On Wed, Feb 14, 2018 at 11:10:55AM +0000, Andrew Cooper wrote:
> DO_OVERWRITE_RSB clobbers %rax, meaning in practice that that the bti_ist_info
                                                           ^dup
> field gets zeroed.  Older versions of this code had the DO_OVERWRITE_RSB
> register selectable, so reintroduce this ability and use it to cause the
> INTR_IST path to use %rdx instead.
> 
> The use of %dl for the %cs.rpl check means that when an IST interrupt hits
> Xen, we try to load 1 into the high 32 bits of MSR_SPEC_CTRL, suffering a #GP
> fault instead.
> 
> Also, drop an unused label which was a copy/paste mistake.
> 
> Reported-by: Boris Ostrovsky <boris.ostrov...@oracle.com>
> Reported-by: Zhenzhong Duan <zhenzhong.d...@oracle.com>
> Signed-off-by: Andrew Cooper <andrew.coop...@citrix.com>

Reviewed-by: Roger Pau Monné <roger....@citrix.com>

> ---
> CC: Jan Beulich <jbeul...@suse.com>
> CC: Zhenzhong Duan <zhenzhong.d...@oracle.com>
> CC: Boris Ostrovsky <boris.ostrov...@oracle.com>
> CC: Wei Liu <wei.l...@citrix.com>
> CC: Roger Pau Monné <roger....@citrix.com>
> ---
>  xen/include/asm-x86/spec_ctrl_asm.h | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/xen/include/asm-x86/spec_ctrl_asm.h 
> b/xen/include/asm-x86/spec_ctrl_asm.h
> index 814f53d..1f78599 100644
> --- a/xen/include/asm-x86/spec_ctrl_asm.h
> +++ b/xen/include/asm-x86/spec_ctrl_asm.h
> @@ -79,10 +79,10 @@
>   *  - SPEC_CTRL_EXIT_TO_GUEST
>   */
>  
> -.macro DO_OVERWRITE_RSB
> +.macro DO_OVERWRITE_RSB tmp=%rax
>  /*
>   * Requires nothing
> - * Clobbers %rax, %rcx
> + * Clobbers \tmp (%rax by default), %rcx

Is it worth changing %rcx to %ecx here...

>   *
>   * 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
> @@ -97,7 +97,7 @@
>   * optimised with mov-elimination in modern cores.
>   */
>      mov $16, %ecx                   /* 16 iterations, two calls per loop */
> -    mov %rsp, %rax                  /* Store the current %rsp */
> +    mov %rsp, \tmp                  /* Store the current %rsp */
>  
>  .L\@_fill_rsb_loop:
>  
> @@ -114,7 +114,7 @@
>  
>      sub $1, %ecx
>      jnz .L\@_fill_rsb_loop
> -    mov %rax, %rsp                  /* Restore old %rsp */
> +    mov \tmp, %rsp                  /* Restore old %rsp */
>  .endm
>  
>  .macro DO_SPEC_CTRL_ENTRY_FROM_VMEXIT ibrs_val:req
> @@ -274,7 +274,7 @@
>      testb $BTI_IST_RSB, %al
>      jz .L\@_skip_rsb
>  
> -    DO_OVERWRITE_RSB
> +    DO_OVERWRITE_RSB tmp=%rdx /* Clobbers %ecx/%rdx */

... to match the comment here?

Thanks, Roger.

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