Re: [PATCH] x86/PoD: defer nested P2M flushes

["Roger Pau Monné" <roger.pau@citrix.com>]

On Tue, Oct 19, 2021 at 01:58:38PM +0200, Jan Beulich wrote:
> On 19.10.2021 12:41, Roger Pau Monné wrote:
> > On Mon, Oct 11, 2021 at 10:17:08AM +0200, Jan Beulich wrote:
> >> With NPT or shadow in use, the p2m_set_entry() -> p2m_pt_set_entry() ->
> >> write_p2m_entry() -> p2m_flush_nestedp2m() call sequence triggers a lock
> >> order violation when the PoD lock is held around it. Hence such flushing
> >> needs to be deferred. Steal the approach from p2m_change_type_range().
> >>
> >> Similarly for EPT I think ept_set_entry() -> ept_sync_domain() ->
> >> ept_sync_domain_prepare() -> p2m_flush_nestedp2m() is affected.
> > 
> > I'm slightly worried by this path because it doesn't seem to
> > acknowledge defer_nested_flush.
> 
> Oh, yes. Iirc I did look at that logic, write down the remark, and
> then forgot to add the conditional to ept_sync_domain_prepare().
> The interactions with the real (host) flush are kind of ugly there,
> though - we then further depend on the ->defer_flush / ->need_flush
> logic, which is EPT-only. But I think I've convinced myself that
> this ought to work out.
> 
> > Maybe the flag should be checked by
> > p2m_flush_nestedp2m instead of leaving it to the callers?
> 
> I'm not sure this would be a good idea, as there are more callers.

We should maybe add an ASSERT to p2m_flush_nestedp2m to make sure it's
not called with defer_nested_flush being set then, or else it's
possible for new callers of p2m_flush_nestedp2m to forget checking
defer_nested_flush.

Thanks, Roger.