x86/virt/tdx: Add SEAMCALL wrapper to enter/exit TDX guest

Intel TDX protects guest VM's from malicious host and certain physical
attacks.  TDX introduces a new operation mode, Secure Arbitration Mode
(SEAM) to isolate and protect guest VM's.  A TDX guest VM runs in SEAM and,
unlike VMX, direct control and interaction with the guest by the host VMM
is not possible.  Instead, Intel TDX Module, which also runs in SEAM,
provides a SEAMCALL API.

The SEAMCALL that provides the ability to enter a guest is TDH.VP.ENTER.
The TDX Module processes TDH.VP.ENTER, and enters the guest via VMX
VMLAUNCH/VMRESUME instructions.  When a guest VM-exit requires host VMM
interaction, the TDH.VP.ENTER SEAMCALL returns to the host VMM (KVM).

Add tdh_vp_enter() to wrap the SEAMCALL invocation of TDH.VP.ENTER.

TDH.VP.ENTER is different from other SEAMCALLS in several ways:
- it may take some time to return as the guest executes
- it uses more arguments
- after it returns some host state may need to be restored

TDH.VP.ENTER arguments are passed through General Purpose Registers (GPRs).
For the special case of the TD guest invoking TDG.VP.VMCALL, nearly any GPR
can be used, as well as XMM0 to XMM15. Notably, RBP is not used, and Linux
mandates the TDX Module feature NO_RBP_MOD, which is enforced elsewhere.
Additionally, XMM registers are not required for the existing Guest
Hypervisor Communication Interface and are handled by existing KVM code
should they be modified by the guest.

There are 2 input formats and 5 output formats for TDH.VP.ENTER arguments.
Input #1 : Initial entry or following a previous async. TD Exit
Input #2 : Following a previous TDCALL(TDG.VP.VMCALL)
Output #1 : On Error (No TD Entry)
Output #2 : Async. Exits with a VMX Architectural Exit Reason
Output #3 : Async. Exits with a non-VMX TD Exit Status
Output #4 : Async. Exits with Cross-TD Exit Details
Output #5 : On TDCALL(TDG.VP.VMCALL)

Currently, to keep things simple, the wrapper function does not attempt
to support different formats, and just passes all the GPRs that could be
used.  The GPR values are held by KVM in the area set aside for guest
GPRs.  KVM code uses the guest GPR area (vcpu->arch.regs[]) to set up for
or process results of tdh_vp_enter().

Therefore changing tdh_vp_enter() to use more complex argument formats
would also alter the way KVM code interacts with tdh_vp_enter().

Signed-off-by: Kai Huang <[email protected]>
---

-Kai:
 - This is not nice, but for now have no clue on improvement.

arch/x86/include/asm/tdx.h

@@ -143,6 +143,7 @@ struct tdx_vp {
 };
 
 u64 tdh_mng_addcx(struct tdx_td *td, struct page *tdcs_page);
+u64 tdh_vp_enter(u64 tdvpr, struct tdx_module_args *args);
 u64 tdh_mem_page_add(u64 tdr, u64 gpa, u64 hpa, u64 source, u64 *rcx, u64 *rdx);
 u64 tdh_mem_sept_add(u64 tdr, u64 gpa, u64 level, u64 hpa, u64 *rcx, u64 *rdx);
 u64 tdh_vp_addcx(struct tdx_vp *vp, struct page *tdcx_page);

arch/x86/virt/vmx/tdx/tdx.c

@@ -1622,6 +1622,14 @@ static inline u64 tdx_seamcall_sept(u64 op, struct tdx_module_args *in)
 	return ret;
 }
 
+u64 tdh_vp_enter(u64 tdvpr, struct tdx_module_args *args)
+{
+	args->rcx = tdvpr;
+
+	return __seamcall_saved_ret(TDH_VP_ENTER, args);
+}
+EXPORT_SYMBOL_GPL(tdh_vp_enter);
+
 u64 tdh_mng_addcx(struct tdx_td *td, struct page *tdcs_page)
 {
 	struct tdx_module_args args = {

arch/x86/virt/vmx/tdx/tdx.h

@@ -17,6 +17,7 @@
 /*
  * TDX module SEAMCALL leaf functions
  */
+#define TDH_VP_ENTER			0
 #define TDH_MNG_ADDCX			1
 #define TDH_MEM_PAGE_ADD		2
 #define TDH_MEM_SEPT_ADD		3