[PATCH v4 12/12] x86/virt/tdx: Adjust 'struct tdx_module_args' to use x86 "register index" layout

[Kai Huang]

For TDX guest, KVM needs to call __seamcall_saved_ret() to make the
TDH.VP.ENTER SEAMCALL to enter the guest, possibly taking all registers
in 'struct tdx_module_args' as input/output.

KVM caches guest's GPRs in 'kvm_vcpu_arch::regs[]', which follows the
"register index" hardware layout of x86 GPRs.  On the other hand, the
__seamcall_saved_ret() takes the pointer of 'struct tdx_module_args' as
argument, thus there's a mismatch.

KVM could choose to copy input registers from 'vcpu::regs[]' to a
'struct tdx_module_args' and use that as argument to make the SEAMCALL,
but such memory copy isn't desired and should be avoided if possible.

It's not feasible to change KVM's 'vcpu::regs[]' layout due to various
reasons (e.g., emulation code uses decoded register index as array index
to access the register).  Therefore, adjust 'struct tdx_module_args' to
match KVM's 'vcpu::regs[]' layout so that KVM can simply do below:

	__seamcall_saved_ret(TDH_VP_ENTER,
			(struct tdx_module_args *)vcpu->arch.regs);

Note RAX/RSP/RBP are not used by the TDX_MODULE_CALL assembly, but they
are necessary in order to match the layout of 'struct tdx_module_args'
to KVM's 'vcpu::regs[]'.  Thus add them to the structure, but name them
as *_unused.  Also don't include them to asm-offset.c so that any misuse
of them in the assembly would result in build error.

Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Sean Christopherson <seanjc@xxxxxxxxxx>
Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Kai Huang <kai.huang@xxxxxxxxx>
Tested-by: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
---
 arch/x86/include/asm/shared/tdx.h | 19 +++++++++++++------
 arch/x86/kernel/asm-offsets.c     |  6 +++---
 2 files changed, 16 insertions(+), 9 deletions(-)

diff --git a/arch/x86/include/asm/shared/tdx.h b/arch/x86/include/asm/shared/tdx.h
index 74fc466dfdcd..8d1427562c63 100644
--- a/arch/x86/include/asm/shared/tdx.h
+++ b/arch/x86/include/asm/shared/tdx.h
@@ -58,24 +58,31 @@
  * Used in __tdcall*() to gather the input/output registers' values of the
  * TDCALL instruction when requesting services from the TDX module. This is a
  * software only structure and not part of the TDX module/VMM ABI
+ *
+ * Note those *_unused are not used by the TDX_MODULE_CALL assembly.
+ * The layout of this structure also matches KVM's kvm_vcpu_arch::regs[]
+ * layout, which follows the "register index" order of x86 GPRs.  KVM
+ * then can simply type cast kvm_vcpu_arch::regs[] to this structure to
+ * avoid the extra memory copy between two structures when making
+ * TDH.VP.ENTER SEAMCALL.
  */
 struct tdx_module_args {
-	/* callee-clobbered */
+	u64 rax_unused;
 	u64 rcx;
 	u64 rdx;
+	u64 rbx;
+	u64 rsp_unused;
+	u64 rbp_unused;
+	u64 rsi;
+	u64 rdi;
 	u64 r8;
 	u64 r9;
-	/* extra callee-clobbered */
 	u64 r10;
 	u64 r11;
-	/* callee-saved + rdi/rsi */
 	u64 r12;
 	u64 r13;
 	u64 r14;
 	u64 r15;
-	u64 rbx;
-	u64 rdi;
-	u64 rsi;
 };
 
 /* Used to communicate with the TDX module */
diff --git a/arch/x86/kernel/asm-offsets.c b/arch/x86/kernel/asm-offsets.c
index 6913b372ccf7..e4ad822d3acd 100644
--- a/arch/x86/kernel/asm-offsets.c
+++ b/arch/x86/kernel/asm-offsets.c
@@ -70,6 +70,9 @@ static void __used common(void)
 	BLANK();
 	OFFSET(TDX_MODULE_rcx, tdx_module_args, rcx);
 	OFFSET(TDX_MODULE_rdx, tdx_module_args, rdx);
+	OFFSET(TDX_MODULE_rbx, tdx_module_args, rbx);
+	OFFSET(TDX_MODULE_rsi, tdx_module_args, rsi);
+	OFFSET(TDX_MODULE_rdi, tdx_module_args, rdi);
 	OFFSET(TDX_MODULE_r8,  tdx_module_args, r8);
 	OFFSET(TDX_MODULE_r9,  tdx_module_args, r9);
 	OFFSET(TDX_MODULE_r10, tdx_module_args, r10);
@@ -78,9 +81,6 @@ static void __used common(void)
 	OFFSET(TDX_MODULE_r13, tdx_module_args, r13);
 	OFFSET(TDX_MODULE_r14, tdx_module_args, r14);
 	OFFSET(TDX_MODULE_r15, tdx_module_args, r15);
-	OFFSET(TDX_MODULE_rbx, tdx_module_args, rbx);
-	OFFSET(TDX_MODULE_rdi, tdx_module_args, rdi);
-	OFFSET(TDX_MODULE_rsi, tdx_module_args, rsi);
 
 	BLANK();
 	OFFSET(BP_scratch, boot_params, scratch);
-- 
2.41.0