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 <l...@kernel.org> --- arch/x86/entry/entry_32.S | 15 ++------------- arch/x86/include/asm/cpufeatures.h | 8 ++++++++ arch/x86/kernel/cpu/common.c | 25 +++++++++++++++++++++++++ 3 files changed, 35 insertions(+), 13 deletions(-) diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S index 6a792603f9f3..4f7615e405a4 100644 --- a/arch/x86/entry/entry_32.S +++ b/arch/x86/entry/entry_32.S @@ -418,6 +418,8 @@ 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 @@ -444,19 +446,6 @@ 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 * diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h index 6663fae71b12..d11a3aaafd96 100644 --- a/arch/x86/include/asm/cpufeatures.h +++ b/arch/x86/include/asm/cpufeatures.h @@ -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 */ diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c index 91ddae732a36..c6ef4da8e4f4 100644 --- a/arch/x86/kernel/cpu/common.c +++ b/arch/x86/kernel/cpu/common.c @@ -979,6 +979,31 @@ static void identify_cpu(struct cpuinfo_x86 *c) #ifdef CONFIG_NUMA numa_add_cpu(smp_processor_id()); #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 } /* -- 2.5.0 _______________________________________________ Xen-devel mailing list Xen-devel@lists.xen.org http://lists.xen.org/xen-devel