Re: [PATCH 4/5] x86/PV: restrict TLB flushing after mod_l[234]_entry()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 03/11/2020 10:57, Jan Beulich wrote:
> Just like we avoid to invoke remote root pt flushes when all uses of an
> L4 table can be accounted for locally, the same can be done for all of
> L[234] for the linear pt flush when the table is a "free floating" one,
> i.e. it is pinned but not hooked up anywhere. While this situation
> doesn't occur very often, it can be observed.
>
> Since this breaks one of the implications of the XSA-286 fix, drop the
> flush_root_pt_local variable again and set ->root_pgt_changed directly,
> just like it was before that change.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> While adjusting the big comment that was added for XSA-286 I wondered
> why it talks about the "construction of 32bit PV guests". How are 64-bit
> PV guests different in this regard?

Because the sole caller is move_l3_below_4G() for 32bit PV guests which
don't support folded CR3's.

It's not impossible that future changes to PV construction might change
this, but it is highly unlikely in practice.

~Andrew