Re: [PATCH v5 02/10] x86/mm: Avoid hard-coding PAT in get_page_from_l1e()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 20.12.2022 02:07, Demi Marie Obenour wrote:
> get_page_from_l1e() relied on Xen's choice of PAT, which is brittle in
> the face of future PAT changes.  Instead, compute the actual cacheability
> used by the CPU and switch on that, as this will work no matter what PAT
> Xen uses.
> 
> No functional change intended.  This code is itself questionable and may
> be removed in the future, but removing it would be an observable
> behavior change and so is out of scope for this patch series.
> 
> Signed-off-by: Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>
> ---
> Changes since v4:
> - Do not add new pte_flags_to_cacheability() helper, as this code may be
>   removed in the near future and so adding a new helper for it is a bad
>   idea.
> - Do not BUG() in the event of an unexpected cacheability.  This cannot
>   happen, but it is simpler to force such types to UC than to prove that
>   the BUG() is not reachable.
> 
> Changes since v3:
> - Compute and use the actual cacheability as seen by the processor.
> 
> Changes since v2:
> - Improve commit message.
> ---
>  xen/arch/x86/mm.c | 14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)
> 
> diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
> index 
> 78b1972e4170cacccc9c37c6e64e76e66a7da87f..dba6c77ef2f7ed7fcb7f7e526583ccadd35e62cc
>  100644
> --- a/xen/arch/x86/mm.c
> +++ b/xen/arch/x86/mm.c
> @@ -959,14 +959,16 @@ get_page_from_l1e(
>              flip = _PAGE_RW;
>          }
>  
> -        switch ( l1f & PAGE_CACHE_ATTRS )
> +        switch ( 0xFF & (XEN_MSR_PAT >> (8 * pte_flags_to_cacheattr(l1f))) )
>          {
> -        case 0: /* WB */
> -            flip |= _PAGE_PWT | _PAGE_PCD;
> +        case X86_MT_UC:
> +        case X86_MT_UCM:
> +        case X86_MT_WC:
> +            /* not cacheable */
>              break;
> -        case _PAGE_PWT: /* WT */
> -        case _PAGE_PWT | _PAGE_PAT: /* WP */
> -            flip |= _PAGE_PCD | (l1f & _PAGE_PAT);
> +        default:
> +            /* cacheable */
> +            flip |= ((l1f & PAGE_CACHE_ATTRS) ^ _PAGE_UC);
>              break;

In v4 the comment here was "cacheable, force to UC". The latter aspect is
quite relevant (and iirc also what Andrew had asked for to have as a
comment). But with this now being the default case, the comment (in either
this or the earlier form) would become stale. A forward compatible way of
wording this would e.g. be "force any other type to UC". With an adjustment
along these lines (which I think could be done while committing)
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

Jan