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[PW_SID:1057504] [v5] KVM: riscv: Skip CSR restore if VCPU is reloaded on the same core #1495

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[PW_SID:1057504] [v5] KVM: riscv: Skip CSR restore if VCPU is reloaded on the same core #1495

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3 changes: 3 additions & 0 deletions arch/riscv/include/asm/kvm_host.h
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Original file line number Diff line number Diff line change
Expand Up @@ -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;
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21 changes: 21 additions & 0 deletions arch/riscv/kvm/vcpu.c
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Expand Up @@ -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),
Expand Down Expand Up @@ -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;
}

Expand Down Expand Up @@ -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);
Expand Down Expand Up @@ -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);
Expand All @@ -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);
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2 changes: 2 additions & 0 deletions arch/riscv/kvm/vcpu_onereg.c
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Expand Up @@ -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;
}

Expand Down