[PATCH v7] KVM: riscv: Skip CSR restore if VCPU is reloaded on the same core
Anup Patel
anup at brainfault.org
Thu Apr 2 06:28:01 PDT 2026
On Fri, Feb 27, 2026 at 5:40 PM Jinyu Tang <tjytimi at 163.com> wrote:
>
> Currently, kvm_arch_vcpu_load() unconditionally restores guest CSRs,
> HGATP, and AIA state. 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 and AIA registers are still valid.
>
> This patch optimizes the vcpu_load path by skipping the expensive CSR
> and AIA writes if all the following conditions are met:
> 1. It is being reloaded on the same CPU (vcpu->arch.last_exit_cpu == cpu).
> 2. The CSRs are not dirty (!vcpu->arch.csr_dirty).
> 3. No other VCPU used this CPU (vcpu == __this_cpu_read(kvm_former_vcpu)).
>
> 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.
> - The 'csr_dirty' flag tracks runtime userspace interventions. If
> userspace modifies guest configurations (e.g., hedeleg via
> KVM_SET_GUEST_DEBUG, or CSRs including AIA via KVM_SET_ONE_REG),
> the flag is set to skip the fast path.
>
> With the 'csr_dirty' safeguard proven effective, it is safe to
> include kvm_riscv_vcpu_aia_load() inside the skip logic now.
>
> Signed-off-by: Jinyu Tang <tjytimi at 163.com>
Queued this patch for Linux-7.1
Thanks,
Anup
> ---
> v6 -> v7:
> - Moved kvm_riscv_vcpu_aia_load() into the fast-path skip logic, as
> suggested by Radim Krčmář.
> - Verified the fix for the IMSIC instability issue reported in v3.
> Testing was conducted on QEMU 10.0.2 with explicitly enabled AIA
> (`-machine virt,aia=aplic-imsic`). The guest boots successfully
> using virtio-mmio devices like virtio-blk and virtio-net.
>
> v5 -> v6:
> As suggested by Andrew Jones, checking 'last_exit_cpu' first (most
> likely to fail on busy hosts) and placing the expensive
> __this_cpu_read() last, skipping __this_cpu_write() in kvm_arch_vcpu_put()
> if kvm_former_vcpu is already set to the current VCPU.
>
> 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 | 24 ++++++++++++++++++++++--
> arch/riscv/kvm/vcpu_onereg.c | 2 ++
> 3 files changed, 27 insertions(+), 2 deletions(-)
>
> 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..2e4dfff07 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 +585,21 @@ 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))
> + __this_cpu_write(kvm_former_vcpu, vcpu);
> + else if (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 +643,9 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>
> kvm_riscv_mmu_update_hgatp(vcpu);
>
> + kvm_riscv_vcpu_aia_load(vcpu, cpu);
> +
> +csr_restore_done:
> kvm_riscv_vcpu_timer_restore(vcpu);
>
> kvm_riscv_vcpu_host_fp_save(&vcpu->arch.host_context);
> @@ -633,8 +655,6 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
> kvm_riscv_vcpu_guest_vector_restore(&vcpu->arch.guest_context,
> vcpu->arch.isa);
>
> - kvm_riscv_vcpu_aia_load(vcpu, cpu);
> -
> kvm_make_request(KVM_REQ_STEAL_UPDATE, vcpu);
>
> vcpu->cpu = cpu;
> 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
>
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