Re: [PATCH v6 2/5] KVM: x86: Dirty quota-based throttling of vcpus

[Sean Christopherson <seanjc@google.com>]

On Thu, Sep 15, 2022, Shivam Kumar wrote:
> index c9b49a09e6b5..140a5511ed45 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -5749,6 +5749,9 @@ static int handle_invalid_guest_state(struct kvm_vcpu *vcpu)
>  		 */
>  		if (__xfer_to_guest_mode_work_pending())
>  			return 1;
> +
> +		if (!kvm_vcpu_check_dirty_quota(vcpu))

A dedicated helper is no longer necessary, this can _test_ the event, a la
KVM_REQ_EVENT above:

		if (kvm_test_request(KVM_REQ_DIRTY_QUOTA_EXIT, vcpu))
			return 1;

> +			return 0;
>  	}
>  
>  	return 1;
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 43a6a7efc6ec..8447e70b26f5 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -10379,6 +10379,15 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  			r = 0;
>  			goto out;
>  		}
> +		if (kvm_check_request(KVM_REQ_DIRTY_QUOTA_EXIT, vcpu)) {
> +			struct kvm_run *run = vcpu->run;
> +

> +			run->exit_reason = KVM_EXIT_DIRTY_QUOTA_EXHAUSTED;
> +			run->dirty_quota_exit.count = vcpu->stat.generic.pages_dirtied;
> +			run->dirty_quota_exit.quota = vcpu->dirty_quota;

As mentioned in patch 1, the code code snippet I suggested is bad.  With a request,
there's no need to snapshot the quota prior to making the request.  If userspace
changes the quota, then using the new quota is perfectly ok since KVM is still
providing sane data.  If userspace lowers the quota, an exit will still ocurr as
expected.  If userspace disables or increases the quota to the point where no exit
is necessary, then userspace can't expect and exit and won't even be aware that KVM
temporarily detected an exhausted quota.

And all of this belongs in a common KVM helper.  Name isn't pefect, but it aligns
with kvm_check_request().

#ifdef CONFIG_HAVE_KVM_DIRTY_QUOTA
static inline bool kvm_check_dirty_quota_request(struct kvm_vcpu *vcpu)
{
	struct kvm_run *run = vcpu->run;

	run->exit_reason = KVM_EXIT_DIRTY_QUOTA_EXHAUSTED;
	run->dirty_quota_exit.count = vcpu->stat.generic.pages_dirtied;
	run->dirty_quota_exit.quota = READ_ONCE(run->dirty_quota);

	/*
	 * Re-check the quota and exit if and only if the vCPU still exceeds its
	 * quota.  If userspace increases (or disables entirely) the quota, then
	 * no exit is required as KVM is still honoring its ABI, e.g. userspace
	 * won't even be aware that KVM temporarily detected an exhausted quota.
	 */
	return run->dirty_quota_exit.count >= run->dirty_quota_exit.quota; 
}
#endif

And then arch usage is simply something like:

		if (kvm_check_dirty_quota_request(vcpu)) {
			r = 0;
			goto out;
		}