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

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Date:   Fri, 6 Jul 2018 11:36:59 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
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 06/07/2018 11:24, Thomas Gleixner wrote:
>> The reason for this is to avoid wasting a lot of BSS memory when KVM is
>> not in use.  Thomas is going to send his take on this!
> Got it working with per cpu variables, but there is a different subtle
> issue with that.
> 
> The pvclock data is mapped into the VDSO as well, i.e. as a full page.
> 
> Right now with the linear array, which is forced to be page sized at least
> this only maps pvclock data or zeroed data (after the last CPU) into the
> VDSO.
> 
> With PER CPU variables this would map arbitraty other per cpu data which
> happens to be in the same page into the VDSO. Not really what we want.
> 
> That means to utilize PER CPU data this requires to allocate page sized
> pvclock data space for each CPU to prevent leaking arbitrary stuff.
> 
> As this data is allocated on demand, i.e. only if kvmclock is used, this
> might be tolerable, but I'm not so sure.

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.

Paolo