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KVM: arm64: nv: Punt stage-2 recycling to a vCPU request

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Currently, when a nested MMU is repurposed for some other MMU context,
KVM unmaps everything during vcpu_load() while holding the MMU lock for
write. This is quite a performance bottleneck for large nested VMs, as
all vCPU scheduling will spin until the unmap completes.

Start punting the MMU cleanup to a vCPU request, where it is then
possible to periodically release the MMU lock and CPU in the presence of
contention.

Ensure that no vCPU winds up using a stale MMU by tracking the pending
unmap on the S2 MMU itself and requesting an unmap on every vCPU that
finds it.

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20241007233028.2236133-4-oliver.upton@linux.dev
Signed-off-by: Marc Zyngier <maz@kernel.org>
This commit is contained in:
Oliver Upton 2024-10-07 23:30:27 +00:00 committed by Marc Zyngier
parent 3c164eb946
commit c268f204f7
4 changed files with 37 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

7
arch/arm64/include/asm/kvm_host.h
View File

@ -51,6 +51,7 @@
#define KVM_REQ_RELOAD_PMU KVM_ARCH_REQ(5)
#define KVM_REQ_SUSPEND KVM_ARCH_REQ(6)
#define KVM_REQ_RESYNC_PMU_EL0 KVM_ARCH_REQ(7)
#define KVM_REQ_NESTED_S2_UNMAP KVM_ARCH_REQ(8)
#define KVM_DIRTY_LOG_MANUAL_CAPS (KVM_DIRTY_LOG_MANUAL_PROTECT_ENABLE | \
KVM_DIRTY_LOG_INITIALLY_SET)
@ -211,6 +212,12 @@ struct kvm_s2_mmu {
*/
bool nested_stage2_enabled;
/*
* true when this MMU needs to be unmapped before being used for a new
* purpose.
*/
bool pending_unmap;
/*
* 0: Nobody is currently using this, check vttbr for validity
* >0: Somebody is actively using this.

2
arch/arm64/include/asm/kvm_nested.h
View File

@ -78,6 +78,8 @@ extern void kvm_s2_mmu_iterate_by_vmid(struct kvm *kvm, u16 vmid,
extern void kvm_vcpu_load_hw_mmu(struct kvm_vcpu *vcpu);
extern void kvm_vcpu_put_hw_mmu(struct kvm_vcpu *vcpu);
extern void check_nested_vcpu_requests(struct kvm_vcpu *vcpu);
struct kvm_s2_trans {
phys_addr_t output;
unsigned long block_size;

2
arch/arm64/kvm/arm.c
View File

@ -1031,6 +1031,8 @@ static int check_vcpu_requests(struct kvm_vcpu *vcpu)
if (kvm_dirty_ring_check_request(vcpu))
return 0;
check_nested_vcpu_requests(vcpu);
}
return 1;

28
arch/arm64/kvm/nested.c
View File

@ -632,9 +632,9 @@ static struct kvm_s2_mmu *get_s2_mmu_nested(struct kvm_vcpu *vcpu)
/* Set the scene for the next search */
kvm->arch.nested_mmus_next = (i + 1) % kvm->arch.nested_mmus_size;
/* Clear the old state */
/* Make sure we don't forget to do the laundry */
if (kvm_s2_mmu_valid(s2_mmu))
kvm_stage2_unmap_range(s2_mmu, 0, kvm_phys_size(s2_mmu), false);
s2_mmu->pending_unmap = true;
/*
* The virtual VMID (modulo CnP) will be used as a key when matching
@ -650,6 +650,16 @@ static struct kvm_s2_mmu *get_s2_mmu_nested(struct kvm_vcpu *vcpu)
out:
atomic_inc(&s2_mmu->refcnt);
/*
* Set the vCPU request to perform an unmap, even if the pending unmap
* originates from another vCPU. This guarantees that the MMU has been
* completely unmapped before any vCPU actually uses it, and allows
* multiple vCPUs to lend a hand with completing the unmap.
*/
if (s2_mmu->pending_unmap)
kvm_make_request(KVM_REQ_NESTED_S2_UNMAP, vcpu);
return s2_mmu;
}
@ -1199,3 +1209,17 @@ int kvm_init_nv_sysregs(struct kvm *kvm)
return 0;
}
void check_nested_vcpu_requests(struct kvm_vcpu *vcpu)
{
if (kvm_check_request(KVM_REQ_NESTED_S2_UNMAP, vcpu)) {
struct kvm_s2_mmu *mmu = vcpu->arch.hw_mmu;
write_lock(&vcpu->kvm->mmu_lock);
if (mmu->pending_unmap) {
kvm_stage2_unmap_range(mmu, 0, kvm_phys_size(mmu), true);
mmu->pending_unmap = false;
}
write_unlock(&vcpu->kvm->mmu_lock);
}
}