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x86/entry/32: Introduce and use X86_BUG_ESPFIX instead of paravirt_enabled 

x86_64 has very clean espfix handling on paravirt: espfix64 is set
up in native_iret, so paravirt systems that override iret bypass
espfix64 automatically.  This is robust and straightforward.

x86_32 is messier.  espfix is set up before the IRET paravirt patch
point, so it can't be directly conditionalized on whether we use
native_iret.  We also can't easily move it into native_iret without
regressing performance due to a bizarre consideration.  Specifically,
on 64-bit kernels, the logic is:

  if (regs->ss & 0x4)
          setup_espfix;

On 32-bit kernels, the logic is:

  if ((regs->ss & 0x4) && (regs->cs & 0x3) == 3 &&
      (regs->flags & X86_EFLAGS_VM) == 0)
          setup_espfix;

The performance of setup_espfix itself is essentially irrelevant, but
the comparison happens on every IRET so its performance matters.  On
x86_64, there's no need for any registers except flags to implement
the comparison, so we fold the whole thing into native_iret.  On
x86_32, we don't do that because we need a free register to
implement the comparison efficiently.  We therefore do espfix setup
before restoring registers on x86_32.

This patch gets rid of the explicit paravirt_enabled check by
introducing X86_BUG_ESPFIX on 32-bit systems and using an ALTERNATIVE
to skip espfix on paravirt systems where iret != native_iret.  This is
also messy, but it's at least in line with other things we do.

This improves espfix performance by removing a branch, but no one
cares.  More importantly, it removes a paravirt_enabled user, which is
good because paravirt_enabled is ill-defined and is going away.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Reviewed-by: Borislav Petkov <bp@suse.de>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Luis R. Rodriguez <mcgrof@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: boris.ostrovsky@oracle.com
Cc: david.vrabel@citrix.com
Cc: konrad.wilk@oracle.com
Cc: lguest@lists.ozlabs.org
Cc: xen-devel@lists.xensource.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Andy Lutomirski 2016-02-29 15:50:19 -08:00 committed by Ingo Molnar
parent ec87e1cf7d
commit 58a5aac533
3 changed files with 35 additions and 13 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

15
arch/x86/entry/entry_32.S
View File

@ -361,6 +361,8 @@ restore_all:
TRACE_IRQS_IRET
restore_all_notrace:
#ifdef CONFIG_X86_ESPFIX32
ALTERNATIVE "jmp restore_nocheck", "", X86_BUG_ESPFIX
movl PT_EFLAGS(%esp), %eax # mix EFLAGS, SS and CS
/*
* Warning: PT_OLDSS(%esp) contains the wrong/random values if we
@ -387,19 +389,6 @@ ENTRY(iret_exc )
#ifdef CONFIG_X86_ESPFIX32
ldt_ss:
#ifdef CONFIG_PARAVIRT
/*
* The kernel can't run on a non-flat stack if paravirt mode
* is active. Rather than try to fixup the high bits of
* ESP, bypass this code entirely. This may break DOSemu
* and/or Wine support in a paravirt VM, although the option
* is still available to implement the setting of the high
* 16-bits in the INTERRUPT_RETURN paravirt-op.
*/
cmpl $0, pv_info+PARAVIRT_enabled
jne restore_nocheck
#endif
/*
* Setup and switch to ESPFIX stack
*

8
arch/x86/include/asm/cpufeatures.h
View File

@ -286,4 +286,12 @@
#define X86_BUG_CLFLUSH_MONITOR X86_BUG(7) /* AAI65, CLFLUSH required before MONITOR */
#define X86_BUG_SYSRET_SS_ATTRS X86_BUG(8) /* SYSRET doesn't fix up SS attrs */
#ifdef CONFIG_X86_32
/*
* 64-bit kernels don't use X86_BUG_ESPFIX. Make the define conditional
* to avoid confusion.
*/
#define X86_BUG_ESPFIX X86_BUG(9) /* "" IRET to 16-bit SS corrupts ESP/RSP high bits */
#endif
#endif /* _ASM_X86_CPUFEATURES_H */

25
arch/x86/kernel/cpu/common.c
View File

@ -802,6 +802,31 @@ static void detect_nopl(struct cpuinfo_x86 *c)
clear_cpu_cap(c, X86_FEATURE_NOPL);
#else
set_cpu_cap(c, X86_FEATURE_NOPL);
#endif
/*
* ESPFIX is a strange bug. All real CPUs have it. Paravirt
* systems that run Linux at CPL > 0 may or may not have the
* issue, but, even if they have the issue, there's absolutely
* nothing we can do about it because we can't use the real IRET
* instruction.
*
* NB: For the time being, only 32-bit kernels support
* X86_BUG_ESPFIX as such. 64-bit kernels directly choose
* whether to apply espfix using paravirt hooks. If any
* non-paravirt system ever shows up that does *not* have the
* ESPFIX issue, we can change this.
*/
#ifdef CONFIG_X86_32
#ifdef CONFIG_PARAVIRT
do {
extern void native_iret(void);
if (pv_cpu_ops.iret == native_iret)
set_cpu_bug(c, X86_BUG_ESPFIX);
} while (0);
#else
set_cpu_bug(c, X86_BUG_ESPFIX);
#endif
#endif
}