Re: [PATCH v15 20/23] KVM: selftests: Add a helper to issue TDVMCALLs from the TDX vm

[Xiaoyao Li]

On 10/2/2026 3:37 AM, Lisa Wang wrote:
> From: Erdem Aktas <erdemaktas@xxxxxxxxxx>
> 
> TDX VMs need to notify the host, e.g. ucall. Add tdvmcall() to allow VMs
> to issue TDG.VP.VMCALL with a single function call instead of
> open-coding TDVMCALL registers setup at every call site.
> 
> The helper makes standard GHCI TDVMCALLs: the caller passes the GHCI
> sub-function and up to four arguments and returns the TDVMCALL status
> from R10.
> 
> Co-developed-by: Sagi Shahar <sagis@xxxxxxxxxx>
> Signed-off-by: Sagi Shahar <sagis@xxxxxxxxxx>
> Co-developed-by: Lisa Wang <wyihan@xxxxxxxxxx>
> Signed-off-by: Lisa Wang <wyihan@xxxxxxxxxx>
> Signed-off-by: Erdem Aktas <erdemaktas@xxxxxxxxxx>
> Reviewed-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>

Reviewed-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>

> ---
>  tools/testing/selftests/kvm/Makefile.kvm          |  1 +
>  tools/testing/selftests/kvm/include/x86/tdx/tdx.h |  9 ++++++
>  tools/testing/selftests/kvm/lib/x86/tdx/tdx.S     | 39 +++++++++++++++++++++++
>  3 files changed, 49 insertions(+)
> 
> diff --git a/tools/testing/selftests/kvm/Makefile.kvm b/tools/testing/selftests/kvm/Makefile.kvm
> index 9464168e361a..495de8ef8166 100644
> --- a/tools/testing/selftests/kvm/Makefile.kvm
> +++ b/tools/testing/selftests/kvm/Makefile.kvm
> @@ -34,6 +34,7 @@ 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..d41a1efc8a63
> --- /dev/null
> +++ b/tools/testing/selftests/kvm/include/x86/tdx/tdx.h
> @@ -0,0 +1,9 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +#ifndef SELFTEST_KVM_TDX_TDX_H
> +#define SELFTEST_KVM_TDX_TDX_H
> +
> +#include <linux/types.h>
> +
> +u64 __tdvmcall(u64 fn, u64 r12, u64 r13, u64 r14, u64 r15);
> +
> +#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..e3f0f7f2232b
> --- /dev/null
> +++ b/tools/testing/selftests/kvm/lib/x86/tdx/tdx.S
> @@ -0,0 +1,39 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +
> +.globl __tdvmcall
> +/*
> + * u64 __tdvmcall(u64 fn, u64 r12, u64 r13, u64 r14, u64 r15)
> + */
> +__tdvmcall:
> +	push %r12
> +	push %r13
> +	push %r14
> +	push %r15
> +
> +	/*
> +	 * Map C ABI fast arguments to TDX GHCI payload registers
> +	 * RDI=fn, RSI=r12, RDX=r13, RCX=r14, R8=r15
> +	 */
> +	mov %rdi, %r11
> +	mov %rsi, %r12
> +	mov %rdx, %r13
> +	mov %rcx, %r14
> +	mov %r8,  %r15
> +
> +	mov $0, %rax		/* TDCALL leaf 0: TDG.VP.VMCALL */
> +	mov $0xFC00, %rcx	/* Expose R10-R15 */
> +	mov $0, %r10		/* Standard GHCI call */
> +
> +	/* TDCALL instruction */
> +	.byte 0x66, 0x0f, 0x01, 0xcc
> +
> +	/* Extract status */
> +	mov %r10, %rax
> +
> +	pop %r15
> +	pop %r14
> +	pop %r13
> +	pop %r12
> +	ret
> +
> +.section .note.GNU-stack,"",%progbits
>