Re: [PATCH v4 02/10] x86/mm: Avoid hard-coding PAT in get_page_from_l1e()

[Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>]

On 15/12/2022 11:57 pm, Demi Marie Obenour wrote:
> diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
> index 
> 78b1972e4170cacccc9c37c6e64e76e66a7da87f..802073a01c5cf4dc3cf1d58d28ea4d4e9e8149c7
>  100644
> --- a/xen/arch/x86/mm.c
> +++ b/xen/arch/x86/mm.c
> @@ -959,15 +959,22 @@ get_page_from_l1e(
>              flip = _PAGE_RW;
>          }
>  
> -        switch ( l1f & PAGE_CACHE_ATTRS )
> +        /* Force cacheable memtypes to UC */
> +        switch ( pte_flags_to_cacheability(l1f) )
>          {
> -        case 0: /* WB */
> -            flip |= _PAGE_PWT | _PAGE_PCD;
> +        case X86_MT_UC:
> +        case X86_MT_UCM:
> +        case X86_MT_WC:
> +            /* not cached */
>              break;
> -        case _PAGE_PWT: /* WT */
> -        case _PAGE_PWT | _PAGE_PAT: /* WP */
> -            flip |= _PAGE_PCD | (l1f & _PAGE_PAT);
> +        case X86_MT_WB:
> +        case X86_MT_WT:
> +        case X86_MT_WP:
> +            /* cacheable, force to UC */
> +            flip |= (l1f & PAGE_CACHE_ATTRS) ^ _PAGE_UC;
>              break;
> +        default:
> +            BUG();

This is guest reachable.

But the more I think about it, the more I'm not sure this logic is
appropriate to begin with.  I think it needs deleting for the same
reasons as the directmap cacheability logic needed deleting in XSA-402.

~Andrew