Re: [PATCH v11 05/20] x86/virt/tdx: Add SEAMCALL infrastructure

[David Hildenbrand]

On 04.06.23 16:27, Kai Huang wrote:
> TDX introduces a new CPU mode: Secure Arbitration Mode (SEAM).  This
> mode runs only the TDX module itself or other code to load the TDX
> module.
>
> The host kernel communicates with SEAM software via a new SEAMCALL
> instruction.  This is conceptually similar to a guest->host hypercall,
> except it is made from the host to SEAM software instead.  The TDX
> module establishes a new SEAMCALL ABI which allows the host to
> initialize the module and to manage VMs.
>
> Add infrastructure to make SEAMCALLs.  The SEAMCALL ABI is very similar
> to the TDCALL ABI and leverages much TDCALL infrastructure.
>
> SEAMCALL instruction causes #GP when TDX isn't BIOS enabled, and #UD
> when CPU is not in VMX operation.  Currently, only KVM code mocks with
> VMX enabling, and KVM is the only user of TDX.  This implementation
> chooses to make KVM itself responsible for enabling VMX before using
> TDX and let the rest of the kernel stay blissfully unaware of VMX.
>
> The current TDX_MODULE_CALL macro handles neither #GP nor #UD.  The
> kernel would hit Oops if SEAMCALL were mistakenly made w/o enabling VMX
> first.  Architecturally, there is no CPU flag to check whether the CPU
> is in VMX operation.  Also, if a BIOS were buggy, it could still report
> valid TDX private KeyIDs when TDX actually couldn't be enabled.
>
> Extend the TDX_MODULE_CALL macro to handle #UD and #GP to return error
> codes.  Introduce two new TDX error codes for them respectively so the
> caller can distinguish.
>
> Also add a wrapper function of SEAMCALL to convert SEAMCALL error code
> to the kernel error code, and print out SEAMCALL error code to help the
> user to understand what went wrong.
>
> Signed-off-by: Kai Huang <kai.huang@xxxxxxxxx>
> ---

I agree with Dave that a buggy bios is not a good motivation for this 
patch. The real strength of this infrastructure IMHO is central error 
handling and expressive error messages. Maybe it makes some corner cases 
(reboot -f) easier to handle. That would make a better justification 
than buggy bios -- and should be spelled out in the patch description.

[...]


> +/*
> + * Wrapper of __seamcall() to convert SEAMCALL leaf function error code
> + * to kernel error code.  @seamcall_ret and @out contain the SEAMCALL
> + * leaf function return code and the additional output respectively if
> + * not NULL.
> + */
> +static int __always_unused seamcall(u64 fn, u64 rcx, u64 rdx, u64 r8, u64 r9,
> +				    u64 *seamcall_ret,
> +				    struct tdx_module_output *out)
> +{
> +	int cpu, ret = 0;
> +	u64 sret;
> +
> +	/* Need a stable CPU id for printing error message */
> +	cpu = get_cpu();
> +
> +	sret = __seamcall(fn, rcx, rdx, r8, r9, out);
> +


Why not

cpu = get_cpu();
sret = __seamcall(fn, rcx, rdx, r8, r9, out);
put_cpu();


> +	/* Save SEAMCALL return code if the caller wants it */
> +	if (seamcall_ret)
> +		*seamcall_ret = sret;
> +
> +	/* SEAMCALL was successful */
> +	if (!sret)
> +		goto out;

Why not move that into the switch statement below to avoid th goto?
If you do the put_cpu() early, you can avoid "ret" as well.

switch (sret) {
case 0:
	/* SEAMCALL was successful */
	return 0;
case TDX_SEAMCALL_GP:
	pr_err_once("[firmware bug]: TDX is not enabled by BIOS.\n");
	return -ENODEV;
...
}

[...]

> +
>   static int __init record_keyid_partitioning(u32 *tdx_keyid_start,
>   					    u32 *nr_tdx_keyids)
>   {
> diff --git a/arch/x86/virt/vmx/tdx/tdx.h b/arch/x86/virt/vmx/tdx/tdx.h
> new file mode 100644
> index 000000000000..48ad1a1ba737
> --- /dev/null
> +++ b/arch/x86/virt/vmx/tdx/tdx.h
> @@ -0,0 +1,10 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _X86_VIRT_TDX_H
> +#define _X86_VIRT_TDX_H
> +
> +#include <linux/types.h>
> +
> +struct tdx_module_output;
> +u64 __seamcall(u64 fn, u64 rcx, u64 rdx, u64 r8, u64 r9,
> +	       struct tdx_module_output *out);
> +#endif
> diff --git a/arch/x86/virt/vmx/tdx/tdxcall.S b/arch/x86/virt/vmx/tdx/tdxcall.S
> index 49a54356ae99..757b0c34be10 100644
> --- a/arch/x86/virt/vmx/tdx/tdxcall.S
> +++ b/arch/x86/virt/vmx/tdx/tdxcall.S
> @@ -1,6 +1,7 @@
>   /* SPDX-License-Identifier: GPL-2.0 */
>   #include <asm/asm-offsets.h>
>   #include <asm/tdx.h>
> +#include <asm/asm.h>
>   
>   /*
>    * TDCALL and SEAMCALL are supported in Binutils >= 2.36.
> @@ -45,6 +46,7 @@
>   	/* Leave input param 2 in RDX */
>   
>   	.if \host
> +1:
>   	seamcall
>   	/*
>   	 * SEAMCALL instruction is essentially a VMExit from VMX root
> @@ -57,10 +59,23 @@
>   	 * This value will never be used as actual SEAMCALL error code as
>   	 * it is from the Reserved status code class.
>   	 */
> -	jnc .Lno_vmfailinvalid
> +	jnc .Lseamcall_out
>   	mov $TDX_SEAMCALL_VMFAILINVALID, %rax
> -.Lno_vmfailinvalid:
> +	jmp .Lseamcall_out
> +2:
> +	/*
> +	 * SEAMCALL caused #GP or #UD.  By reaching here %eax contains
> +	 * the trap number.  Convert the trap number to the TDX error
> +	 * code by setting TDX_SW_ERROR to the high 32-bits of %rax.
> +	 *
> +	 * Note cannot OR TDX_SW_ERROR directly to %rax as OR instruction
> +	 * only accepts 32-bit immediate at most.

Not sure if that comment is really helpful here. It's a common pattern 
for large immediates, no?

> +	 */
> +	mov $TDX_SW_ERROR, %r12
> +	orq %r12, %rax
>   
> +	_ASM_EXTABLE_FAULT(1b, 2b)
> +.Lseamcall_out:
>   	.else
>   	tdcall
>   	.endif

--
Cheers,

David / dhildenb