Re: [PATCH v14 20/22] KVM: selftests: Implement MMIO WRITE for the TDX VM

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 7/23/2026 7:13 AM, Lisa Wang wrote:
> From: Erdem Aktas <erdemaktas@google.com>
> 
> Implement the tdx_mmio_write() to allow TDX VMs to request MMIO
> emulation.
> 
> Follow the Intel Guest-Hypervisor Communication Interface (GHCI) spec
> to the minimum extent that a spec-abiding TDX module will pass the
> request to KVM. 

I don't get what it wants to say with this.

> Skip implementing the #VE handler as described in the
> GHCI spec to reduce selftests dependency.
> 
> To perform emulated I/O, VMs use the TDG.VP.VMCALL instruction to

To perform emulated MMIO?

> request MMIO.
> 
> Signed-off-by: Erdem Aktas <erdemaktas@google.com>
> Co-developed-by: Sagi Shahar <sagis@google.com>
> Signed-off-by: Sagi Shahar <sagis@google.com>
> Co-developed-by: Lisa Wang <wyihan@google.com>
> Signed-off-by: Lisa Wang <wyihan@google.com>
> ---
>   tools/testing/selftests/kvm/Makefile.kvm          |  1 +
>   tools/testing/selftests/kvm/include/x86/tdx/tdx.h | 17 +++++++++++
>   tools/testing/selftests/kvm/lib/x86/tdx/tdx.S     | 37 +++++++++++++++++++++++
>   3 files changed, 55 insertions(+)
> 
> diff --git a/tools/testing/selftests/kvm/Makefile.kvm b/tools/testing/selftests/kvm/Makefile.kvm
> index 645d9aac61db..28b33ed8c2e6 100644
> --- a/tools/testing/selftests/kvm/Makefile.kvm
> +++ b/tools/testing/selftests/kvm/Makefile.kvm
> @@ -33,6 +33,7 @@ LIBKVM_x86 += lib/x86/tdx/tdx_util.c
>   LIBKVM_x86 += lib/x86/ucall.c
>   LIBKVM_x86 += lib/x86/vmx.c
>   LIBKVM_x86 += lib/x86/tdx/td_boot.S
> +LIBKVM_x86 += lib/x86/tdx/tdx.S
>   
>   LIBKVM_arm64 += lib/arm64/gic.c
>   LIBKVM_arm64 += lib/arm64/gic_v3.c
> diff --git a/tools/testing/selftests/kvm/include/x86/tdx/tdx.h b/tools/testing/selftests/kvm/include/x86/tdx/tdx.h
> new file mode 100644
> index 000000000000..6355a30bb47f
> --- /dev/null
> +++ b/tools/testing/selftests/kvm/include/x86/tdx/tdx.h
> @@ -0,0 +1,17 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +#ifndef SELFTEST_KVM_TDX_TDX_H
> +#define SELFTEST_KVM_TDX_TDX_H
> +
> +#include <linux/types.h>
> +
> +#define TDG_VP_VMCALL_VE_REQUEST_MMIO    48

I know it is the name in GHCI spec. But MMIO doesn't have to be invoked 
by #VE. I think we can directly call it

TDG_VP_VMCALL_MMIO

> +#define TDVMCALL_MMIO_WRITE		  1

So we are defining TDG_VP_VMCALL_* and TDVMCALL_*. We'd better to make 
it consistent.


> +u64 __tdcall(u64 leaf, u64 r12, u64 r13, u64 r14, u64 r15);
> +
> +static inline u64 tdx_mmio_write(u64 address, u32 size, u64 data_in)
> +{
> +	return __tdcall(TDG_VP_VMCALL_VE_REQUEST_MMIO, size,
> +			TDVMCALL_MMIO_WRITE, address, data_in);
> +}
> +#endif /* SELFTEST_KVM_TDX_TDX_H */
> diff --git a/tools/testing/selftests/kvm/lib/x86/tdx/tdx.S b/tools/testing/selftests/kvm/lib/x86/tdx/tdx.S
> new file mode 100644
> index 000000000000..dff48a3624a1
> --- /dev/null
> +++ b/tools/testing/selftests/kvm/lib/x86/tdx/tdx.S
> @@ -0,0 +1,37 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +
> +.globl __tdcall
> +/*
> + * u64 __tdcall(u64 leaf, u64 r12, u64 r13, u64 r14, u64 r15)

this funtion is specific to TDG.VP.VMCALL. Please just name it 
tdg_vp_vmcall()

> + */
> +__tdcall:
> +	push %r12
> +	push %r13
> +	push %r14
> +	push %r15
> +
> +	/* Map C ABI fast arguments to TDX GHCI payload registers */
> +	/* RDI=leaf, RSI=r12, RDX=r13, RCX=r14, R8=r15 */

/*
  *
  */

for multiple-line comments?

> +	mov %rdi, %r11
> +	mov %rsi, %r12
> +	mov %rdx, %r13
> +	mov %rcx, %r14
> +	mov %r8,  %r15
> +
> +	/* TDCALL boilerplate */

it's not TDCALL boilerplate but TDG.VP.VMCALL since below rax is 
hardcoded to 0.

> +	mov $0, %rax
> +	mov $0xFC00, %rcx	/* Expose R10-R15 */
> +	mov $0, %r10
> +
> +	/* tdcall instruction */
> +	.byte 0x66, 0x0f, 0x01, 0xcc
> +
> +	/* Extract status */
> +	mov %r10, %rax
> +
> +	pop %r15
> +	pop %r14
> +	pop %r13
> +	pop %r12
> +	ret
>