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

From: Huang, Kai
Date: Fri Jul 21 2023 - 07:44:55 EST

On Fri, 2023-07-21 at 11:02 +0000, Huang, Kai wrote:
> 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)

Sorry should be __seamcall_saved_ret.

> --
> 2.41.0
>
>

Sorry missing the declaration of __seamcall_saved_ret():

--- a/arch/x86/include/asm/tdx.h
+++ b/arch/x86/include/asm/tdx.h
@@ -80,6 +80,7 @@ static inline long tdx_kvm_hypercall(unsigned int nr, unsigned
long p1,
#ifdef CONFIG_INTEL_TDX_HOST
u64 __seamcall(u64 fn, struct tdx_module_args *args);
u64 __seamcall_ret(u64 fn, struct tdx_module_args *args);
+u64 __seamcall_saved_ret(u64 fn, struct tdx_module_args *args);

And perhaps we should adjust the order of registers in asm-offset.c to match the
change to the 'struct tdx_module_args'?

--- 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);