On Fri, Jul 03, 2020 at 09:48:14PM +0000, Andersen, John wrote: > > > Is there a plan for fixing this for real? I'm wondering if there is a > > > sane weakening of this feature that still allows things like kexec. > > > > > > > I'm pretty sure kexec can be fixed. I had it working at one point, I'm > > currently in the process of revalidating this. The issue was though that > > kexec only worked within the guest, not on the physical host, which I suspect > > is related to the need for supervisor pages to be mapped, which seems to be > > required before enabling SMAP (based on what I'd seen with the selftests and > > unittests). I was also just blindly turning on the bits without checking for > > support when I'd tried this, so that could have been the issue too. > > > > I think most of the changes for just blindly enabling the bits were in > > relocate_kernel, secondary_startup_64, and startup_32. > > > > So I have a naive fix for kexec which has only been tested to work under KVM. > When tested on a physical host, it did not boot when SMAP or UMIP were set. > Undoubtedly it's not the correct way to do this, as it skips CPU feature > identification, opting instead for blindly setting the bits. The physical host > I tested this on does not have UMIP so that's likely why it failed to boot when > UMIP gets set blindly. Within kvm-unit-tests, the test for SMAP maps memory as > supervisor pages before enabling SMAP. I suspect this is why setting SMAP > blindly causes the physical host not to boot. > > Within trampoline_32bit_src() if I add more instructions I get an error > about "attempt to move .org backwards", which as I understand it means > there are only so many instructions allowed in each of those functions. > > My suspicion is that someone with more knowledge of this area has a good > idea on how best to handle this. Feedback would be much appreciated. You can simply increase the value of TRAMPOLINE_32BIT_CODE_SIZE in pgtable.h, assuming you don't need a very large increase. There's one page available for code + stack at present.
