Re: [Xen-devel] [PATCH v9 08/11] x86/entry: Clobber the Return Stack Buffer/Return Address Stack on entry to Xen

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 22/01/18 16:49, Jan Beulich wrote:
>>>> On 22.01.18 at 16:51, <andrew.cooper3@xxxxxxxxxx> wrote:
>> On 19/01/18 15:02, Jan Beulich wrote:
>>>>>> On 19.01.18 at 15:24, <andrew.cooper3@xxxxxxxxxx> wrote:
>>>> On 19/01/18 12:47, Jan Beulich wrote:
>>>>>>>> On 18.01.18 at 16:46, <andrew.cooper3@xxxxxxxxxx> wrote:
>>>>>> + * %rsp is preserved by using an extra GPR because a) we've got plenty 
>>>>>> spare,
>>>>>> + * b) the two movs are shorter to encode than `add $32*8, %rsp`, and c) 
>>>>>> can be
>>>>>> + * optimised with mov-elimination in modern cores.
>>>>>> + */
>>>>>> +    mov $16, %ecx   /* 16 iterations, two calls per loop */
>>>>>> +    mov %rsp, %rax  /* Store the current %rsp */
>>>>>> +
>>>>>> +.L\@_fill_rsb_loop:
>>>>>> +
>>>>>> +    .rept 2         /* Unrolled twice. */
>>>>>> +    call 2f         /* Create an RSB entry. */
>>>>>> +1:  pause
>>>>>> +    jmp 1b          /* Capture rogue speculation. */
>>>>>> +2:
>>>>> I won't further insist on changing away from numeric labels here, but
>>>>> I'd still like to point out an example of a high risk use of such labels 
>>>>> in
>>>>> mainline code: There's a "jz 1b" soon after
>>>>> exception_with_ints_disabled, leading across _two_ other labels and
>>>>> quite a few insns and macro invocations. May I at the very least
>>>>> suggest that you don't use 1 and 2 here?
>>>> I spent ages trying to get .L labels working here, but they don't
>>>> function inside a rept, as you end up with duplicate local symbols.
>>>>
>>>> Even using irp to inject a unique number into the loop doesn't appear to
>>>> work, because the \ escape gets interpreted as a token separator. 
>>>> AFAICT, \@ is special by virtue of the fact that it doesn't count as a
>>>> token separator.
>>>>
>>>> If you've got a better suggestion then I'm all ears.
>>>>
>>>> Alternatively, I could manually unroll the loop, or pick some arbitrary
>>>> other numbers to use.
>>> Since the unroll number is just 2, this is what I would have
>>> suggested primarily. .rept of course won't work, as it's not a
>>> macro invocation, and hence doesn't increment the internal
>>> counter. With .irp I can get things to work:
>>>
>>>     .macro m
>>>     .irp n, 1, 2
>>> .Lxyz_\@_\n:        mov     $\@, %eax
>>>     .endr
>>>     .endm
>> This appears to only work when \n is at the end of the label.  None of:
>>
>> .Lxyz_\@_\n_:    mov    $\@, %eax
>> .Lxyz_\@_\n\()_:    mov    $\@, %eax
>> .Lxyz_\n\@:    mov    $\@, %eax
>>
>> work.
> .Lxyz_\n\(\)()_\(\)@: mov     $\@, %ecx
>
> (There are two rounds of expansion, so \-s you want expanded
> in the second round need escaping for the first one.

Eww.  That's too much like Perl IMO.

I'll see if I can arrange things to work neatly with \n at the end of
the label.  If not, I'll manually unroll the loop; it is only twice
after all.

~Andrew

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