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[PATCH v6] KVM: riscv: Skip CSR restore if VCPU is reloaded on the same core

Jinyu Tang posted 1 patch 1 month, 1 week ago
There is a newer version of this series
arch/riscv/include/asm/kvm_host.h |  3 +++
arch/riscv/kvm/vcpu.c             | 22 ++++++++++++++++++++++
arch/riscv/kvm/vcpu_onereg.c      |  2 ++
3 files changed, 27 insertions(+)
[PATCH v6] KVM: riscv: Skip CSR restore if VCPU is reloaded on the same core
Posted by Jinyu Tang 1 month, 1 week ago 
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. 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.
- 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@163.com>
---
 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             | 22 ++++++++++++++++++++++
 arch/riscv/kvm/vcpu_onereg.c      |  2 ++
 3 files changed, 27 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..ff51a312d 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->arch.last_exit_cpu == cpu && !vcpu->arch.csr_dirty &&
+	    vcpu == __this_cpu_read(kvm_former_vcpu))
+		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 +664,9 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
 	void *nsh;
 	struct kvm_vcpu_csr *csr = &vcpu->arch.guest_csr;
 
+	if (vcpu != __this_cpu_read(kvm_former_vcpu))
+		__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
Re: [PATCH v6] KVM: riscv: Skip CSR restore if VCPU is reloaded on the same core
Posted by Radim Krčmář 1 month, 1 week ago 
2026-02-26T20:38:02+08:00, Jinyu Tang <tjytimi@163.com>:
> 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. 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.
> - 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@163.com>
> ---
>  v3 -> v4:
>  - 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.

Excluding AIA is disturbing as we're writing only vsiselect, hviprio1,
and hviprio2...  It seems to me that it should be fine to optimize the
AIA CSRs too.

Wasn't the issue that you originally didn't track csr_dirty, and the bug
just manifested through IMSICs?

> diff --git a/arch/riscv/kvm/vcpu.c b/arch/riscv/kvm/vcpu.c
> @@ -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->arch.last_exit_cpu == cpu && !vcpu->arch.csr_dirty &&
> +	    vcpu == __this_cpu_read(kvm_former_vcpu))
> +		goto csr_restore_done;

I see a small optimization if we set the per-cpu variable here, instead
of doing that in kvm_arch_vcpu_put:

	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;

This means we never have to read the per-cpu twice in the get/put
sequence: faster put at the cost of slightly slower get.

Thanks.

Patchew 2.1-devel-ea86937

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