Re: [PATCH v13 20/22] KVM: selftests: Implement MMIO WRITE for the TDX VM

[Binbin Wu <binbin.wu@linux.intel.com>]

On 5/22/2026 7:17 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. Skip implementing the #VE handler as described in the
> GHCI spec so selftests will not take a dependency on having a working
                                                                       ^
Something was cut off?

> 
> To perform emulated I/O, VMs use the TDG.VP.VMCALL instruction to
> 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 | 16 ++++++++++++
>  tools/testing/selftests/kvm/lib/x86/tdx/tdx.c     | 30 +++++++++++++++++++++++
>  3 files changed, 47 insertions(+)
> 
> diff --git a/tools/testing/selftests/kvm/Makefile.kvm b/tools/testing/selftests/kvm/Makefile.kvm
> index a651a876c522..489324cecf83 100644
> --- a/tools/testing/selftests/kvm/Makefile.kvm
> +++ b/tools/testing/selftests/kvm/Makefile.kvm
> @@ -33,6 +33,7 @@ LIBKVM_x86 += lib/x86/ucall.c
>  LIBKVM_x86 += lib/x86/vmx.c
>  LIBKVM_x86 += lib/x86/tdx/tdx_util.c
>  LIBKVM_x86 += lib/x86/tdx/td_boot.S
> +LIBKVM_x86 += lib/x86/tdx/tdx.c
>  
>  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..810ca7423c84
> --- /dev/null
> +++ b/tools/testing/selftests/kvm/include/x86/tdx/tdx.h
> @@ -0,0 +1,16 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +#ifndef SELFTESTS_TDX_TDX_H
> +#define SELFTESTS_TDX_TDX_H

Nit:
The headers in tools/testing/selftests/kvm use SELFTEST_KVM_XXX.

> +
> +#include <linux/types.h>
> +
> +enum mmio_size {
> +	MMIO_SIZE_1B = 1,
> +	MMIO_SIZE_2B = 2,
> +	MMIO_SIZE_4B = 4,
> +	MMIO_SIZE_8B = 8
> +};
> +
> +u64 tdx_mmio_write(u64 address, enum mmio_size size, u64 data_in);
> +
> +#endif // SELFTESTS_TDX_TDX_H
> diff --git a/tools/testing/selftests/kvm/lib/x86/tdx/tdx.c b/tools/testing/selftests/kvm/lib/x86/tdx/tdx.c
> new file mode 100644
> index 000000000000..f19be79fe11f
> --- /dev/null
> +++ b/tools/testing/selftests/kvm/lib/x86/tdx/tdx.c
> @@ -0,0 +1,30 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +
> +#include "tdx/tdx.h"
> +
> +#define TDG_VP_VMCALL 0
> +#define TDG_VP_VMCALL_VE_REQUEST_MMIO    48
> +#define TDVMCALL_MMIO_WRITE		  1
> +#define TDVMCALL_EXPOSE_REGS_MASK    0xFC00
> +
> +u64 tdx_mmio_write(u64 address, enum mmio_size size, u64 data_in)
> +{
> +	register u64 r10_reg asm("r10") = TDG_VP_VMCALL;

I think this should just be 0 instead of TDG_VP_VMCALL, although
TDG_VP_VMCALL is also 0.
Per GHCI spec about R10:
 : Set to 0 indicates that TDG.VP.VMCALL leaf used in R11 is defined
 : in this specification.
 : All other values 0x1 to 0xFFFFFFFFFFFFFFFF indicate TDG.VP.VMCALL
 : is vendor-specific (both R10 and R11).


> +	register u64 r11_reg asm("r11") = TDG_VP_VMCALL_VE_REQUEST_MMIO;
> +	register u64 r12_reg asm("r12") = size;
> +	register u64 r13_reg asm("r13") = TDVMCALL_MMIO_WRITE;
> +	register u64 r14_reg asm("r14") = address;
> +	register u64 r15_reg asm("r15") = data_in;
> +	register u64 rax_reg asm("rax") = TDG_VP_VMCALL;
> +	register u64 rcx_reg asm("rcx") = TDVMCALL_EXPOSE_REGS_MASK;
> +
> +	asm volatile(
> +	 ".byte 0x66,0x0f,0x01,0xcc" /* tdcall */
> +	 : "+r" (r10_reg), "+r" (r11_reg)
> +	 : "r" (r12_reg), "r" (r13_reg), "r" (r14_reg), "r" (r15_reg),
> +	   "r" (rax_reg), "r" (rcx_reg)
> +	 : "cc", "memory"
> +	);
> +
> +	return r10_reg;
> +}
>