Re: [PATCH v9 10/19] KVM: selftests: Set up TDX boot code region

[Yan Zhao <yan.y.zhao@intel.com>]

On Wed, Aug 20, 2025 at 09:29:03PM -0700, Sagi Shahar wrote:
> Add memory for TDX boot code in a separate memslot.
> 
> Use virt_map() to get identity map in this memory region to allow for
> seamless transition from paging disabled to paging enabled code.
> 
> Copy the boot code into the memory region and set up the reset vectors
> at this point. While it's possible to separate the memory allocation and
> boot code initialization into separate functions, having all the
> calculations for memory size and offsets in one place simplifies the
> code and avoids duplications.
> 
> Handcode the reset vector as suggested by Sean Christopherson.
> 
> Suggested-by: Sean Christopherson <seanjc@google.com>
> Co-developed-by: Erdem Aktas <erdemaktas@google.com>
> Signed-off-by: Erdem Aktas <erdemaktas@google.com>
> Signed-off-by: Sagi Shahar <sagis@google.com>
> ---
>  tools/testing/selftests/kvm/Makefile.kvm      |  1 +
>  .../selftests/kvm/include/x86/tdx/tdx_util.h  |  2 +
>  .../selftests/kvm/lib/x86/tdx/tdx_util.c      | 54 +++++++++++++++++++
>  3 files changed, 57 insertions(+)
>  create mode 100644 tools/testing/selftests/kvm/lib/x86/tdx/tdx_util.c
> 
> diff --git a/tools/testing/selftests/kvm/Makefile.kvm b/tools/testing/selftests/kvm/Makefile.kvm
> index 03754ce2e983..c42b579fb7c5 100644
> --- a/tools/testing/selftests/kvm/Makefile.kvm
> +++ b/tools/testing/selftests/kvm/Makefile.kvm
> @@ -31,6 +31,7 @@ LIBKVM_x86 += lib/x86/sev.c
>  LIBKVM_x86 += lib/x86/svm.c
>  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_arm64 += lib/arm64/gic.c
> diff --git a/tools/testing/selftests/kvm/include/x86/tdx/tdx_util.h b/tools/testing/selftests/kvm/include/x86/tdx/tdx_util.h
> index 286d5e3c24b1..ec05bcd59145 100644
> --- a/tools/testing/selftests/kvm/include/x86/tdx/tdx_util.h
> +++ b/tools/testing/selftests/kvm/include/x86/tdx/tdx_util.h
> @@ -11,4 +11,6 @@ static inline bool is_tdx_vm(struct kvm_vm *vm)
>  	return vm->type == KVM_X86_TDX_VM;
>  }
>  
> +void vm_tdx_setup_boot_code_region(struct kvm_vm *vm);
> +
>  #endif // SELFTESTS_TDX_TDX_UTIL_H
> diff --git a/tools/testing/selftests/kvm/lib/x86/tdx/tdx_util.c b/tools/testing/selftests/kvm/lib/x86/tdx/tdx_util.c
> new file mode 100644
> index 000000000000..15833b9eb5d5
> --- /dev/null
> +++ b/tools/testing/selftests/kvm/lib/x86/tdx/tdx_util.c
> @@ -0,0 +1,54 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +
> +#include <stdint.h>
> +
> +#include "kvm_util.h"
> +#include "processor.h"
> +#include "tdx/td_boot.h"
> +#include "tdx/tdx_util.h"
> +
> +/* Arbitrarily selected to avoid overlaps with anything else */
> +#define TD_BOOT_CODE_SLOT	20
> +
> +#define X86_RESET_VECTOR	0xfffffff0ul
> +#define X86_RESET_VECTOR_SIZE	16
> +
> +void vm_tdx_setup_boot_code_region(struct kvm_vm *vm)
> +{
> +	size_t total_code_size = TD_BOOT_CODE_SIZE + X86_RESET_VECTOR_SIZE;
> +	vm_paddr_t boot_code_gpa = X86_RESET_VECTOR - TD_BOOT_CODE_SIZE;
> +	vm_paddr_t alloc_gpa = round_down(boot_code_gpa, PAGE_SIZE);
> +	size_t nr_pages = DIV_ROUND_UP(total_code_size, PAGE_SIZE);
> +	vm_paddr_t gpa;
> +	uint8_t *hva;
> +
> +	vm_userspace_mem_region_add(vm, VM_MEM_SRC_ANONYMOUS,
> +				    alloc_gpa,
> +				    TD_BOOT_CODE_SLOT, nr_pages,
> +				    KVM_MEM_GUEST_MEMFD);
> +
> +	gpa = vm_phy_pages_alloc(vm, nr_pages, alloc_gpa, TD_BOOT_CODE_SLOT);
> +	TEST_ASSERT(gpa == alloc_gpa, "Failed vm_phy_pages_alloc\n");
> +
> +	virt_map(vm, alloc_gpa, alloc_gpa, nr_pages);
> +	hva = addr_gpa2hva(vm, boot_code_gpa);
> +	memcpy(hva, td_boot, TD_BOOT_CODE_SIZE);
> +
> +	hva += TD_BOOT_CODE_SIZE;
> +	TEST_ASSERT(hva == addr_gpa2hva(vm, X86_RESET_VECTOR),
> +		    "Expected RESET vector at hva 0x%lx, got %lx",
> +		    (unsigned long)addr_gpa2hva(vm, X86_RESET_VECTOR), (unsigned long)hva);
> +
> +	/*
> +	 * Handcode "JMP rel8" at the RESET vector to jump back to the TD boot
> +	 * code, as there are only 16 bytes at the RESET vector before RIP will
> +	 * wrap back to zero.  Insert a trailing int3 so that the vCPU crashes
> +	 * in case the JMP somehow falls through.  Note!  The target address is
> +	 * relative to the end of the instruction!
> +	 */
> +	TEST_ASSERT(TD_BOOT_CODE_SIZE < 256,
Looks TD_BOOT_CODE_SIZE needs to be <= 126, as the jump range is limited to -128
to +127 for JMP rel8.

> +		    "TD boot code not addressable by 'JMP rel8'");
> +	hva[0] = 0xeb;
> +	hva[1] = 256 - 2 - TD_BOOT_CODE_SIZE;
> +	hva[2] = 0xcc;
> +}
> -- 
> 2.51.0.rc1.193.gad69d77794-goog
> 
>