Re: [PATCH 11/16] x86/spec-ctrl: Adjust DO_OVERWRITE_RSB to be shadow stack compatible

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 07/05/2020 14:22, Jan Beulich wrote:
> On 02.05.2020 00:58, Andrew Cooper wrote:
>> @@ -114,6 +114,16 @@
>>      sub $1, %ecx
>>      jnz .L\@_fill_rsb_loop
>>      mov %\tmp, %rsp                 /* Restore old %rsp */
>> +
>> +#ifdef CONFIG_XEN_SHSTK
>> +    mov $1, %ecx
>> +    rdsspd %ecx
>> +    cmp $1, %ecx
>> +    je .L\@_shstk_done
>> +    mov $64, %ecx                   /* 64 * 4 bytes, given incsspd */
>> +    incsspd %ecx                    /* Restore old SSP */
>> +.L\@_shstk_done:
>> +#endif
> The latest here I wonder why you don't use alternatives patching.
> I thought that's what you've introduced the synthetic feature
> flag for.

We're already in the middle of an alternative and they don't nest.  More
importantly, this path gets used on the BSP, after patching and before
CET gets enabled.

~Andrew