On Fri, 6 Jul 2018, Paolo Bonzini wrote: > 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. 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. Thanks, tglx -- To unsubscribe from this list: send the line "unsubscribe linux-s390" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
