have a different strategy that is specific to our platform.

Paolo

> 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.