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

[Paolo Bonzini]

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