Re: [RESEND RFC 2/2] X86: Use KVM CR pin MSRs

Andersen, John S
2019-12-24T21:18:32Z

On Sun, 2019-12-22 at 23:39 -0800, Andy Lutomirski wrote:
> On Fri, Dec 20, 2019 at 11:27 AM John Andersen
> <john.s.andersen@intel.com> wrote:
> > Strengthen existing control register pinning when running
> > paravirtualized under KVM. Check which bits KVM supports pinning
> > for
> > each control register and only pin supported bits which are already
> > pinned via the existing native protection. Write to KVM CR0 and CR4
> > pinned MSRs to enable pinning.
> >
> > Initiate KVM assisted pinning directly following the setup of
> > native
> > pinning on boot CPU. For non-boot CPUs initiate paravirtualized
> > pinning
> > on CPU identification.
> >
> > Identification of non-boot CPUs takes place after the boot CPU has
> > setup
> > native CR pinning. Therefore, non-boot CPUs access pinned bits
> > setup by
> > the boot CPU and request that those be pinned. All CPUs request
> > paravirtualized pinning of the same bits which are already pinned
> > natively.
> >
> > Guests using the kexec system call currently do not support
> > paravirtualized control register pinning. This is due to early boot
> > code writing known good values to control registers, these values
> > do
> > not contain the protected bits. This is due to CPU feature
> > identification being done at a later time, when the kernel properly
> > checks if it can enable protections.
>
> Is hibernation supported? How about suspend-to-RAM?
>

Something is writing to CR4 during resume which is breaking
hibernation. Unfortunately I hadn't been able to get my hibernation
test working before sending this out. I will investigate.

> FWIW, I think that handling these details through Kconfig is the
> wrong
> choice. Distribution kernels should enable this, and they're not
> going to turn off kexec. Arguably kexec should be made to work --
> there is no fundamental reason that kexec should need to fiddle with
> CR0.WP, for example. But a boot option could also work as a
> short-term option.

Given the situation with hibernation. I think I'll implement the kexec
discovery Liran suggested, and then investigate the hibernate situation
further.

Thanks,
John