linux-kernel - Re: [PATCH v12 04/11] kvm/x86: remove kvm memblock dependency

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Message-ID: <alpine.DEB.2.21.1807061248410.1664@nanos.tec.linutronix.de>
Date:   Fri, 6 Jul 2018 12:50:07 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Paolo Bonzini <pbonzini@...hat.com>
cc:     Pavel Tatashin <pasha.tatashin@...cle.com>,
        steven.sistare@...cle.com, daniel.m.jordan@...cle.com,
        linux@...linux.org.uk, schwidefsky@...ibm.com,
        heiko.carstens@...ibm.com, john.stultz@...aro.org,
        sboyd@...eaurora.org, x86@...nel.org, linux-kernel@...r.kernel.org,
        mingo@...hat.com, hpa@...or.com, douly.fnst@...fujitsu.com,
        peterz@...radead.org, prarit@...hat.com, feng.tang@...el.com,
        pmladek@...e.com, gnomes@...rguk.ukuu.org.uk,
        linux-s390@...r.kernel.org
Subject: Re: [PATCH v12 04/11] kvm/x86: remove kvm memblock dependency

On Fri, 6 Jul 2018, Thomas Gleixner wrote:
> On Fri, 6 Jul 2018, Paolo Bonzini wrote:
> > On 06/07/2018 11:45, Thomas Gleixner wrote:
> > >> One possibility is to introduce another layer of indirection: in
> > >> addition to the percpu pvclock data, add a percpu pointer to the pvclock
> > >> data and initialize it to point to a page-aligned variable in BSS.  CPU0
> > >> (used by vDSO) doesn't touch the pointer and keeps using the BSS
> > >> variable, APs instead redirect the pointer to the percpu data.
> > > Yeah, thought about that, but the extra indirection is ugly. Instead of
> > > using per cpu data, I just can allocate the memory _after_ the allocators
> > > are up and running and use a single page sized static __initdata for the
> > > early boot.
> > 
> > Either works for me.  Assembly-wise, the indirection should be more or
> > less the same as what we have now; even more efficient because it
> > accesses a percpu pointer instead of computing it based on
> > smp_processor_id().
> 
> Good point. Let me try that.

Duh, that either requires a static key or a static PER_CPU pointer thingy,
which then wastes 8 bytes per cpu instead of 64 byte if unused.

Thanks,

	tglx