On 17/01/2024 9:37 am, Jan Beulich wrote:
> --- a/xen/arch/x86/ioport_emulate.c
> +++ b/xen/arch/x86/ioport_emulate.c
> @@ -8,11 +8,10 @@
> #include <xen/sched.h>
> #include <xen/dmi.h>
>
> -unsigned int (*__read_mostly ioemul_handle_quirk)(
> - uint8_t opcode, char *io_emul_stub, struct cpu_user_regs *regs);
> +bool __ro_after_init ioemul_handle_quirk;
>
> -static unsigned int cf_check ioemul_handle_proliant_quirk(
> - u8 opcode, char *io_emul_stub, struct cpu_user_regs *regs)
> +unsigned int ioemul_handle_proliant_quirk(
> + uint8_t opcode, char *io_emul_stub, const struct cpu_user_regs *regs)
> {
> static const char stub[] = {
> 0x9c, /* pushf */
Something occurred to me.
diff --git a/xen/arch/x86/ioport_emulate.c b/xen/arch/x86/ioport_emulate.c
index 23cba842b22e..70f94febe255 100644
--- a/xen/arch/x86/ioport_emulate.c
+++ b/xen/arch/x86/ioport_emulate.c
@@ -13,7 +13,7 @@ bool __ro_after_init ioemul_handle_quirk;
unsigned int ioemul_handle_proliant_quirk(
uint8_t opcode, char *io_emul_stub, const struct cpu_user_regs *regs)
{
- static const char stub[] = {
+ const char stub[] = {
0x9c, /* pushf */
0xfa, /* cli */
0xee, /* out %al, %dx */
is an improvement, confirmed by bloat-o-meter:
add/remove: 0/1 grow/shrink: 1/0 up/down: 1/-9 (-8)
Function old new delta
ioemul_handle_proliant_quirk 58 59 +1
stub 9 - -9
The reason is that we've got a 9 byte object that's decomposed into two
rip-relative accesses. i.e. we've got more pointer than data in this case.
But this adjustment seems to tickle a GCC bug. With that change in
place, GCC emits:
<ioemul_handle_proliant_quirk>:
48 83 ec 10 sub $0x10,%rsp
...
48 83 c4 10 add $0x10,%rsp
c3 retq
i.e. we get a stack frame (space at least, no initialisation) despite
the object having been converted entirely to instruction immediates.
Or in other words, there's a further 12 byte saving available when GCC
can be persuaded to not even emit the stack frame.
What is even more weird is that I see this GCC-10, and a build of gcc
master from last week, but not when I try to reproduce in
https://godbolt.org/z/PnachbznW so there's probably some other setting
used by Xen which tickles this bug.
~Andrew
