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Re: [PATCH 7/8] x86/mm: adjust type check around _get_page_type()'s TLB flush
- To: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Fri, 12 Aug 2022 10:51:35 +0200
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- Cc: Wei Liu <wl@xxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>, George Dunlap <George.Dunlap@xxxxxxxxxx>, "Tim (Xen.org)" <tim@xxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Fri, 12 Aug 2022 08:51:45 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 27.07.2022 20:25, Andrew Cooper wrote:
> On 26/07/2022 17:06, Jan Beulich wrote:
>> While "type" can include PGT_pae_xen_l2, "x" can't, as the bit is
>> cleared upon de-validation (see also the respective assertion earlier in
>> the function).
>
> While this statement is true, it doesn't really explain why this is
> relevant (or not) to TLB flushing.
>
> As far as the change goes, it's safe on 64bit builds of Xen (I think),
> but would not be on 32bit builds when this logic was first written.
Actually no, I don't think it's safe, and I therefore withdraw the
patch. (I'll re-base the subsequent one accordingly, which you did
give R-b for already.) Whatever content may have been in an L2 table
which is to become a PGT_pae_xen_l2 one may still be in TLBs / paging
structure caches, yet we'll replace those entries without further
flushing.
The opposite direction (previously PGT_pae_xen_l2 trying to become an
ordinary L2) is "fine": Due to the Xen entries still there, its
validation will simply fail.
Jan
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