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
