Re: [RFC 08/17] x86/asm/64: De-Xen-ify our NMI code

[Andy Lutomirski <luto@kernel.org>]

On Thu, Sep 7, 2017 at 2:34 AM, Juergen Gross <jgross@suse.com> wrote:
> On 06/09/17 23:36, Andy Lutomirski wrote:
>> Xen PV is fundamentally incompatible with our fancy NMI code: it
>> doesn't use IST at all, and Xen entries clobber two stack slots
>> below the hardware frame.
>>
>> Drop Xen PV support from our NMI code entirely.
>>
>> XXX: Juergen: could you write and test the tiny patch needed to
>> make Xen PV have a xen_nmi entry that handles NMIs?  I don't know
>> how to test it.
>
> You mean something like the attached one?

Yes.  Mind if I add it to my series?

>
> Seems to work at least for the "normal" case of a NMI coming in at
> a random point in time.
>
> Regarding testing: in case you have a Xen setup you can easily send
> a NMI to a domain from dom0:
>
> xl trigger <domain> nmi

Thanks!

>
>
> Juergen
>
>>
>> Cc: Juergen Gross <jgross@suse.com>
>> Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
>> Signed-off-by: Andy Lutomirski <luto@kernel.org>
>> ---
>>  arch/x86/entry/entry_64.S | 41 +++++++++++++++++------------------------
>>  1 file changed, 17 insertions(+), 24 deletions(-)
>>
>> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
>> index a9e318f7cc9b..c81e05fb999e 100644
>> --- a/arch/x86/entry/entry_64.S
>> +++ b/arch/x86/entry/entry_64.S
>> @@ -1171,21 +1171,12 @@ ENTRY(error_exit)
>>       jmp     retint_user
>>  END(error_exit)
>>
>> -/* Runs on exception stack */
>> +/*
>> + * Runs on exception stack.  Xen PV does not go through this path at all,
>> + * so we can use real assembly here.
>> + */
>>  ENTRY(nmi)
>>       /*
>> -      * Fix up the exception frame if we're on Xen.
>> -      * PARAVIRT_ADJUST_EXCEPTION_FRAME is guaranteed to push at most
>> -      * one value to the stack on native, so it may clobber the rdx
>> -      * scratch slot, but it won't clobber any of the important
>> -      * slots past it.
>> -      *
>> -      * Xen is a different story, because the Xen frame itself overlaps
>> -      * the "NMI executing" variable.
>> -      */
>> -     PARAVIRT_ADJUST_EXCEPTION_FRAME
>> -
>> -     /*
>>        * We allow breakpoints in NMIs. If a breakpoint occurs, then
>>        * the iretq it performs will take us out of NMI context.
>>        * This means that we can have nested NMIs where the next
>> @@ -1240,7 +1231,7 @@ ENTRY(nmi)
>>        * stacks lest we corrupt the "NMI executing" variable.
>>        */
>>
>> -     SWAPGS_UNSAFE_STACK
>> +     swapgs
>>       cld
>>       movq    %rsp, %rdx
>>       movq    PER_CPU_VAR(cpu_current_top_of_stack), %rsp
>> @@ -1402,7 +1393,7 @@ nested_nmi_out:
>>       popq    %rdx
>>
>>       /* We are returning to kernel mode, so this cannot result in a fault. */
>> -     INTERRUPT_RETURN
>> +     iretq
>>
>>  first_nmi:
>>       /* Restore rdx. */
>> @@ -1432,7 +1423,7 @@ first_nmi:
>>       pushfq                  /* RFLAGS */
>>       pushq   $__KERNEL_CS    /* CS */
>>       pushq   $1f             /* RIP */
>> -     INTERRUPT_RETURN        /* continues at repeat_nmi below */
>> +     iretq                   /* continues at repeat_nmi below */
>>  1:
>>  #endif
>>
>> @@ -1502,20 +1493,22 @@ nmi_restore:
>>       /*
>>        * Clear "NMI executing".  Set DF first so that we can easily
>>        * distinguish the remaining code between here and IRET from
>> -      * the SYSCALL entry and exit paths.  On a native kernel, we
>> -      * could just inspect RIP, but, on paravirt kernels,
>> -      * INTERRUPT_RETURN can translate into a jump into a
>> -      * hypercall page.
>> +      * the SYSCALL entry and exit paths.
>> +      *
>> +      * We arguably should just inspect RIP instead, but I (Andy) wrote
>> +      * this code when I had the misapprehension that Xen PV supported
>> +      * NMIs, and Xen PV would break that approach.
>>        */
>>       std
>>       movq    $0, 5*8(%rsp)           /* clear "NMI executing" */
>>
>>       /*
>> -      * INTERRUPT_RETURN reads the "iret" frame and exits the NMI
>> -      * stack in a single instruction.  We are returning to kernel
>> -      * mode, so this cannot result in a fault.
>> +      * iretq reads the "iret" frame and exits the NMI stack in a
>> +      * single instruction.  We are returning to kernel mode, so this
>> +      * cannot result in a fault.  Similarly, we don't need to worry
>> +      * about espfix64 on the way back to kernel mode.
>>        */
>> -     INTERRUPT_RETURN
>> +     iretq
>>  END(nmi)
>>
>>  ENTRY(ignore_sysret)
>>
>