Re: [Xen-devel] 回复: [PATCH] x86/spec_ctrl: Fix several bugs in SPEC_CTRL_ENTRY_FROM_INTR_IST

2018-02-14 Thread Andrew Cooper 
On 14/02/18 12:08, zhenzhong.duan wrote:
>
> > @@ -286,13 +286,13 @@
> >  setz %dl
> >  and %dl, STACK_CPUINFO_FIELD(use_shadow_spec_ctrl)(%r14)
> Is it safe to remove the 'xor %edx, %edx' above? setz set whole byte 1
> or 0.
>

It is safe, but it is not a good idea.

Using setz is an 8bit operation, which will suffer a register merge
stall in the pipeline as we know for certain at this point that the
upper bits of %edx are nonzero at this point.  (An encoding which
allowed setz %eax would have been far more useful in 64bit code.)

~Andrew

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[Xen-devel] 回复: [PATCH] x86/spec_ctrl: Fix several bugs in SPEC_CTRL_ENTRY_FROM_INTR_IST

2018-02-14 Thread zhenzhong.duan 

2018年2月14日 19:10于 Andrew Cooper 写道：
>
> DO_OVERWRITE_RSB clobbers %rax, meaning in practice that that the 
> bti_ist_info 
> 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  
> Reported-by: Zhenzhong Duan  
> Signed-off-by: Andrew Cooper  
> --- 
> CC: Jan Beulich  
> CC: Zhenzhong Duan  
> CC: Boris Ostrovsky  
> CC: Wei Liu  
> CC: Roger Pau Monné  
> --- 
> 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 
>   * 
>   * 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 */ 
>
> .L\@_skip_rsb: 
>
> @@ -286,13 +286,13 @@ 
>  setz %dl 
>  and %dl, STACK_CPUINFO_FIELD(use_shadow_spec_ctrl)(%r14) 
Is it safe to remove the 'xor %edx, %edx' above? setz set whole byte 1 or 0.
---
thanks
zduan
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