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Re: [PATCH] x86/PoD: defer nested P2M flushes
- To: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Tue, 19 Oct 2021 13:58:38 +0200
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- Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, George Dunlap <george.dunlap@xxxxxxxxxx>
- Delivery-date: Tue, 19 Oct 2021 11:58:53 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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.
Jan
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