Re: [PATCH v10 06/11] x86/entry: Organise the clobbering of the RSB/RAS on entry to Xen

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 25/01/18 13:19, Jan Beulich wrote:
>>>> On 24.01.18 at 14:12, <andrew.cooper3@citrix.com> wrote:
>> --- a/xen/include/asm-x86/spec_ctrl_asm.h
>> +++ b/xen/include/asm-x86/spec_ctrl_asm.h
>> @@ -74,6 +74,43 @@
>>   *  - SPEC_CTRL_EXIT_TO_GUEST
>>   */
>>  
>> +.macro DO_OVERWRITE_RSB
>> +/*
>> + * Requires nothing
>> + * Clobbers %rax, %rcx
>> + *
>> + * Requires 256 bytes of stack space, but %rsp has no net change. Based on
>> + * Google's performance numbers, the loop is unrolled to 16 iterations and two
>> + * calls per iteration.
>> + *
>> + * The call filling the RSB needs a nonzero displacement.  A nop would do, but
>> + * we use "1: pause, jmp 1b" to safely contains any ret-based speculation,
>> + * even if the loop is speculatively executed prematurely.
>> + *
>> + * %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:
>> +
>> +    .irp n, 1, 2                    /* Unrolled twice. */
>> +    call .L\@_insert_rsb_entry_\n   /* Create an RSB entry. */
>> +
>> +.L\@_capture_speculation_\n:
>> +    pause
>> +    jmp .L\@_capture_speculation_\n /* Capture rogue speculation. */
> Have you seen Linux commit 28d437d550e1e39f805d99f9f8ac399c778827b7
> ("x86/retpoline: Add LFENCE to the retpoline/RSB filling RSB
> macros")?

I saw the patch, but hadn't realised it had gone in.  There was quite a
long discussion.

>  I think we want to have the same, to please AMD. I'd
> suggest to use alternative patching though (except again on the
> IST paths), but then again maybe in a follow-up patch.

I trust the later patches are suitable in the IST regard?

~Andrew

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