Re: [PATCH v2 00/11] Unify TDCALL/SEAMCALL and TDVMCALL assembly

[Huang, Kai]

On Fri, 2023-07-21 at 10:06 +0200, Peter Zijlstra wrote:
> On Fri, Jul 21, 2023 at 12:18:23AM +0000, Huang, Kai wrote:
> 
> > Unfortunately I don't think it's feasible.  Sean pointed out that
> > kvm_vcpu_arch::regs[] do follow the "register index" hardware layout in x86 (for
> > which I missed sorry), so we cannot re-order KVM part.  
> > 
> > And unfortunately RBP (5) is in middle of those registers:
> > 
> > 	0 = RAX
> > 	1 = RCX
> > 	2 = RDX
> > 	3 = RBX
> > 	4 = RSP
> > 	5 = RBP
> > 	6 = RSI
> > 	7 = RDI
> > 	8–15 represent R8–R15, respectively...
> > 
> > Thus unless we add RBP to 'struct tdx_module_args', it's impossible to re-order
> > the structure to match KVM's layout.
> 
> Adding RBP to the struct shouldn't be a problem, we'll just not use it.
> Same as RSP, nobody sane would expect that to be used. If you worry
> about it you can give them funny names like:
> 
> struct tdx_module_args {
> 	unsigned long ax;
> 	unsigned long cx;
> 	unsigned long dx;
> 	unsigned long bx;
> 	unsigned long __sp_unused;
> 	unsigned long __bp_unused;
> 	unsigned long si;
> 	unsigned long di;
> 	...
> };
> 
> I mean, at this point the whole thing is just 128 bytes of data anyway.

Sure I can do.

How about adding an additional patch on top of this series?  

For instance, below patch (compile only):

From c63d01bf91fe23f1e2d2e085644326bdee114d49 Mon Sep 17 00:00:00 2001
From: Kai Huang <kai.huang@xxxxxxxxx>
Date: Fri, 21 Jul 2023 22:06:31 +1200
Subject: [PATCH] x86/tdx: Add __seamcall_saved_ret() for TDH.VP.ENTER

For TDX guest, KVM uses TDH.VP.ENTER SEAMCALL leaf to enter the guest.
When TDH.VP.ENTER returns to KVM due to TDG.VP.VMCALL from the TDX
guest, all RCX/RDX/RBX/RDI/RSI/R8-R15 are valid output registers, and
the following TDH.VP.ENTER also uses all those registers as input.

Introduce __seamcall_saved_ret(), which uses all above registers as
both input/output, for KVM to support TDH.VP.ENTER.

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

One option is KVM to copy input registers from 'vcpu::regs[]' to a
'struct tdx_module_args' on the stack and use that 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[]' 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 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.

Signed-off-by: Kai Huang <kai.huang@xxxxxxxxx>
---
 arch/x86/include/asm/shared/tdx.h | 19 +++++++++++++------
 arch/x86/virt/vmx/tdx/seamcall.S  | 19 +++++++++++++++++++
 2 files changed, 32 insertions(+), 6 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/virt/vmx/tdx/seamcall.S b/arch/x86/virt/vmx/tdx/seamcall.S
index b32934837f16..f251ebadb014 100644
--- a/arch/x86/virt/vmx/tdx/seamcall.S
+++ b/arch/x86/virt/vmx/tdx/seamcall.S
@@ -40,3 +40,22 @@ SYM_FUNC_END(__seamcall)
 SYM_FUNC_START(__seamcall_ret)
        TDX_MODULE_CALL host=1 ret=1
 SYM_FUNC_END(__seamcall_ret)
+
+/*
+ * __seamcall_saved_ret() - Host-side interface functions to SEAM software
+ * (the P-SEAMLDR or the TDX module), with saving output registers to the
+ * 'struct tdx_module_args' used as input.
+ *
+ * __seamcall_saved_ret() function ABI:
+ *
+ * @fn   (RDI)  - SEAMCALL Leaf number, moved to RAX
+ * @args (RSI)  - struct tdx_module_args for input and output
+ *
+ * All registers in @args are used as input/output registers.
+ *
+ * Return (via RAX) TDX_SEAMCALL_VMFAILINVALID if the SEAMCALL itself
+ * fails, or the completion status of the SEAMCALL leaf function.
+ */
+SYM_FUNC_START(__seamcall_ret)
+       TDX_MODULE_CALL host=1 ret=1 saved=1
+SYM_FUNC_END(__seamcall_ret)
-- 
2.41.0