Re: [PATCH 4/4] X86: Use KVM CR pin MSRs

[Arvind Sankar]

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.