Re: SEV guest regression in 4.18

[Brijesh Singh]

On 08/21/2018 10:19 AM, Borislav Petkov wrote:
> On Tue, Aug 21, 2018 at 09:37:56AM -0500, Brijesh Singh wrote:
> > Those variables are accessed immediately by the tsc calibration code
> > path hence we will not able to delay the allocation.
>
> If you mean, check_tsc_sync_source/_target(), those are called pretty
> late, AFAICT.
>
> And I see a setup_arch() -> init_mem_mapping -> ... ->
> kernel_physical_mapping_init() call which happens much earlier.
>
> Again, AFAICT. You probably should try it though, to make sure.

Here is a typical flow:

setup_arch()
  ....

  init_hypervisor_platform()
    kvmclock_init()
      kvm_register_clock("primary cpu clock")
        /* this shares the physical address of variable with HV.
         * Ideally you want C-bit to be cleared before we give
         * the address to HV because HV may start using the
         * variable as soon as we issue wrmsrl(MSR_KVM_SYSTEM_TIME).
         *
         * we may able to relax rules that variable should be mapped
         * with C=0 before making the wrmsrl(). Since HV will anyways
         * write the data with C=0. The important thing is when guest
         * tries to read it must map the address as C=0.
         */
       Later part of kvmclock_init provides a hooks for tsc calibration

  tsc_early_init()
     determine_cpu_tsc_frequencies()
       /* this will call tsc calibration hooks registered by the
        * kvmclock_init. The hooks basically reads those variables.
        * These variable must be mapped with C=0 otherwise we will
        * not be able to get the data written by the hypervisor.
        */

  ...
  ...
  ...
  ...

  trim_low_memory_range
  init_mem_mapping()

  ...

The tsc_early_init() is called before setup_arch() -> init_mem_mapping.

-Brijesh