On Tue, 2022-09-27 at 17:07 +0000, Sean Christopherson wrote: > On Mon, Sep 26, 2022, Paolo Bonzini wrote: > > 32-bit KVM has extra complications in the code due to: > > > > - different ways to write 64-bit values in VMCS > > > > - different handling of DS and ES selectors as well as FS/GS bases > > > > - lack of CR8 and EFER > > > > - lack of XFD > > > > More for the list: > > - SVM is effectively restricted to PAE kernels due to NX requirements > > > - impossibility of writing 64-bit PTEs atomically > > It's not impossible, just ugly. KVM could use CMPXCHG8B to do all of the accesses > for the TDP MMU, including the non-atomic reads and writes. > > > The last is the big one, because it prevents from using the TDP MMU > > unconditionally. > > As above, if the TDP MMU really is the sticking point, that's solvable. > > The real justification for deprecating 32-bit KVM is that, outside of KVM developers, > literally no one uses 32-bit KVM. I.e. any amount of effort that is required to > continue supporting 32-bit kernels is a complete waste of resources. > I also think that outside KVM developers nobody should be using KVM on 32 bit host. However for _developement_ I think that 32 bit KVM support is very useful, as it allows to smoke test the support for 32 bit nested hypervisors, which I do once in a while, and can even probably be useful to some users (e.g running some legacy stuff in a VM, which includes a hypervisor, especially to run really legacy OSes / custom bare metal software, using an old hypervisor) - or in other words, 32 bit nested KVM is mostly useless, but other 32 bit nested hypervisors can be useful. Yes, I can always use an older 32 bit kernel in a guest with KVM support, but as long as current kernel works, it is useful to use the same kernel on host and guest. Best regards, Maxim Levitsky
