Re: [RFC PATCH 02/15] x86/tdx: Add TDCM hypercall wrapper for TDX Connect

[Xiaoyao Li]

On 9/24/2026 12:10 PM, Zhenzhong Duan wrote:
> TDX Connect is a TDX feature that enables secure device assignment,
> allowing a TDX guest to directly interact with TEE-IO capable devices.
> 
> The TEE-IO Device Control and Management (TDCM) hypercall
> (TDG.VP.VMCALL<TDCM>, defined in TDX GHCI v2.0 specification) is the
> primary interface through which a TDX guest issues device management
> commands to the host. The host processes these commands and returns
> responses via a shared memory buffer.

Could we add some clarification of why this guest-host hypercall is necessary?
i.e., explain that some device management operations can only be performed by
the host VMM.

> Abstract the underlying leaf call and pass the shared buffer physical
> address as a decrypted mapping. Add a runtime page alignment check
> for the buffer to enforce correct usage.

This doesn't match the implementation. The code only WARNs instead of enforcing
correct usage.

> Place the implementation in a separate tdx_connect.c file to keep TDX
> Connect specific code isolated from core tdx.c.
> 
> CONFIG_TDX_CONNECT_GUEST referenced in the header guard will be
> introduced in a later patch in this series.

I'm not sure why the CONFIG cannot be introduced in this patch...

> Signed-off-by: Zhenzhong Duan <zhenzhong.duan@xxxxxxxxx>
> ---
>  arch/x86/coco/tdx/Makefile        |  2 ++
>  arch/x86/coco/tdx/tdx_connect.c   | 32 +++++++++++++++++++++++++++++++
>  arch/x86/include/asm/shared/tdx.h |  1 +
>  arch/x86/include/asm/tdx.h        |  4 ++++
>  4 files changed, 39 insertions(+)
>  create mode 100644 arch/x86/coco/tdx/tdx_connect.c
> 
> diff --git a/arch/x86/coco/tdx/Makefile b/arch/x86/coco/tdx/Makefile
> index b3c47d3700e2..2ab2dbbdd3d2 100644
> --- a/arch/x86/coco/tdx/Makefile
> +++ b/arch/x86/coco/tdx/Makefile
> @@ -1,3 +1,5 @@
>  # SPDX-License-Identifier: GPL-2.0
>  
>  obj-y += debug.o tdcall.o tdx.o tdx-shared.o
> +
> +obj-$(CONFIG_TDX_CONNECT_GUEST) += tdx_connect.o
> diff --git a/arch/x86/coco/tdx/tdx_connect.c b/arch/x86/coco/tdx/tdx_connect.c
> new file mode 100644
> index 000000000000..0c6ca650771a
> --- /dev/null
> +++ b/arch/x86/coco/tdx/tdx_connect.c
> @@ -0,0 +1,32 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/* Copyright (C) 2026 Intel Corporation */
> +
> +#undef pr_fmt
> +#define pr_fmt(fmt)     "tdx_connect: " fmt

I think this is file is only intended to hold guest stuff. Thus maybe call out
"guest" as

  #define pr_fmt(fmt)     "tdx_connect guest: " fmt

though arch/x86/coco/tdx/tdx.c doesn't print "guest":

  #define pr_fmt(fmt)     "tdx: " fmt

So this is just my preference, feel free to ignore it.

> +
> +#include <linux/io.h>
> +#include <asm/tdx.h>
> +#include <linux/mm.h>
> +
> +/*
> + * tdx_hcall_tdcm - Send a TDCM command to the host via TDG.VP.VMCALL<TDCM>.
> + *
> + * @devid:  Device identifier of the target TEE-IO device.
> + * @buf:    Shared, directly mapped buffer containing the TDCM command on
> + *          input and the host response on output.
> + * @size:   Size of @buf in bytes.
> + * @vector: Event notification interrupt vector, or 0 for synchronous operation.
> + *

"0 for synchronous operation" isn't correct.

vector 0 means "no vector. TD polls the status for completion". If the command
doesn't finish when TDG.VP.VMCALL<TDCM> returns. It's not a synchronous operation.

> + * The buffer physical address is passed to the host with decrypted/shared
> + * mark.
> + *
> + * Returns 0 on success or a TDX VMCALL status code on failure. See
> + * TDG.VP.VMCALL<TDCM> in the TDX GHCI v2.0 specification for status codes.
> + */
> +u64 tdx_hcall_tdcm(u16 devid, void *buf, size_t size, u8 vector)
> +{
> +	WARN_ON_ONCE(!PAGE_ALIGNED(buf) || !PAGE_ALIGNED(size));
> +
> +	return _tdx_hypercall(TDVMCALL_TDCM, devid, cc_mkdec(virt_to_phys(buf)), size, vector);
> +}
> +EXPORT_SYMBOL_FOR_MODULES(tdx_hcall_tdcm, "tdx-guest");
> diff --git a/arch/x86/include/asm/shared/tdx.h b/arch/x86/include/asm/shared/tdx.h
> index f20e91d7ac35..f46a3171a512 100644
> --- a/arch/x86/include/asm/shared/tdx.h
> +++ b/arch/x86/include/asm/shared/tdx.h
> @@ -74,6 +74,7 @@
>  #define TDVMCALL_GET_QUOTE		0x10002
>  #define TDVMCALL_REPORT_FATAL_ERROR	0x10003
>  #define TDVMCALL_SETUP_EVENT_NOTIFY_INTERRUPT 0x10004ULL
> +#define TDVMCALL_TDCM			0x10007
>  
>  /*
>   * TDG.VP.VMCALL Status Codes (returned in R10)
> diff --git a/arch/x86/include/asm/tdx.h b/arch/x86/include/asm/tdx.h
> index 884bb8067521..df4496ef8a6a 100644
> --- a/arch/x86/include/asm/tdx.h
> +++ b/arch/x86/include/asm/tdx.h
> @@ -85,6 +85,10 @@ int tdx_mcall_extend_rtmr(u8 index, u8 *data);
>  
>  u64 tdx_hcall_get_quote(u8 *buf, size_t size);
>  
> +#ifdef CONFIG_TDX_CONNECT_GUEST
> +u64 tdx_hcall_tdcm(u16 devid, void *buf, size_t size, u8 vector);
> +#endif
> +
>  void __init tdx_dump_attributes(u64 td_attr);
>  void __init tdx_dump_td_ctls(u64 td_ctls);
>