Re: [PATCH v2 for-4.18?] x86: support data operand independent timing mode

[Henry Wang <Henry.Wang@xxxxxxx>]

Hi Jan,

> On Sep 11, 2023, at 23:01, Jan Beulich <jbeulich@xxxxxxxx> wrote:
> 
> [1] specifies a long list of instructions which are intended to exhibit
> timing behavior independent of the data they operate on. On certain
> hardware this independence is optional, controlled by a bit in a new
> MSR. Provide a command line option to control the mode Xen and its
> guests are to operate in, with a build time control over the default.
> Longer term we may want to allow guests to control this.
> 
> Since Arm64 supposedly also has such a control, put command line option
> and Kconfig control in common files.
> 
> [1] 
> https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/best-practices/data-operand-independent-timing-isa-guidance.html
> 
> Requested-by: Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> This may be viewed as a new feature, and hence be too late for 4.18. It
> may, however, also be viewed as security relevant, which is why I'd like
> to propose to at least consider it.

Fine with me if this patch can be properly reviewed on time, because of
the security relevance. 

> 
> Slightly RFC, in particular for whether the Kconfig option should
> default to Y or N.
> 
> I would have wanted to invoke setup_doitm() from cpu_init(), but that
> works only on the BSP. On APs cpu_init() runs before ucode loading.
> Plus recheck_cpu_features() invoking identify_cpu() takes care of the
> BSP during S3 resume.
> ---
> v2: Introduce and use cpu_has_doitm. Add comment "borrowed" from the
>    XenServer patch queue patch providing similar functionality.
>    Re-base.
> 
> --- a/docs/misc/xen-command-line.pandoc
> +++ b/docs/misc/xen-command-line.pandoc
> @@ -788,6 +788,14 @@ Specify the size of the console debug tr
> additionally a trace buffer of the specified size is allocated per cpu.
> The debug trace feature is only enabled in debugging builds of Xen.
> 
> +### dit (x86)
> +> `= <boolean>`
> +
> +> Default: `CONFIG_DIT_DEFAULT`
> +
> +Specify whether Xen and guests should operate in Data Independent Timing
> +mode.
> +

Since a new cmdline interface is added, I am wondering would such
addtion deserves a CHANGELOG entry?

Kind regards,
Henry