Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Fri, 6 Jul 2018 12:50:07 +0200 (CEST)
From:   Thomas Gleixner <tglx@linutronix.de>
To:     Paolo Bonzini <pbonzini@redhat.com>
cc:     Pavel Tatashin <pasha.tatashin@oracle.com>,
        steven.sistare@oracle.com, daniel.m.jordan@oracle.com,
        linux@armlinux.org.uk, schwidefsky@de.ibm.com,
        heiko.carstens@de.ibm.com, john.stultz@linaro.org,
        sboyd@codeaurora.org, x86@kernel.org, linux-kernel@vger.kernel.org,
        mingo@redhat.com, hpa@zytor.com, douly.fnst@cn.fujitsu.com,
        peterz@infradead.org, prarit@redhat.com, feng.tang@intel.com,
        pmladek@suse.com, gnomes@lxorguk.ukuu.org.uk,
        linux-s390@vger.kernel.org
Subject: Re: [PATCH v12 04/11] kvm/x86: remove kvm memblock dependency
In-Reply-To: <alpine.DEB.2.21.1807061243400.1664@nanos.tec.linutronix.de>
Message-ID: <alpine.DEB.2.21.1807061248410.1664@nanos.tec.linutronix.de>
References: <20180621212518.19914-1-pasha.tatashin@oracle.com> <20180621212518.19914-5-pasha.tatashin@oracle.com> <fc67276e-643d-8a78-29e3-273c6dc79275@redhat.com> <alpine.DEB.2.21.1807061101040.1664@nanos.tec.linutronix.de> <52117b6e-cbdc-8583-494b-5e8e5d6d4265@redhat.com>
 <alpine.DEB.2.21.1807061142320.1664@nanos.tec.linutronix.de> <585b3646-5515-240a-db57-406d6c311a43@redhat.com> <alpine.DEB.2.21.1807061243400.1664@nanos.tec.linutronix.de>
User-Agent: Alpine 2.21 (DEB 202 2017-01-01)
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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