From: Jan Beulich <jbeulich@suse.com>
To: Ian Jackson <iwj@xenproject.org>,
George Dunlap <George.Dunlap@citrix.com>
Cc: "xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
"Kevin Tian" <kevin.tian@intel.com>,
"Julien Grall" <julien@xen.org>,
"Andrew Cooper" <andrew.cooper3@citrix.com>,
"Wei Liu" <wl@xen.org>, "Roger Pau Monné" <roger.pau@citrix.com>
Subject: Re: [PATCH for-4.15 v2] VMX: use a single, global APIC access page
Date: Mon, 22 Feb 2021 08:51:59 +0100 [thread overview]
Message-ID: <dfdd4440-3c37-8cb5-b7d3-a86b7c697b2e@suse.com> (raw)
In-Reply-To: <24623.61403.440917.434@mariner.uk.xensource.com>
On 19.02.2021 18:05, Ian Jackson wrote:
> Jan Beulich writes ("Re: [PATCH v2] VMX: use a single, global APIC access page"):
>> While this patch was triggered not just by Julien's observation of
>> the early p2m insertion being a problem, but also many earlier
>> times of running into this odd code, it is - especially at this
>> stage - perhaps a possible option to split the change into just
>> the movement of the set_mmio_p2m_entry() invocation and all the
>> rest, in order to defer that rest until after 4.15.
>
> I infer that this contains a bugfix, but perhaps other
> changes/improvements too.
>
> George, I think you're our expert on this refcounting stuff - what do
> you think of this ?
>
> I guess my key question is whether this change will introduce risk by
> messing with the complex refcounting machineryt - or remove it by
> removing an interaction with the refcounting.
If anything, then the latter, but largely neither afaict - there's no
change in this regard here at all as far as the guest could affect
behavior, due to the page getting inserted as p2m_mmio_direct, and
guest_remove_page() having
if ( p2mt == p2m_mmio_direct )
{
rc = clear_mmio_p2m_entry(d, gmfn, mfn, PAGE_ORDER_4K);
goto out_put_gfn;
}
before any refcounting logic is reached. The removal of interaction
is because now the page doesn't get associated with a domain (and
hence doesn't become subject to refcounting) at all.
The risk of the change stems from going from using a per-domain
page to using a single, system-wide one, which indeed was the subject
of v1 discussion. In any event the consideration towards splitting
the change would cover either concern. Perhaps I should really do so
and submit as v3 ...
Jan
next prev parent reply other threads:[~2021-02-22 7:52 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-02-19 16:45 [PATCH v2] VMX: use a single, global APIC access page Jan Beulich
2021-02-19 16:59 ` Jan Beulich
2021-02-19 17:05 ` [PATCH for-4.15 v2] VMX: use a single, global APIC access page [and 1 more messages] Ian Jackson
2021-02-22 7:51 ` Jan Beulich [this message]
2021-02-22 10:09 ` [PATCH for-4.15 v2] VMX: use a single, global APIC access page Roger Pau Monné
2021-02-22 11:22 ` [PATCH for-4.15 v2] VMX: use a single, global APIC access page [and 1 more messages] Ian Jackson
2021-02-22 10:26 ` [PATCH for-4.15 v2] VMX: use a single, global APIC access page George Dunlap
2021-02-22 10:50 ` Jan Beulich
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