On 26.11.2021 17:37, Andrew Cooper wrote:
> Each of MSR_{L,C}STAR and MSR_SYSENTER_EIP need to land on an endbr64
> instruction. For sysenter, this is easy.
>
> Unfortunately for syscall, the stubs are already 29 byte long with a limit of
> 32. endbr64 is 4 bytes. Luckily, there is a 1 byte instruction which can
> move from the stubs into the main handlers.
Oh, you actually found a different solution to the space problem. I should
probably have looked here first, but this being a v1.1 it sits lower in my
mailbox ...
> Move the push %rax out of the stub and into {l,c}star_entry(), allowing room
> for the endbr64 instruction when appropriate. Update the comment describing
> the entry state.
>
> Signed-off-by: Andrew Cooper <andrew.coop...@citrix.com>
Reviewed-by: Jan Beulich <jbeul...@suse.com>
> @@ -295,6 +296,12 @@ static unsigned int write_stub_trampoline(
> {
> unsigned char *p = stub;
>
> + if ( cpu_has_xen_ibt )
> + {
> + place_endbr64(p);
> + p += 4;
> + }
> +
> /* Store guest %rax into %ss slot */
> /* movabsq %rax, stack_bottom - 8 */
> *p++ = 0x48;
> @@ -315,10 +322,6 @@ static unsigned int write_stub_trampoline(
> *(uint64_t *)p = stack_bottom - 8;
> p += 8;
>
> - /* Store guest %rsp into %rsp slot */
> - /* pushq %rax */
> - *p++ = 0x50;
> -
> /* jmp target_va */
> *p++ = 0xe9;
> *(int32_t *)p = target_va - (stub_va + (p - stub) + 4);
With this it's even less clear to me why you did remove the 16-byte
alignment in the other patch. Initially I expected you would extend
the "first half" of stub space by 2 bytes.
Jan
