Re: [RFC PATCH v3 16/21] KVM: x86: Factor out kvm_use_master_clock()

[Sean Christopherson <seanjc@google.com>]

The shortlog is rather misleading.  This is more than just a refactor, and I
would argue the refactor aspect is secondary, i.e. the main goal of this patch
is to apply the exceptons to kvm_track_tsc_matching().

On Wed, May 22, 2024, David Woodhouse wrote:
> From: David Woodhouse <dwmw@amazon.co.uk>
> 
> Both kvm_track_tsc_matching() and pvclock_update_vm_gtod_copy() make a
> decision about whether the KVM clock should be in master clock mode.
> 
> They use *different* criteria for the decision though. This isn't really
> a problem; it only has the potential to cause unnecessary invocations of
> KVM_REQ_MASTERCLOCK_UPDATE if the masterclock was disabled due to TSC
> going backwards, or the guest using the old MSR. But it isn't pretty.
> 
> Factor the decision out to a single function. And document the historical
> reason why it's disabled for guests that use the old MSR_KVM_SYSTEM_TIME.
> 
> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
> ---
>  arch/x86/kvm/x86.c | 27 +++++++++++++++++++++++----
>  1 file changed, 23 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index e21b8c075bf6..437412b36cae 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2518,6 +2518,27 @@ static inline bool gtod_is_based_on_tsc(int mode)
>  }
>  #endif
>  
> +static bool kvm_use_master_clock(struct kvm *kvm)

Maybe kvm_can_use_master_clock() so that this isn't misconstrued with the actual
ka->user_master_clock field.

> +{
> +	struct kvm_arch *ka = &kvm->arch;
> +
> +	/*
> +	 * The 'old kvmclock' check is a workaround (from 2015) for a
> +	 * SUSE 2.6.16 kernel that didn't boot if the system_time in
> +	 * its kvmclock was too far behind the current time. So the
> +	 * mode of just setting the reference point and allowing time
> +	 * to proceed linearly from there makes it fail to boot.
> +	 * Despite that being kind of the *point* of the way the clock
> +	 * is exposed to the guest. By coincidence, the offending
> +	 * kernels used the old MSR_KVM_SYSTEM_TIME, which was moved
> +	 * only because it resided in the wrong number range. So the
> +	 * workaround is activated for *all* guests using the old MSR.
> +	 */
> +	return ka->all_vcpus_matched_tsc &&
> +		!ka->backwards_tsc_observed &&
> +		!ka->boot_vcpu_runs_old_kvmclock;

Please align indentation:

	return ka->all_vcpus_matched_tsc &&
	       !ka->backwards_tsc_observed &&
	       !ka->boot_vcpu_runs_old_kvmclock;

> +}
> +
>  static void kvm_track_tsc_matching(struct kvm_vcpu *vcpu)
>  {
>  #ifdef CONFIG_X86_64
> @@ -2550,7 +2571,7 @@ static void kvm_track_tsc_matching(struct kvm_vcpu *vcpu)
>  	 * To use the masterclock, the host clocksource must be based on TSC
>  	 * and all vCPUs must have matching TSC frequencies.
>  	 */
> -	bool use_master_clock = ka->all_vcpus_matched_tsc &&
> +	bool use_master_clock = kvm_use_master_clock(vcpu->kvm) &&
>  				gtod_is_based_on_tsc(gtod->clock.vclock_mode);
>  
>  	/*
> @@ -3096,9 +3117,7 @@ static void pvclock_update_vm_gtod_copy(struct kvm *kvm)
>  					&ka->master_cycle_now);
>  
>  	ka->use_master_clock = host_tsc_clocksource
> -				&& ka->all_vcpus_matched_tsc
> -				&& !ka->backwards_tsc_observed
> -				&& !ka->boot_vcpu_runs_old_kvmclock;
> +				&& kvm_use_master_clock(kvm);

Perfect opportuity to put the "&&" on the preceding line.
>  
>  	/*
>  	 * When TSC scaling is in use (which can thankfully only happen
> -- 
> 2.44.0
>