Ping²: [PATCH] x86/PV: make post-migration page state consistent

[Jan Beulich <jbeulich@xxxxxxxx>]

On 11.09.2020 12:34, Jan Beulich wrote:
> When a page table page gets de-validated, its type reference count drops
> to zero (and PGT_validated gets cleared), but its type remains intact.
> XEN_DOMCTL_getpageframeinfo3, therefore, so far reported prior usage for
> such pages. An intermediate write to such a page via e.g.
> MMU_NORMAL_PT_UPDATE, however, would transition the page's type to
> PGT_writable_page, thus altering what XEN_DOMCTL_getpageframeinfo3 would
> return. In libxc the decision which pages to normalize / localize
> depends solely on the type returned from the domctl. As a result without
> further precautions the guest won't be able to tell whether such a page
> has had its (apparent) PTE entries transitioned to the new MFNs.
> 
> Add a check of PGT_validated, thus consistently avoiding normalization /
> localization in the tool stack.
> 
> Alongside using XEN_DOMCTL_PFINFO_NOTAB instead of plain zero for the
> change at hand, also change the variable's initializer to use this
> constant, too. Take the opportunity and also adjust its type.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

I think I did address all questions here.

Jan

> --- a/xen/arch/x86/domctl.c
> +++ b/xen/arch/x86/domctl.c
> @@ -215,7 +215,8 @@ long arch_do_domctl(
>  
>          for ( i = 0; i < num; ++i )
>          {
> -            unsigned long gfn = 0, type = 0;
> +            unsigned long gfn = 0;
> +            unsigned int type = XEN_DOMCTL_PFINFO_NOTAB;
>              struct page_info *page;
>              p2m_type_t t;
>  
> @@ -255,6 +256,8 @@ long arch_do_domctl(
>  
>                  if ( page->u.inuse.type_info & PGT_pinned )
>                      type |= XEN_DOMCTL_PFINFO_LPINTAB;
> +                else if ( !(page->u.inuse.type_info & PGT_validated) )
> +                    type = XEN_DOMCTL_PFINFO_NOTAB;
>  
>                  if ( page->count_info & PGC_broken )
>                      type = XEN_DOMCTL_PFINFO_BROKEN;
>