Re: [PATCH 5/5] x86/p2m: split write_p2m_entry() hook

[Tim Deegan <tim@xxxxxxx>]

At 15:04 +0100 on 12 Nov (1605193496), Jan Beulich wrote:
> On 12.11.2020 14:07, Roger Pau Monné wrote:
> > On Thu, Nov 12, 2020 at 01:29:33PM +0100, Jan Beulich wrote:
> >> I agree with all this. If only it was merely about TLB flushes. In
> >> the shadow case, shadow_blow_all_tables() gets invoked, and that
> >> one - looking at the other call sites - wants the paging lock held.
[...]
> > The post hook for shadow could pick the lock again, as I don't think
> > the removal of the tables needs to be strictly done inside of the same
> > locked region?
> 
> I think it does, or else a use of the now stale tables may occur
> before they got blown away. Tim?

Is this the call to shadow_blow_tables() in the write_p2m_entry path?
I think it would be safe to drop and re-take the paging lock there as
long as the call happens before the write is considered to have
finished.

But it would not be a useful performance improvement - any update that
takes this path is going to be very slow regardless.  So unless you
have another pressing reason to split it up, I would be inclined to
leave it as it is.  That way it's easier to see that the locking is
correct.

Cheers,

Tim.