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Kirill A. Shutemov authored
The HLT instruction is a privileged instruction, executing it stops instruction execution and places the processor in a HALT state. It is used in kernel for cases like reboot, idle loop and exception fixup handlers. For the idle case, interrupts will be enabled (using STI) before the HLT instruction (this is also called safe_halt()). To support the HLT instruction in TDX guests, it needs to be emulated using TDVMCALL (hypercall to VMM). More details about it can be found in Intel Trust Domain Extensions (Intel TDX) Guest-Host-Communication Interface (GHCI) specification, section TDVMCALL[Instruction.HLT]. In TDX guests, executing HLT instruction will generate a #VE, which is used to emulate the HLT instruction. But #VE based emulation will not work for the safe_halt() flavor, because it requires STI instruction to be executed just before the TDCALL. Since idle loop is the only user of safe_halt() variant, handle it as a special case. To avoid *safe_halt() call in the idle function, define the tdx_guest_idle() and use it to override the "x86_idle" function pointer for a valid TDX guest. Alternative choices like PV ops have been considered for adding safe_halt() support. But it was rejected because HLT paravirt calls only exist under PARAVIRT_XXL, and enabling it in TDX guest just for safe_halt() use case is not worth the cost. Co-developed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com> Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com> Reviewed-by: Andi Kleen <ak@linux.intel.com> Reviewed-by: Tony Luck <tony.luck@intel.com> Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com> Link: https://lkml.kernel.org/r/20220405232939.73860-9-kirill.shutemov@linux.intel.com
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