x86/tdx: Reimplement __tdx_hypercall() using TDX_MODULE_CALL asm

Now the TDX_HYPERCALL asm is basically identical to the TDX_MODULE_CALL
with both '\saved' and '\ret' enabled, with two minor things though:

1) The way to restore the structure pointer is different

The TDX_HYPERCALL uses RCX as spare to restore the structure pointer,
but the TDX_MODULE_CALL assumes no spare register can be used. In other
words, TDX_MODULE_CALL already covers what TDX_HYPERCALL does.

2) TDX_MODULE_CALL only clears shared registers for TDH.VP.ENTER

For this just need to make that code available for the non-host case.

Thus, remove the TDX_HYPERCALL and reimplement the __tdx_hypercall()
using the TDX_MODULE_CALL.

Extend the TDX_MODULE_CALL to cover "clear shared registers" for
TDG.VP.VMCALL. Introduce a new __tdcall_saved_ret() to replace the
temporary __tdcall_hypercall().

The __tdcall_saved_ret() can also be used for those new TDCALLs which
require more input/output registers than the basic TDCALLs do.

Suggested-by: Peter Zijlstra <[e-mail]>
Signed-off-by: Kai Huang <[e-mail]>
Signed-off-by: Dave Hansen <[e-mail]>
Reviewed-by: Kirill A. Shutemov <[e-mail]>
Acked-by: Peter Zijlstra (Intel) <[e-mail]>
Link: https://lore.kernel.org/all/e68a2473fb6f5bcd78b078cae7510e9d0753b3df.1692096753.git.kai.huang%40intel.com

arch/x86/coco/tdx/tdcall.S

···
 /* SPDX-License-Identifier: GPL-2.0 */
 #include <asm/asm-offsets.h>
 #include <asm/asm.h>
-#include <asm/frame.h>
 
 #include <linux/linkage.h>
 #include <linux/errno.h>
···
 SYM_FUNC_END(__tdcall_ret)
 
 /*
- * TDX_HYPERCALL - Make hypercalls to a TDX VMM using TDVMCALL leaf of TDCALL
- * instruction
+ * __tdcall_saved_ret() - Used by TDX guests to request services from the
+ * TDX module (including VMM services) using TDCALL instruction, with
+ * saving output registers to the 'struct tdx_module_args' used as input.
  *
- * Transforms values in  function call argument struct tdx_module_args @args
- * into the TDCALL register ABI. After TDCALL operation, VMM output is saved
- * back in @args, if \ret is 1.
- *
- * Depends on the caller to pass TDG.VP.VMCALL as the TDCALL leaf, and set
- * @args::rcx to TDVMCALL_EXPOSE_REGS_MASK.
- *
- *-------------------------------------------------------------------------
- * TD VMCALL ABI:
- *-------------------------------------------------------------------------
- *
- * Input Registers:
- *
- * RAX                 - TDCALL instruction leaf number (0 - TDG.VP.VMCALL)
- * RCX                 - BITMAP which controls which part of TD Guest GPR
- *                       is passed as-is to the VMM and back.
- * R10                 - Set 0 to indicate TDCALL follows standard TDX ABI
- *                       specification. Non zero value indicates vendor
- *                       specific ABI.
- * R11                 - VMCALL sub function number
- * RBX, RDX, RDI, RSI  - Used to pass VMCALL sub function specific arguments.
- * R8-R9, R12-R15      - Same as above.
- *
- * Output Registers:
- *
- * RAX                 - TDCALL instruction status (Not related to hypercall
- *                        output).
- * RBX, RDX, RDI, RSI  - Hypercall sub function specific output values.
- * R8-R15              - Same as above.
- *
- */
-.macro TDX_HYPERCALL
-	FRAME_BEGIN
-
-	/* Save callee-saved GPRs as mandated by the x86_64 ABI */
-	push %r15
-	push %r14
-	push %r13
-	push %r12
-	push %rbx
-
-	/* Move Leaf ID to RAX */
-	movq %rdi, %rax
-
-	/* Move bitmap of shared registers to RCX */
-	movq TDX_MODULE_rcx(%rsi), %rcx
-
-	/* Copy hypercall registers from arg struct: */
-	movq TDX_MODULE_r8(%rsi),  %r8
-	movq TDX_MODULE_r9(%rsi),  %r9
-	movq TDX_MODULE_r10(%rsi), %r10
-	movq TDX_MODULE_r11(%rsi), %r11
-	movq TDX_MODULE_r12(%rsi), %r12
-	movq TDX_MODULE_r13(%rsi), %r13
-	movq TDX_MODULE_r14(%rsi), %r14
-	movq TDX_MODULE_r15(%rsi), %r15
-	movq TDX_MODULE_rdi(%rsi), %rdi
-	movq TDX_MODULE_rbx(%rsi), %rbx
-	movq TDX_MODULE_rdx(%rsi), %rdx
-
-	pushq %rsi
-	movq TDX_MODULE_rsi(%rsi), %rsi
-
-	tdcall
-
-	/*
-	 * Restore the pointer of the structure to save output registers.
-	 *
-	 * RCX is used as bitmap of shared registers and doesn't hold any
-	 * value provided by the VMM, thus it can be used as spare to
-	 * restore the structure pointer.
-	 */
-	popq %rcx
-	movq %rsi, TDX_MODULE_rsi(%rcx)
-	movq %rcx, %rsi
-
-	movq %r8,  TDX_MODULE_r8(%rsi)
-	movq %r9,  TDX_MODULE_r9(%rsi)
-	movq %r10, TDX_MODULE_r10(%rsi)
-	movq %r11, TDX_MODULE_r11(%rsi)
-	movq %r12, TDX_MODULE_r12(%rsi)
-	movq %r13, TDX_MODULE_r13(%rsi)
-	movq %r14, TDX_MODULE_r14(%rsi)
-	movq %r15, TDX_MODULE_r15(%rsi)
-	movq %rdi, TDX_MODULE_rdi(%rsi)
-	movq %rbx, TDX_MODULE_rbx(%rsi)
-	movq %rdx, TDX_MODULE_rdx(%rsi)
-
-	/*
-	 * Zero out registers exposed to the VMM to avoid speculative execution
-	 * with VMM-controlled values. This needs to include all registers
-	 * present in TDVMCALL_EXPOSE_REGS_MASK, except RBX, and R12-R15 which
-	 * will be restored.
-	 */
-	xor %r8d,  %r8d
-	xor %r9d,  %r9d
-	xor %r10d, %r10d
-	xor %r11d, %r11d
-	xor %rdi,  %rdi
-	xor %rsi,  %rsi
-	xor %rdx,  %rdx
-
-	/* Restore callee-saved GPRs as mandated by the x86_64 ABI */
-	pop %rbx
-	pop %r12
-	pop %r13
-	pop %r14
-	pop %r15
-
-	FRAME_END
-
-	RET
-.endm
-
-/*
- *
- * __tdcall_hypercall() function ABI:
+ * __tdcall_saved_ret() function ABI:
  *
  * @fn   (RDI)	- TDCALL leaf ID, moved to RAX
  * @args (RSI)	- struct tdx_module_args for input/output
  *
- * @fn and @args::rcx from the caller must be TDG_VP_VMCALL and
- * TDVMCALL_EXPOSE_REGS_MASK respectively.
+ * All registers in @args are used as input/output registers.
  *
  * On successful completion, return the hypercall error code.
  */
-SYM_FUNC_START(__tdcall_hypercall)
-	TDX_HYPERCALL
-SYM_FUNC_END(__tdcall_hypercall)
+SYM_FUNC_START(__tdcall_saved_ret)
+	TDX_MODULE_CALL host=0 ret=1 saved=1
+SYM_FUNC_END(__tdcall_saved_ret)

arch/x86/coco/tdx/tdx-shared.c

···
 	};
 
 	/*
-	 * Failure of __tdcall_hypercall() indicates a failure of the TDVMCALL
+	 * Failure of __tdcall_saved_ret() indicates a failure of the TDVMCALL
 	 * mechanism itself and that something has gone horribly wrong with
 	 * the TDX module.  __tdx_hypercall_failed() never returns.
 	 */
-	if (__tdcall_hypercall(TDG_VP_VMCALL, &margs))
+	if (__tdcall_saved_ret(TDG_VP_VMCALL, &margs))
 		__tdx_hypercall_failed();
 
 	args->r8  = margs.r8;

arch/x86/include/asm/shared/tdx.h

···
 /* Used to communicate with the TDX module */
 u64 __tdcall(u64 fn, struct tdx_module_args *args);
 u64 __tdcall_ret(u64 fn, struct tdx_module_args *args);
+u64 __tdcall_saved_ret(u64 fn, struct tdx_module_args *args);
 
 /*
  * Used in __tdx_hypercall() to pass down and get back registers' values of
···
 };
 
 /* Used to request services from the VMM */
-u64 __tdcall_hypercall(u64 fn, struct tdx_module_args *args);
 u64 __tdx_hypercall(struct tdx_hypercall_args *args);
 
 /*

arch/x86/virt/vmx/tdx/tdxcall.S

···
 	movq %r11, TDX_MODULE_r11(%rsi)
 .endif	/* \ret */
 
-.if \host && \saved && \ret
+.if \saved && \ret
 	/*
-	 * Clear registers shared by guest for VP.ENTER to prevent
-	 * speculative use of guest's values, including those are
+	 * Clear registers shared by guest for VP.VMCALL/VP.ENTER to prevent
+	 * speculative use of guest's/VMM's values, including those are
 	 * restored from the stack.
 	 *
 	 * See arch/x86/kvm/vmx/vmenter.S:
···
 	xorl %r15d, %r15d
 	xorl %ebx,  %ebx
 	xorl %edi,  %edi
-.endif	/* \host && \ret && \host */
+.endif	/* \ret && \host */
 
 .if \host
 .Lout\@: