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Re: [PATCH v2] x86/PV: make post-migration page state consistent
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Date: Mon, 23 Nov 2020 12:48:43 +0100
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On Wed, Nov 04, 2020 at 08:56:50AM +0100, 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.
>
> Also use XEN_DOMCTL_PFINFO_NOTAB in the variable's initializer instead
> open coding it.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Maybe the switch could be avoided if the page is not validated or
broken? Not that I care that much.
Thanks, Roger.
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