[PATCH v5] KVM: riscv: Skip CSR restore if VCPU is reloaded on the same core

[Jinyu Tang]

Currently, kvm_arch_vcpu_load() unconditionally restores guest CSRs and
HGATP. However, when a VCPU is loaded back on the same physical CPU, 
and no other KVM VCPU has run on this CPU since it was last put,
the hardware CSRs are still valid.

This patch optimizes the vcpu_load path by skipping the expensive CSR
writes if all the following conditions are met:
1. It is being reloaded on the same CPU (vcpu->arch.last_exit_cpu == cpu).
2. No other VCPU used this CPU (vcpu == __this_cpu_read(kvm_former_vcpu)).
3. The CSRs are not dirty (!vcpu->arch.csr_dirty).

To ensure this fast-path doesn't break corner cases:
- Live migration and VCPU reset are naturally safe. KVM initializes
  last_exit_cpu to -1, which guarantees the fast-path won't trigger.
- A new 'csr_dirty' flag is introduced to track runtime userspace
  interventions. If userspace modifies guest configurations (e.g.,
  hedeleg via KVM_SET_GUEST_DEBUG, or CSRs via KVM_SET_ONE_REG) while
  the VCPU is preempted, the flag is set to skip fast path.

Note that kvm_riscv_vcpu_aia_load() is kept outside the skip logic
to ensure IMSIC/AIA interrupt states are always properly
synchronized.

Signed-off-by: Jinyu Tang <tjytimi@xxxxxxx>
---
 v4 -> v5:
 - Dropped the 'vcpu->scheduled_out' check as Andrew Jones pointed out,
   relying on 'last_exit_cpu', 'former_vcpu', and '!csr_dirty'
   is sufficient and safe. This expands the optimization to cover many
   userspace exits (e.g., MMIO) as well.
 - Added a block comment in kvm_arch_vcpu_load() to warn future
   developers about maintaining the 'csr_dirty' dependency, as Andrew's
   suggestion to reduce fragility.
 - Removed unnecessary single-line comments and fixed indentation nits.

 v3 -> v4:
 - Addressed Anup Patel's review regarding hardware state inconsistency.
 - Introduced 'csr_dirty' flag to track dynamic userspace CSR/CONFIG
   modifications (KVM_SET_ONE_REG, KVM_SET_GUEST_DEBUG), forcing a full
   restore when debugging or modifying states at userspace.
 - Kept kvm_riscv_vcpu_aia_load() out of the skip block to resolve IMSIC
   VS-file instability.

 v2 -> v3:
 v2 was missing a critical check because I generated the patch from my
 wrong (experimental) branch. This is fixed in v3. Sorry for my trouble.

 v1 -> v2:
 Apply the logic to aia csr load. Thanks for Andrew Jones's advice.
---
 arch/riscv/include/asm/kvm_host.h |  3 +++
 arch/riscv/kvm/vcpu.c             | 21 +++++++++++++++++++++
 arch/riscv/kvm/vcpu_onereg.c      |  2 ++
 3 files changed, 26 insertions(+)

diff --git a/arch/riscv/include/asm/kvm_host.h b/arch/riscv/include/asm/kvm_host.h
index 24585304c..7ee47b83c 100644
--- a/arch/riscv/include/asm/kvm_host.h
+++ b/arch/riscv/include/asm/kvm_host.h
@@ -273,6 +273,9 @@ struct kvm_vcpu_arch {
 	/* 'static' configurations which are set only once */
 	struct kvm_vcpu_config cfg;
 
+	/* Indicates modified guest CSRs */
+	bool csr_dirty;
+
 	/* SBI steal-time accounting */
 	struct {
 		gpa_t shmem;
diff --git a/arch/riscv/kvm/vcpu.c b/arch/riscv/kvm/vcpu.c
index a55a95da5..578f8693a 100644
--- a/arch/riscv/kvm/vcpu.c
+++ b/arch/riscv/kvm/vcpu.c
@@ -24,6 +24,8 @@
 #define CREATE_TRACE_POINTS
 #include "trace.h"
 
+static DEFINE_PER_CPU(struct kvm_vcpu *, kvm_former_vcpu);
+
 const struct _kvm_stats_desc kvm_vcpu_stats_desc[] = {
 	KVM_GENERIC_VCPU_STATS(),
 	STATS_DESC_COUNTER(VCPU, ecall_exit_stat),
@@ -537,6 +539,8 @@ int kvm_arch_vcpu_ioctl_set_guest_debug(struct kvm_vcpu *vcpu,
 		vcpu->arch.cfg.hedeleg |= BIT(EXC_BREAKPOINT);
 	}
 
+	vcpu->arch.csr_dirty = true;
+
 	return 0;
 }
 
@@ -581,6 +585,20 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 	struct kvm_vcpu_csr *csr = &vcpu->arch.guest_csr;
 	struct kvm_vcpu_config *cfg = &vcpu->arch.cfg;
 
+	/*
+	 * If VCPU is being reloaded on the same physical CPU and no
+	 * other KVM VCPU has run on this CPU since it was last put,
+	 * we can skip the expensive CSR and HGATP writes.
+	 *
+	 * Note: If a new CSR is added to this fast-path skip block,
+	 * make sure that 'csr_dirty' is set to true in any
+	 * ioctl (e.g., KVM_SET_ONE_REG) that modifies it.
+	 */
+	if (vcpu == __this_cpu_read(kvm_former_vcpu) &&
+	    vcpu->arch.last_exit_cpu == cpu && !vcpu->arch.csr_dirty)
+		goto csr_restore_done;
+
+	vcpu->arch.csr_dirty = false;
 	if (kvm_riscv_nacl_sync_csr_available()) {
 		nsh = nacl_shmem();
 		nacl_csr_write(nsh, CSR_VSSTATUS, csr->vsstatus);
@@ -624,6 +642,7 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 
 	kvm_riscv_mmu_update_hgatp(vcpu);
 
+csr_restore_done:
 	kvm_riscv_vcpu_timer_restore(vcpu);
 
 	kvm_riscv_vcpu_host_fp_save(&vcpu->arch.host_context);
@@ -645,6 +664,8 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
 	void *nsh;
 	struct kvm_vcpu_csr *csr = &vcpu->arch.guest_csr;
 
+	__this_cpu_write(kvm_former_vcpu, vcpu);
+
 	vcpu->cpu = -1;
 
 	kvm_riscv_vcpu_aia_put(vcpu);
diff --git a/arch/riscv/kvm/vcpu_onereg.c b/arch/riscv/kvm/vcpu_onereg.c
index e7ab6cb00..fc08bf833 100644
--- a/arch/riscv/kvm/vcpu_onereg.c
+++ b/arch/riscv/kvm/vcpu_onereg.c
@@ -652,6 +652,8 @@ static int kvm_riscv_vcpu_set_reg_csr(struct kvm_vcpu *vcpu,
 	if (rc)
 		return rc;
 
+	vcpu->arch.csr_dirty = true;
+
 	return 0;
 }
 
-- 
2.43.0