On Tue, Jan 06, 2015 at 11:49:09AM -0800, Andy Lutomirski wrote: > > What is the point with the new flags bit though? > > To try to work around the problem on old hosts. I'm not at all > convinced that this is worthwhile or that it helps, though. Don't think so. Just fix the host bug. > >> Also, if you do this, can you also make setting and clearing > >> STABLE_BIT properly atomic across all vCPUs? Or at least do something > >> like setting it last and clearing it first on vPCU 0? > > > > If the version "seqlock" works properly across vCPUs, why do you need > > STABLE_BIT "properly atomic" ? > > > > Please define what you mean by "properly atomic". > > > > I'd like to be able to rely using vCPU 0's pvti even from other vCPUs > in the vdso if the stable bit is set. That means that the host should > avoid doing things like migrating the guest, clearing the stable bit > for vCPU 1, resuming vCPU 1, and waiting long enough to clear the > stable bit for vCPU 0 that vCPU 1's vdso code could see invalid data > and return a bad timestamp. > > Maybe this scenario is impossible, but getting rid of any getcpu-like > operation in the vdso has really nice benefits. You can park every vCPU in host while updating vCPU-0's timestamp. See kvm_gen_update_masterclock: + /* no guest entries from this point */ + pvclock_update_vm_gtod_copy(kvm); - touch guest memory + /* guest entries allowed */ + kvm_for_each_vcpu(i, vcpu, kvm) + clear_bit(KVM_REQ_MCLOCK_INPROGRESS, &vcpu->requests); > It's faster and it > lets us guarantee that the vdso's pvti data fits in a single page. > The latter means that we can easily make it work like the hpet > mapping, which gets us 32-bit support and will *finally* let us turn > off user access to the fixmap if vsyscall=none. > > (We can, of course, still do this if the pvti data needs to be an > array, but it's messier.) > > --Andy -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
