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 -- 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
