On Tue, Aug 26, 2025 at 09:38:15AM -0700, Sean Christopherson wrote: > 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'"); I like this version, which's is much clearer than asserting TD_BOOT_CODE_SIZE alone. nit: TD_BOOT_CODE_SIZE + 2 can be equal to 128, i.e., 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? Yes, this one is correct :) > > > + hva[2] = 0xcc; > > > +} > > > -- > > > 2.51.0.rc1.193.gad69d77794-goog > > > > > >
