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Date:	Mon, 28 Jul 2014 20:47:21 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andy Lutomirski <luto@...capital.net>,
	Ingo Molnar <mingo@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	stable <stable@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...ux.intel.com>
CC:	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>
Subject: Re: [tip:x86/urgent] x86_64/entry/xen: Do not invoke espfix64 on Xen

Would like to  absolutely minimize changes at this point.

On July 28, 2014 8:39:43 PM PDT, Andy Lutomirski <luto@...capital.net> wrote:
>On Mon, Jul 28, 2014 at 3:33 PM, tip-bot for Andy Lutomirski
><tipbot@...or.com> wrote:
>> Commit-ID:  7209a75d2009dbf7745e2fd354abf25c3deb3ca3
>> Gitweb:    
>http://git.kernel.org/tip/7209a75d2009dbf7745e2fd354abf25c3deb3ca3
>> Author:     Andy Lutomirski <luto@...capital.net>
>> AuthorDate: Wed, 23 Jul 2014 08:34:11 -0700
>> Committer:  H. Peter Anvin <hpa@...ux.intel.com>
>> CommitDate: Mon, 28 Jul 2014 15:25:40 -0700
>>
>> x86_64/entry/xen: Do not invoke espfix64 on Xen
>>
>> This moves the espfix64 logic into native_iret.  To make this work,
>> it gets rid of the native patch for INTERRUPT_RETURN:
>> INTERRUPT_RETURN on native kernels is now 'jmp native_iret'.
>>
>> This changes the 16-bit SS behavior on Xen from OOPSing to leaking
>> some bits of the Xen hypervisor's RSP (I think).
>>
>> [ hpa: this is a nonzero cost on native, but probably not enough to
>>   measure. Xen needs to fix this in their own code, probably doing
>>   something equivalent to espfix64. ]
>
>Any plan to either fix or remove the buggy bsp-only initialization of
>espfix64 on paravirt?  It's harmless now, but IMO it's still ugly.  Or
>should this wait for 3.17?
>
>--Andy
>
>>
>> Signed-off-by: Andy Lutomirski <luto@...capital.net>
>> Link:
>http://lkml.kernel.org/r/7b8f1d8ef6597cb16ae004a43c56980a7de3cf94.1406129132.git.luto@amacapital.net
>> Signed-off-by: H. Peter Anvin <hpa@...ux.intel.com>
>> Cc: <stable@...r.kernel.org>
>> ---
>>  arch/x86/include/asm/irqflags.h     |  2 +-
>>  arch/x86/kernel/entry_64.S          | 28
>++++++++++------------------
>>  arch/x86/kernel/paravirt_patch_64.c |  2 --
>>  3 files changed, 11 insertions(+), 21 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/irqflags.h
>b/arch/x86/include/asm/irqflags.h
>> index bba3cf8..0a8b519 100644
>> --- a/arch/x86/include/asm/irqflags.h
>> +++ b/arch/x86/include/asm/irqflags.h
>> @@ -129,7 +129,7 @@ static inline notrace unsigned long
>arch_local_irq_save(void)
>>
>>  #define PARAVIRT_ADJUST_EXCEPTION_FRAME        /*  */
>>
>> -#define INTERRUPT_RETURN       iretq
>> +#define INTERRUPT_RETURN       jmp native_iret
>>  #define USERGS_SYSRET64                                \
>>         swapgs;                                 \
>>         sysretq;
>> diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
>> index b25ca96..c844f08 100644
>> --- a/arch/x86/kernel/entry_64.S
>> +++ b/arch/x86/kernel/entry_64.S
>> @@ -830,27 +830,24 @@ restore_args:
>>         RESTORE_ARGS 1,8,1
>>
>>  irq_return:
>> +       INTERRUPT_RETURN
>> +
>> +ENTRY(native_iret)
>>         /*
>>          * Are we returning to a stack segment from the LDT?  Note:
>in
>>          * 64-bit mode SS:RSP on the exception stack is always valid.
>>          */
>>  #ifdef CONFIG_X86_ESPFIX64
>>         testb $4,(SS-RIP)(%rsp)
>> -       jnz irq_return_ldt
>> +       jnz native_irq_return_ldt
>>  #endif
>>
>> -irq_return_iret:
>> -       INTERRUPT_RETURN
>> -       _ASM_EXTABLE(irq_return_iret, bad_iret)
>> -
>> -#ifdef CONFIG_PARAVIRT
>> -ENTRY(native_iret)
>> +native_irq_return_iret:
>>         iretq
>> -       _ASM_EXTABLE(native_iret, bad_iret)
>> -#endif
>> +       _ASM_EXTABLE(native_irq_return_iret, bad_iret)
>>
>>  #ifdef CONFIG_X86_ESPFIX64
>> -irq_return_ldt:
>> +native_irq_return_ldt:
>>         pushq_cfi %rax
>>         pushq_cfi %rdi
>>         SWAPGS
>> @@ -872,7 +869,7 @@ irq_return_ldt:
>>         SWAPGS
>>         movq %rax,%rsp
>>         popq_cfi %rax
>> -       jmp irq_return_iret
>> +       jmp native_irq_return_iret
>>  #endif
>>
>>         .section .fixup,"ax"
>> @@ -956,13 +953,8 @@ __do_double_fault:
>>         cmpl $__KERNEL_CS,CS(%rdi)
>>         jne do_double_fault
>>         movq RIP(%rdi),%rax
>> -       cmpq $irq_return_iret,%rax
>> -#ifdef CONFIG_PARAVIRT
>> -       je 1f
>> -       cmpq $native_iret,%rax
>> -#endif
>> +       cmpq $native_irq_return_iret,%rax
>>         jne do_double_fault             /* This shouldn't happen...
>*/
>> -1:
>>         movq PER_CPU_VAR(kernel_stack),%rax
>>         subq $(6*8-KERNEL_STACK_OFFSET),%rax    /* Reset to original
>stack */
>>         movq %rax,RSP(%rdi)
>> @@ -1428,7 +1420,7 @@ error_sti:
>>   */
>>  error_kernelspace:
>>         incl %ebx
>> -       leaq irq_return_iret(%rip),%rcx
>> +       leaq native_irq_return_iret(%rip),%rcx
>>         cmpq %rcx,RIP+8(%rsp)
>>         je error_swapgs
>>         movl %ecx,%eax  /* zero extend */
>> diff --git a/arch/x86/kernel/paravirt_patch_64.c
>b/arch/x86/kernel/paravirt_patch_64.c
>> index 3f08f34..a1da673 100644
>> --- a/arch/x86/kernel/paravirt_patch_64.c
>> +++ b/arch/x86/kernel/paravirt_patch_64.c
>> @@ -6,7 +6,6 @@ DEF_NATIVE(pv_irq_ops, irq_disable, "cli");
>>  DEF_NATIVE(pv_irq_ops, irq_enable, "sti");
>>  DEF_NATIVE(pv_irq_ops, restore_fl, "pushq %rdi; popfq");
>>  DEF_NATIVE(pv_irq_ops, save_fl, "pushfq; popq %rax");
>> -DEF_NATIVE(pv_cpu_ops, iret, "iretq");
>>  DEF_NATIVE(pv_mmu_ops, read_cr2, "movq %cr2, %rax");
>>  DEF_NATIVE(pv_mmu_ops, read_cr3, "movq %cr3, %rax");
>>  DEF_NATIVE(pv_mmu_ops, write_cr3, "movq %rdi, %cr3");
>> @@ -50,7 +49,6 @@ unsigned native_patch(u8 type, u16 clobbers, void
>*ibuf,
>>                 PATCH_SITE(pv_irq_ops, save_fl);
>>                 PATCH_SITE(pv_irq_ops, irq_enable);
>>                 PATCH_SITE(pv_irq_ops, irq_disable);
>> -               PATCH_SITE(pv_cpu_ops, iret);
>>                 PATCH_SITE(pv_cpu_ops, irq_enable_sysexit);
>>                 PATCH_SITE(pv_cpu_ops, usergs_sysret32);
>>                 PATCH_SITE(pv_cpu_ops, usergs_sysret64);

-- 
Sent from my mobile phone.  Please pardon brevity and lack of formatting.
--
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