|| ||Ingo Molnar <mingo-AT-elte.hu>|
|| ||Linus Torvalds <torvalds-AT-linux-foundation.org>|
|| ||Re: ABI coupling to hypervisors via CONFIG_PARAVIRT|
|| ||Fri, 9 Mar 2007 21:12:57 +0100|
|| ||Jeremy Fitzhardinge <jeremy-AT-goop.org>,
Zachary Amsden <zach-AT-vmware.com>,
Thomas Gleixner <tglx-AT-linutronix.de>,
john stultz <johnstul-AT-us.ibm.com>, akpm-AT-linux-foundation.org,
Rusty Russell <rusty-AT-rustcorp.com.au>, Andi Kleen <ak-AT-suse.de>,
Chris Wright <chrisw-AT-sous-sol.org>,
Alan Cox <alan-AT-lxorguk.ukuu.org.uk>|
* Linus Torvalds <email@example.com> wrote:
> Similarly, maybe the VMI ABI doesn't allow for something that the
> kernel wants to do efficiently. Big deal. What relevance does that
> have to do with anything, except the fact that if true, the VMWare
> people are screwed? It's *their* problem.
i wont hold you up for long, but i think this is the key difference, and
if i understand your point correctly i think you are really wrong here.
This is 'enterprise Linux compatibility and ABI 101', really - i dare to
bet blindly that you wont see anyone here from distros arguing against
this simple point.
Once this thing is released upstream, it creates a new compatibility
_new kernel must not break on an older hypervisor_
due to a new paravirt_ops design. Ever. It's really that simple. (I
think i never said this explicitly because this requirement of backwards
compatibility was so obvious to me.)
And it doesnt matter whether we think that it was VMWare who messed up.
Users/customers _will_ blame us: "v2.6.25 regresses, it wont run under
ESX v1.12 anymore". Distro will yield and will undo whatever change
breaks backwards compatibility with older hypervisors. (most likely it
will be undone upstream already) Backwards compatibility acts as a very
heavy barrier against certain types of paravirt_ops design changes.
Once v2.6.21 is released, and a bigger distro releases a kernel with
CONFIG_PARAVIRT+CONFIG_VMI enabled: backwards compatibility in future
versions becomes mainly /that/ distro's problem (and upstream's
problem), _NOT_ WMware's problem.
That's why i mentioned CONFIG_COMPAT_VDSO as an example. One major
distro (SuSE 9.0) came out with that particular glibc version that had a
bug that depended on a particular and totally unintentional ABI detail
in the vDSO. As a result we had to do several iterations of
CONFIG_COMPAT_VDSO to keep backwards compatibility. And glibc is perhaps
_the_ most kernel-friendly external software project in existence.
Still, the ABI dependency was there, and we cannot break users who run
old userspace. The same rule holds here: we cannot break users who run
an old hypervisor.
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