On Mon, May 7, 2018 at 5:16 AM Jan Beulich <jbeul...@suse.com> wrote:
> While on native entry into the kernel happens on the trampoline stack, > PV Xen kernels are being entered with the current thread stack right > away. Hence source and destination stacks are identical in that case, > and special care is needed. > Other than in sync_regs() the copying done on the INT80 path as well as > on the NMI path itself isn't NMI / #MC safe, as either of these events > occurring in the middle of the stack copying would clobber data on the > (source) stack. (Of course, in the NMI case only #MC could break > things.) I think I'd rather fix this by changing the stack switch code or alternativing around it on non-stack-switching kernels. Or make Xen use a trampoline stack just like native. > I'm not altering the similar code in interrupt_entry(), as that code > path is unreachable when running an PV Xen guest afaict. > Signed-off-by: Jan Beulich <jbeul...@suse.com> > Cc: sta...@kernel.org > --- > There would certainly have been the option of using alternatives > patching, but afaict the patching code isn't NMI / #MC safe, so I'd > rather stay away from patching the NMI path. And I thought it would be > better to use similar code in both cases. I would hope we do the patching before we enable any NMIs. > Another option would be to make the Xen case match the native one, by > going through the trampoline stack, but to me this would look like extra > overhead for no gain. Avoiding even more complexity in the nmi code seems like a big gain to me.
