x86/ACPI/cstate: Allow ACPI C1 FFH MWAIT use on AMD systems
AMD systems support the Monitor/Mwait instructions and these can be used for ACPI C1 in the same way as on Intel systems. Three things are needed: 1) This patch. 2) BIOS that declares a C1 state in _CST to use FFH, with correct values. 3) CPUID_Fn00000005_EDX is non-zero on the system. The BIOS on AMD systems have historically not defined a C1 state in _CST, so the acpi_idle driver uses HALT for ACPI C1. Currently released systems have CPUID_Fn00000005_EDX as reserved/RAZ. If a BIOS is released for these systems that requests a C1 state with FFH, the FFH implementation in Linux will fail since CPUID_Fn00000005_EDX is 0. The acpi_idle driver will then fallback to using HALT for ACPI C1. Future systems are expected to have non-zero CPUID_Fn00000005_EDX and BIOS support for using FFH for ACPI C1. Allow ffh_cstate_init() to succeed on AMD systems. Tested on Fam15h and Fam17h systems. Signed-off-by: NYazen Ghannam <yazen.ghannam@amd.com> Acked-by: NBorislav Petkov <bp@suse.de> Signed-off-by: NRafael J. Wysocki <rafael.j.wysocki@intel.com>
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