Re: [PATCH v2 4/6] x86/tdx: Expand __tdx_hypercall() to handle more arguments

[Sathyanarayanan Kuppuswamy <sathyanarayanan.kuppuswamy@linux.intel.com>]

On 12/6/22 4:33 PM, Dexuan Cui wrote:
> From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
> 
> So far __tdx_hypercall() only handles six arguments for VMCALL.
> Expanding it to six more register would allow to cover more use-cases.
> 
> Using RDI and RSI as VMCALL arguments requires more register shuffling.
> RAX is used to hold tdx_hypercall_args pointer and RBP stores flags.
> 
> While there, fix typo in the comment on panic branch.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Reviewed-by: Dexuan Cui <decui@microsoft.com>
> Tested-by: Dexuan Cui <decui@microsoft.com>
> ---
> 
> This patch is from Kirill. I'm posting the patch on behalf him:
> https://lwn.net/ml/linux-kernel/20221202214741.7vfmqgvgubxqffen@box.shutemov.name/
> 
> This is actually v1, but let's use v2 in the Subject to be consistent
> with the Subjects of the other patches.
> 
>  arch/x86/coco/tdx/tdcall.S        | 82 ++++++++++++++++++++++---------
>  arch/x86/include/asm/shared/tdx.h |  6 +++
>  arch/x86/kernel/asm-offsets.c     |  6 +++
>  3 files changed, 70 insertions(+), 24 deletions(-)
> 
> diff --git a/arch/x86/coco/tdx/tdcall.S b/arch/x86/coco/tdx/tdcall.S
> index f9eb1134f22d..64e57739dc9d 100644
> --- a/arch/x86/coco/tdx/tdcall.S
> +++ b/arch/x86/coco/tdx/tdcall.S
> @@ -13,6 +13,12 @@
>  /*
>   * Bitmasks of exposed registers (with VMM).
>   */
> +#define TDX_RDX		BIT(2)
> +#define TDX_RBX		BIT(3)
> +#define TDX_RSI		BIT(6)
> +#define TDX_RDI		BIT(7)
> +#define TDX_R8		BIT(8)
> +#define TDX_R9		BIT(9)
>  #define TDX_R10		BIT(10)
>  #define TDX_R11		BIT(11)
>  #define TDX_R12		BIT(12)
> @@ -27,9 +33,9 @@
>   * details can be found in TDX GHCI specification, section
>   * titled "TDCALL [TDG.VP.VMCALL] leaf".
>   */
> -#define TDVMCALL_EXPOSE_REGS_MASK	( TDX_R10 | TDX_R11 | \
> -					  TDX_R12 | TDX_R13 | \
> -					  TDX_R14 | TDX_R15 )
> +#define TDVMCALL_EXPOSE_REGS_MASK	\
> +	( TDX_RDX | TDX_RBX | TDX_RSI | TDX_RDI | TDX_R8  | TDX_R9  | \
> +	  TDX_R10 | TDX_R11 | TDX_R12 | TDX_R13 | TDX_R14 | TDX_R15 )
>  

You seem to have expanded the list to include all VMCALL supported
registers except RBP. Why not include it as well? That way, it will be
a complete support.

>  /*
>   * __tdx_module_call()  - Used by TDX guests to request services from
> @@ -124,19 +130,32 @@ SYM_FUNC_START(__tdx_hypercall)
>  	push %r14
>  	push %r13
>  	push %r12
> +	push %rbx
> +	push %rbp
> +
> +	movq %rdi, %rax
> +	movq %rsi, %rbp
> +
> +	/* Copy hypercall registers from arg struct: */
> +	movq TDX_HYPERCALL_r8(%rax),  %r8
> +	movq TDX_HYPERCALL_r9(%rax),  %r9
> +	movq TDX_HYPERCALL_r10(%rax), %r10
> +	movq TDX_HYPERCALL_r11(%rax), %r11
> +	movq TDX_HYPERCALL_r12(%rax), %r12
> +	movq TDX_HYPERCALL_r13(%rax), %r13
> +	movq TDX_HYPERCALL_r14(%rax), %r14
> +	movq TDX_HYPERCALL_r15(%rax), %r15
> +	movq TDX_HYPERCALL_rdi(%rax), %rdi
> +	movq TDX_HYPERCALL_rsi(%rax), %rsi
> +	movq TDX_HYPERCALL_rbx(%rax), %rbx
> +	movq TDX_HYPERCALL_rdx(%rax), %rdx
> +
> +	push %rax
>  
>  	/* Mangle function call ABI into TDCALL ABI: */
>  	/* Set TDCALL leaf ID (TDVMCALL (0)) in RAX */
>  	xor %eax, %eax
>  
> -	/* Copy hypercall registers from arg struct: */
> -	movq TDX_HYPERCALL_r10(%rdi), %r10
> -	movq TDX_HYPERCALL_r11(%rdi), %r11
> -	movq TDX_HYPERCALL_r12(%rdi), %r12
> -	movq TDX_HYPERCALL_r13(%rdi), %r13
> -	movq TDX_HYPERCALL_r14(%rdi), %r14
> -	movq TDX_HYPERCALL_r15(%rdi), %r15
> -
>  	movl $TDVMCALL_EXPOSE_REGS_MASK, %ecx
>  
>  	/*
> @@ -148,14 +167,14 @@ SYM_FUNC_START(__tdx_hypercall)
>  	 * HLT operation indefinitely. Since this is the not the desired
>  	 * result, conditionally call STI before TDCALL.
>  	 */
> -	testq $TDX_HCALL_ISSUE_STI, %rsi
> +	testq $TDX_HCALL_ISSUE_STI, %rbp
>  	jz .Lskip_sti
>  	sti
>  .Lskip_sti:
>  	tdcall
>  
>  	/*
> -	 * RAX==0 indicates a failure of the TDVMCALL mechanism itself and that
> +	 * RAX!=0 indicates a failure of the TDVMCALL mechanism itself and that
>  	 * something has gone horribly wrong with the TDX module.
>  	 *
>  	 * The return status of the hypercall operation is in a separate
> @@ -165,30 +184,45 @@ SYM_FUNC_START(__tdx_hypercall)
>  	testq %rax, %rax
>  	jne .Lpanic
>  
> -	/* TDVMCALL leaf return code is in R10 */
> -	movq %r10, %rax
> +	pop %rax
>  
>  	/* Copy hypercall result registers to arg struct if needed */
> -	testq $TDX_HCALL_HAS_OUTPUT, %rsi
> +	testq $TDX_HCALL_HAS_OUTPUT, %rbp
>  	jz .Lout
>  
> -	movq %r10, TDX_HYPERCALL_r10(%rdi)
> -	movq %r11, TDX_HYPERCALL_r11(%rdi)
> -	movq %r12, TDX_HYPERCALL_r12(%rdi)
> -	movq %r13, TDX_HYPERCALL_r13(%rdi)
> -	movq %r14, TDX_HYPERCALL_r14(%rdi)
> -	movq %r15, TDX_HYPERCALL_r15(%rdi)
> +	movq %r8,  TDX_HYPERCALL_r8(%rax)
> +	movq %r9,  TDX_HYPERCALL_r9(%rax)
> +	movq %r10, TDX_HYPERCALL_r10(%rax)
> +	movq %r11, TDX_HYPERCALL_r11(%rax)
> +	movq %r12, TDX_HYPERCALL_r12(%rax)
> +	movq %r13, TDX_HYPERCALL_r13(%rax)
> +	movq %r14, TDX_HYPERCALL_r14(%rax)
> +	movq %r15, TDX_HYPERCALL_r15(%rax)
> +	movq %rdi, TDX_HYPERCALL_rdi(%rax)
> +	movq %rsi, TDX_HYPERCALL_rsi(%rax)
> +	movq %rbx, TDX_HYPERCALL_rbx(%rax)
> +	movq %rdx, TDX_HYPERCALL_rdx(%rax)
>  .Lout:
> +	/* TDVMCALL leaf return code is in R10 */
> +	movq %r10, %rax
> +
>  	/*
>  	 * 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 R12-R15). R12-R15
> -	 * context will be restored.
> +	 * 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 %rbp
> +	pop %rbx
>  	pop %r12
>  	pop %r13
>  	pop %r14
> diff --git a/arch/x86/include/asm/shared/tdx.h b/arch/x86/include/asm/shared/tdx.h
> index e53f26228fbb..8068faa52de1 100644
> --- a/arch/x86/include/asm/shared/tdx.h
> +++ b/arch/x86/include/asm/shared/tdx.h
> @@ -22,12 +22,18 @@
>   * This is a software only structure and not part of the TDX module/VMM ABI.
>   */
>  struct tdx_hypercall_args {
> +	u64 r8;
> +	u64 r9;
>  	u64 r10;
>  	u64 r11;
>  	u64 r12;
>  	u64 r13;
>  	u64 r14;
>  	u64 r15;
> +	u64 rdi;
> +	u64 rsi;
> +	u64 rbx;
> +	u64 rdx;
>  };
>  
>  /* Used to request services from the VMM */
> diff --git a/arch/x86/kernel/asm-offsets.c b/arch/x86/kernel/asm-offsets.c
> index 437308004ef2..9f09947495e2 100644
> --- a/arch/x86/kernel/asm-offsets.c
> +++ b/arch/x86/kernel/asm-offsets.c
> @@ -75,12 +75,18 @@ static void __used common(void)
>  	OFFSET(TDX_MODULE_r11, tdx_module_output, r11);
>  
>  	BLANK();
> +	OFFSET(TDX_HYPERCALL_r8,  tdx_hypercall_args, r8);
> +	OFFSET(TDX_HYPERCALL_r9,  tdx_hypercall_args, r9);
>  	OFFSET(TDX_HYPERCALL_r10, tdx_hypercall_args, r10);
>  	OFFSET(TDX_HYPERCALL_r11, tdx_hypercall_args, r11);
>  	OFFSET(TDX_HYPERCALL_r12, tdx_hypercall_args, r12);
>  	OFFSET(TDX_HYPERCALL_r13, tdx_hypercall_args, r13);
>  	OFFSET(TDX_HYPERCALL_r14, tdx_hypercall_args, r14);
>  	OFFSET(TDX_HYPERCALL_r15, tdx_hypercall_args, r15);
> +	OFFSET(TDX_HYPERCALL_rdi, tdx_hypercall_args, rdi);
> +	OFFSET(TDX_HYPERCALL_rsi, tdx_hypercall_args, rsi);
> +	OFFSET(TDX_HYPERCALL_rbx, tdx_hypercall_args, rbx);
> +	OFFSET(TDX_HYPERCALL_rdx, tdx_hypercall_args, rdx);
>  
>  	BLANK();
>  	OFFSET(BP_scratch, boot_params, scratch);

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer