On Thu, Jul 13, 2023 at 08:02:54AM +0000, Huang, Kai wrote: > Sorry I am ignorant here. Won't "clearing ECX only" leave high bits of > registers still containing guest's value? architecture zero-extends 32bit stores > I see KVM code uses: > > xor %eax, %eax > xor %ecx, %ecx > xor %edx, %edx > xor %ebp, %ebp > xor %esi, %esi > xor %edi, %edi > #ifdef CONFIG_X86_64 > xor %r8d, %r8d > xor %r9d, %r9d > xor %r10d, %r10d > xor %r11d, %r11d > xor %r12d, %r12d > xor %r13d, %r13d > xor %r14d, %r14d > xor %r15d, %r15d > #endif > > Which makes sense because KVM wants to support 32-bit too. Encoding for the first lot is shorter, the 64bit regs obviously need the RAX byte anyway. > However for TDX is 64-bit only. > > And I also see the current TDVMCALL code has: > > xor %r8d, %r8d > xor %r9d, %r9d > xor %r10d, %r10d > xor %r11d, %r11d > xor %rdi, %rdi > xor %rdx, %rdx > > Why does it need to use "d" postfix for all r* registers? That's the name of the 32bit subword, r#[bwd] for byte, word, double-word. SDM v1 3.7.2.1 has the whole list, I couldn't quicky find one for the zero-extention thing. > Sorry for those questions but I struggled when I wrote those assembly and am > hoping to get my mind cleared on this. :-) No problem.
