[RFC PATCH 1/4] KVM: arm/arm64: Introduce kvm_arch_vcpu_run_pid_change

[Dave Martin]

From: Christoffer Dall <christoffer.dall at linaro.org>

KVM/ARM differs from other architectures in having to maintain an
additional virtual address space from that of the host and the guest,
because we split the execution of KVM across both EL1 and EL2.

This results in a need to explicitly map data structures into EL2 (hyp)
which are accessed from the hyp code.  As we are about to be more clever
with our FPSIMD handling, which stores data on the task struct and uses
thread_info flags, we have to map the currently executing task struct
into the EL2 virtual address space.

However, we don't want to do this on every KVM_RUN, because it is a
fairly expensive operation to walk the page tables, and the common
execution mode is to map a single thread to a VCPU.  By introducing a
hook that architectures can select with HAVE_KVM_VCPU_RUN_PID_CHANGE, we
do not introduce overhead for other architectures, but have a simple way
to only map the data we need when required for arm64.

Signed-off-by: Christoffer Dall <christoffer.dall at linaro.org>
Signed-off-by: Dave Martin <Dave.Martin at arm.com>

---

The use of hyp_current to pass the value of "current" to hyp is backed
out compared with Chritoffer's original patch.

Instead, I do most of the grubbing around in task_struct to the host
side to reduce the amount of stuff that needs to be duplicated in hyp.
---
 arch/arm64/kvm/Kconfig   |  1 +
 include/linux/kvm_host.h |  9 +++++++++
 virt/kvm/Kconfig         |  3 +++
 virt/kvm/arm/arm.c       | 10 ++++++++++
 virt/kvm/kvm_main.c      |  7 ++++++-
 5 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/Kconfig b/arch/arm64/kvm/Kconfig
index 2257dfc..5b2c8d8 100644
--- a/arch/arm64/kvm/Kconfig
+++ b/arch/arm64/kvm/Kconfig
@@ -39,6 +39,7 @@ config KVM
 	select HAVE_KVM_IRQ_ROUTING
 	select IRQ_BYPASS_MANAGER
 	select HAVE_KVM_IRQ_BYPASS
+	select HAVE_KVM_VCPU_RUN_PID_CHANGE
 	---help---
 	  Support hosting virtualized guest machines.
 	  We don't support KVM with 16K page tables yet, due to the multiple
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index ac0062b..10a37b1 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -1272,4 +1272,13 @@ static inline long kvm_arch_vcpu_async_ioctl(struct file *filp,
 }
 #endif /* CONFIG_HAVE_KVM_VCPU_ASYNC_IOCTL */
 
+#ifdef CONFIG_HAVE_KVM_VCPU_RUN_PID_CHANGE
+int kvm_arch_vcpu_run_pid_change(struct kvm_vcpu *vcpu);
+#else
+static inline int kvm_arch_vcpu_run_pid_change(struct kvm_vcpu *vcpu)
+{
+	return 0;
+}
+#endif /* CONFIG_HAVE_KVM_VCPU_RUN_PID_CHANGE */
+
 #endif
diff --git a/virt/kvm/Kconfig b/virt/kvm/Kconfig
index cca7e06..72143cf 100644
--- a/virt/kvm/Kconfig
+++ b/virt/kvm/Kconfig
@@ -54,3 +54,6 @@ config HAVE_KVM_IRQ_BYPASS
 
 config HAVE_KVM_VCPU_ASYNC_IOCTL
        bool
+
+config HAVE_KVM_VCPU_RUN_PID_CHANGE
+       bool
diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
index 86941f6..02a153a 100644
--- a/virt/kvm/arm/arm.c
+++ b/virt/kvm/arm/arm.c
@@ -822,6 +822,16 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
 	return ret;
 }
 
+#ifdef CONFIG_ARM64
+int kvm_arch_vcpu_run_pid_change(struct kvm_vcpu *vcpu)
+{
+	struct task_struct *tsk = current;
+
+	/* Make sure the host task fpsimd state is visible to hyp: */
+	return create_hyp_mappings(tsk, tsk + 1, PAGE_HYP);
+}
+#endif
+
 static int vcpu_interrupt_line(struct kvm_vcpu *vcpu, int number, bool level)
 {
 	int bit_index;
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 4501e65..dbd35ab 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2551,8 +2551,13 @@ static long kvm_vcpu_ioctl(struct file *filp,
 		oldpid = rcu_access_pointer(vcpu->pid);
 		if (unlikely(oldpid != current->pids[PIDTYPE_PID].pid)) {
 			/* The thread running this VCPU changed. */
-			struct pid *newpid = get_task_pid(current, PIDTYPE_PID);
+			struct pid *newpid;
 
+			r = kvm_arch_vcpu_run_pid_change(vcpu);
+			if (r)
+				break;
+
+			newpid = get_task_pid(current, PIDTYPE_PID);
 			rcu_assign_pointer(vcpu->pid, newpid);
 			if (oldpid)
 				synchronize_rcu();
-- 
2.1.4