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

[Sean Christopherson <seanjc@google.com>]

On Thu, May 21, 2026, Lisa Wang wrote:
> 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
> +
> +#include <linux/types.h>
> +
> +enum mmio_size {
> +	MMIO_SIZE_1B = 1,
> +	MMIO_SIZE_2B = 2,
> +	MMIO_SIZE_4B = 4,
> +	MMIO_SIZE_8B = 8

This is absurd.  Either open code the literals or use sizeof() where appropriate.

> +};
> +
> +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;
> +	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;

This needs to be proper assembly, i.e. in a .S file.  Using register like this
is *extremely* dangerous, because the compiler is (stupidly) allowed to clobber
registers between their declarations/initialization and their consumption in
the asm() blob.

> +
> +	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;
> +}
> 
> -- 
> 2.54.0.746.g67dd491aae-goog
>