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

On Mon, Aug 25, 2025, Yan Zhao wrote:

...

• /*

• * 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.

Gah, I managed to forget that relative targets obviously need to be signed values, and I also forgot to account for the size of the JMP in the assert. Go me.

Maybe express this as:

TEST_ASSERT(TD_BOOT_CODE_SIZE + 2 < 128, "TD boot code not addressable by 'JMP rel8'");

...

•     "TD boot code not addressable by 'JMP rel8'");
  

• hva[0] = 0xeb;
• hva[1] = 256 - 2 - TD_BOOT_CODE_SIZE;

I think I lucked into getting this right though?

...

• hva[2] = 0xcc;

+}

2.51.0.rc1.193.gad69d77794-goog