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
> >
> >
