KVM: VMX: Cap VMX preemption timer to work around Intel erratum

Due to a widespread Intel erratum (e.g. EMR158), programming the
VMX-preemption timer with certain large values may cause the timer to
expire earlier than expected.  The recommended workaround is to cap the
VMX-preemption timer value to strictly less than:

   2^25 * CPUID.15H:EBX[31:0] / CPUID.15H:EAX[31:0].

Calculate the maximum "safe" preemption timer value during hardware setup
based on CPUID 15H when available, and use the adjusted max value in all
locations where KVM currently hardcodes the max architectural value,
including in the subtle case where KVM soft-disables the timer.

Don't apply the workaround when running as a VM, because absent explicit
enumeration to state the bug is present (or not), it's L0's responsibility
to faithfully emulate/virtualize the VMX preemption timer.

WARN if the above logic would result in a max value of zero and fall back
to the maximum architectural value, as the expectation is that real
hardware will never provide problematic EAX/EBX values (which is another
reason to ignore the erratum when running as a VM; there's less chance of
a false positive on the WARN due to L0 providing an unanticipated ratio).

Reported-by: Sean Christopherson <seanjc@google.com>
Closes: https://lore.kernel.org/all/Zn9X0yFxZi_Mrlnt@google.com/
Suggested-by: Chao Gao <chao.gao@intel.com>
Assisted-by: Gemini:Gemini-Next
Reviewed-by: Chao Gao <chao.gao@intel.com>
Signed-off-by: Jim Mattson <jmattson@google.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
[sean: track inclusive max instead of exclusive limit, massage changelog]
Link: https://patch.msgid.link/20260731171926.2629627-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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