Re: [PATCH 22/22] x86/mm: zero stack on stack switch or reset

[Jan Beulich]

On 26.07.2024 17:22, Roger Pau Monne wrote:
> With the stack mapped on a per-CPU basis there's no risk of other CPUs being
> able to read the stack contents, but vCPUs running on the current pCPU could
> read stack rubble from operations of previous vCPUs.
> 
> The #DF stack is not zeroed because handling of #DF results in a panic.
> 
> Signed-off-by: Roger Pau Monné <roger....@citrix.com>
> ---
>  xen/arch/x86/include/asm/current.h | 30 +++++++++++++++++++++++++++++-
>  1 file changed, 29 insertions(+), 1 deletion(-)
> 
> diff --git a/xen/arch/x86/include/asm/current.h 
> b/xen/arch/x86/include/asm/current.h
> index 75b9a341f814..02b4118b03ef 100644
> --- a/xen/arch/x86/include/asm/current.h
> +++ b/xen/arch/x86/include/asm/current.h
> @@ -177,6 +177,14 @@ unsigned long get_stack_dump_bottom (unsigned long sp);
>  # define SHADOW_STACK_WORK ""
>  #endif
>  
> +#define ZERO_STACK                                              \
> +    "test %[stk_size], %[stk_size];"                            \
> +    "jz .L_skip_zeroing.%=;"                                    \
> +    "std;"                                                      \
> +    "rep stosb;"                                                \
> +    "cld;"                                                      \

Is ERMS actually helping with backwards copies? I didn't think so, and hence
it may be that REP STOSQ might be more efficient here?

Jan