Is GET_CR0_INTO_EAX macro unused?

Is GET_CR0_INTO_EAX macro unused?

[Denys Vlasenko]

Hi Andy,

 From by git archaeology, looks like last use of GET_CR0_INTO_EAX
was removed long ago, in 2008 (see commit below).

Right now, I only grep it here in a comment in entry_32.S:
/*
  * We use macros for low-level operations which need to be overridden
  * for paravirtualization.  The following will never clobber any registers:
  *   INTERRUPT_RETURN (aka. "iret")
  *   GET_CR0_INTO_EAX (aka. "movl %cr0, %eax")

To support GET_CR0_INTO_EAX, paravirt has PV_CPU_read_cr0 a.k.a.
struct pv_cpu_ops::read_cr0 and a bunch of code to support it
for each hypervisor.

Can we delete it?
Or am I missing a non-obvious place where this macro is still in use?



commit 7643e9b936b4af31ba4851eb7d5b3a3bfad52502
Author: Alexander van Heukelum <heukelum@fastmail.fm>
Date:   Tue Sep 9 21:56:02 2008 +0200

     i386: convert hardware exception 7 to an interrupt gate

     Handle no coprocessor exception with interrupt initially off.

     device_not_available in entry_32.S calls either math_state_restore
     or math_emulate. This patch adds an extra indirection to be
     able to re-enable interrupts explicitly in traps_32.c

     Signed-off-by: Alexander van Heukelum <heukelum@fastmail.fm>
     Signed-off-by: Ingo Molnar <mingo@elte.hu>

diff --git a/arch/x86/kernel/entry_32.S b/arch/x86/kernel/entry_32.S
index 109792b..5a88585 100644
--- a/arch/x86/kernel/entry_32.S
+++ b/arch/x86/kernel/entry_32.S
@@ -760,20 +760,9 @@ ENTRY(device_not_available)
         RING0_INT_FRAME
         pushl $-1                       # mark this as an int
         CFI_ADJUST_CFA_OFFSET 4
-       SAVE_ALL
-       GET_CR0_INTO_EAX
-       testl $0x4, %eax                # EM (math emulation bit)
-       jne device_not_available_emulate
-       preempt_stop(CLBR_ANY)
-       call math_state_restore
-       jmp ret_from_exception
-device_not_available_emulate:
-       pushl $0                        # temporary storage for ORIG_EIP
+       pushl $do_device_not_available
         CFI_ADJUST_CFA_OFFSET 4
-       call math_emulate
-       addl $4, %esp
-       CFI_ADJUST_CFA_OFFSET -4
-       jmp ret_from_exception
+       jmp error_code
         CFI_ENDPROC
  END(device_not_available)

Re: Is GET_CR0_INTO_EAX macro unused?

[Thomas Gleixner]

On Mon, 2 Oct 2017, Denys Vlasenko wrote:

> Hi Andy,
> 
> From by git archaeology, looks like last use of GET_CR0_INTO_EAX
> was removed long ago, in 2008 (see commit below).
> 
> Right now, I only grep it here in a comment in entry_32.S:
> /*
>  * We use macros for low-level operations which need to be overridden
>  * for paravirtualization.  The following will never clobber any registers:
>  *   INTERRUPT_RETURN (aka. "iret")
>  *   GET_CR0_INTO_EAX (aka. "movl %cr0, %eax")
> 
> To support GET_CR0_INTO_EAX, paravirt has PV_CPU_read_cr0 a.k.a.
> struct pv_cpu_ops::read_cr0 and a bunch of code to support it
> for each hypervisor.
> 
> Can we delete it?
> Or am I missing a non-obvious place where this macro is still in use?

That thing was simply forgotten and bitrots happily since 2008.

Thanks,

	tglx