Re: [Xen-devel] backporting considerations (Re: [PATCH v9 0/9] xen/x86: various XPTI speedups)

[George Dunlap <dunlapg@xxxxxxxxx>]

On Wed, May 16, 2018 at 10:06 AM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
>>>> On 26.04.18 at 13:33, <jgross@xxxxxxxx> wrote:
>> Juergen Gross (9):
>>   x86/xpti: avoid copying L4 page table contents when possible
>>   xen/x86: add a function for modifying cr3
>>   xen/x86: support per-domain flag for xpti
>>   xen/x86: use invpcid for flushing the TLB
>>   xen/x86: disable global pages for domains with XPTI active
>>   xen/x86: use flag byte for decision whether xen_cr3 is valid
>>   xen/x86: convert pv_guest_cr4_to_real_cr4() to a function
>>   xen/x86: add some cr3 helpers
>>   xen/x86: use PCID feature
>
> This being a performance improvement rather than a plain bug fix series,
> I'm not entirely certain about backporting here. My current thinking is to
> put this into 4.10 (Jürgen was kind enough to do the backporting work
> already), but not into any older trees. Otoh at SUSE we already have
> this in our 4.9-based branch as well, and if other consumers of that or
> the 4.8 branch would mostly agree it should go there, I could certainly
> be convinced.

Turning on XPTI causes a pretty large regression in performance; and
these reduce that regression significantly.  I'm pretty sure that most
downstreams will end up backporting these anyway (I'm sure CentOS
will); it's much better to do it officially once, rather than have
individual downstreams (most of whom do not have hypervisor developers
maintaining packages) all do it separately.

> It is imo out of question of putting it into 4.7 or older.

Is that because of the complexity?  Or because 4.7 is in "security-only" mode?

If the latter, I think the same argument applies: turning on XPTI is a
requirement for many people, and thus represents a pretty hefty
performance regression.  While we don't need to backport normal fixes
to security-only releases, we should certainly try to avoid
regressions.

(Note that this doesn't mean you personally have to do the work if you
think it's not worth your time.)

 -George

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