[PATCH] KVM: x86: async_pf: check earlier if can deliver async pf

[Nikita Kalyazin <kalyazin@amazon.com>]

On x86, async pagefault events can only be delivered if the page fault
was triggered by guest userspace, not kernel.  This is because
the guest may be in non-sleepable context and will not be able
to reschedule.

However existing implementation pays the following overhead even for the
kernel-originated faults, even though it is known in advance that they
cannot be processed asynchronously:
 - allocate async PF token
 - create and schedule an async work

This patch avoids the overhead above in case of kernel-originated faults
by moving the `kvm_can_deliver_async_pf` check from
`kvm_arch_async_page_not_present` to `__kvm_faultin_pfn`.

Note that the existing check `kvm_can_do_async_pf` already calls
`kvm_can_deliver_async_pf` internally, however it only does that if the
`kvm_hlt_in_guest` check is true, ie userspace requested KVM not to exit
on guest halts via `KVM_CAP_X86_DISABLE_EXITS`.  In that case the code
proceeds with the async fault processing with the following
justification in 1dfdb45ec510ba27e366878f97484e9c9e728902 ("KVM: x86:
clean up conditions for asynchronous page fault handling"):

"Even when asynchronous page fault is disabled, KVM does not want to pause
the host if a guest triggers a page fault; instead it will put it into
an artificial HLT state that allows running other host processes while
allowing interrupt delivery into the guest."

Signed-off-by: Nikita Kalyazin <kalyazin@amazon.com>
---
 arch/x86/kvm/mmu/mmu.c | 3 ++-
 arch/x86/kvm/x86.c     | 5 ++---
 arch/x86/kvm/x86.h     | 2 ++
 3 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 22e7ad235123..11d29d15b6cd 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -4369,7 +4369,8 @@ static int __kvm_mmu_faultin_pfn(struct kvm_vcpu *vcpu,
 			trace_kvm_async_pf_repeated_fault(fault->addr, fault->gfn);
 			kvm_make_request(KVM_REQ_APF_HALT, vcpu);
 			return RET_PF_RETRY;
-		} else if (kvm_arch_setup_async_pf(vcpu, fault)) {
+		} else if (kvm_can_deliver_async_pf(vcpu) &&
+			kvm_arch_setup_async_pf(vcpu, fault)) {
 			return RET_PF_RETRY;
 		}
 	}
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 2e713480933a..8edae75b39f7 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -13355,7 +13355,7 @@ static inline bool apf_pageready_slot_free(struct kvm_vcpu *vcpu)
 	return !val;
 }
 
-static bool kvm_can_deliver_async_pf(struct kvm_vcpu *vcpu)
+bool kvm_can_deliver_async_pf(struct kvm_vcpu *vcpu)
 {
 
 	if (!kvm_pv_async_pf_enabled(vcpu))
@@ -13406,8 +13406,7 @@ bool kvm_arch_async_page_not_present(struct kvm_vcpu *vcpu,
 	trace_kvm_async_pf_not_present(work->arch.token, work->cr2_or_gpa);
 	kvm_add_async_pf_gfn(vcpu, work->arch.gfn);
 
-	if (kvm_can_deliver_async_pf(vcpu) &&
-	    !apf_put_user_notpresent(vcpu)) {
+	if (!apf_put_user_notpresent(vcpu)) {
 		fault.vector = PF_VECTOR;
 		fault.error_code_valid = true;
 		fault.error_code = 0;
diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index ec623d23d13d..9647f41e5c49 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -387,6 +387,8 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
 fastpath_t handle_fastpath_set_msr_irqoff(struct kvm_vcpu *vcpu);
 fastpath_t handle_fastpath_hlt(struct kvm_vcpu *vcpu);
 
+bool kvm_can_deliver_async_pf(struct kvm_vcpu *vcpu);
+
 extern struct kvm_caps kvm_caps;
 extern struct kvm_host_values kvm_host;
 

base-commit: d96c77bd4eeba469bddbbb14323d2191684da82a
-- 
2.40.1