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Message-ID: <YJGqzZ/8CS8mSx2c@google.com>
Date: Tue, 4 May 2021 20:13:01 +0000
From: Sean Christopherson <seanjc@...gle.com>
To: Ben Gardon <bgardon@...gle.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
Paolo Bonzini <pbonzini@...hat.com>,
Peter Xu <peterx@...hat.com>, Peter Shier <pshier@...gle.com>,
Junaid Shahid <junaids@...gle.com>,
Jim Mattson <jmattson@...gle.com>,
Yulei Zhang <yulei.kernel@...il.com>,
Wanpeng Li <kernellwp@...il.com>,
Vitaly Kuznetsov <vkuznets@...hat.com>,
Xiao Guangrong <xiaoguangrong.eric@...il.com>
Subject: Re: [PATCH v2 7/7] KVM: x86/mmu: Lazily allocate memslot rmaps
On Thu, Apr 29, 2021, Ben Gardon wrote:
> If the TDP MMU is in use, wait to allocate the rmaps until the shadow
> MMU is actually used. (i.e. a nested VM is launched.) This saves memory
> equal to 0.2% of guest memory in cases where the TDP MMU is used and
> there are no nested guests involved.
>
> Signed-off-by: Ben Gardon <bgardon@...gle.com>
> ---
> arch/x86/include/asm/kvm_host.h | 11 +++++++
> arch/x86/kvm/mmu/mmu.c | 21 +++++++++++--
> arch/x86/kvm/mmu/mmu_internal.h | 2 +-
> arch/x86/kvm/x86.c | 54 ++++++++++++++++++++++++++++++---
> 4 files changed, 80 insertions(+), 8 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 3900dcf2439e..b8633ed00a6a 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1124,6 +1124,15 @@ struct kvm_arch {
> #endif /* CONFIG_X86_64 */
>
> bool shadow_mmu_active;
> +
> + /*
> + * If set, the rmap should be allocated for any newly created or
> + * modified memslots. If allocating rmaps lazily, this may be set
> + * before the rmaps are allocated for existing memslots, but
> + * shadow_mmu_active will not be set until after the rmaps are fully
> + * allocated.
> + */
> + bool alloc_memslot_rmaps;
Maybe "need_rmaps" or "need_memslot_rmaps"?
> };
>
> struct kvm_vm_stat {
> @@ -1855,4 +1864,6 @@ static inline int kvm_cpu_get_apicid(int mps_cpu)
>
> int kvm_cpu_dirty_log_size(void);
>
> +int alloc_all_memslots_rmaps(struct kvm *kvm);
> +
> #endif /* _ASM_X86_KVM_HOST_H */
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index e252af46f205..b2a6585bd978 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3125,9 +3125,17 @@ static int fast_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
> return ret;
> }
>
> -void activate_shadow_mmu(struct kvm *kvm)
> +int activate_shadow_mmu(struct kvm *kvm)
> {
> + int r;
> +
> + r = alloc_all_memslots_rmaps(kvm);
> + if (r)
> + return r;
> +
> kvm->arch.shadow_mmu_active = true;
If shadow_mmu_active goes away, so does this helper.
> +
> + return 0;
> }
>
> static void mmu_free_root_page(struct kvm *kvm, hpa_t *root_hpa,
> @@ -3300,7 +3308,9 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
> }
> }
>
> - activate_shadow_mmu(vcpu->kvm);
> + r = activate_shadow_mmu(vcpu->kvm);
> + if (r)
> + return r;
>
> write_lock(&vcpu->kvm->mmu_lock);
> r = make_mmu_pages_available(vcpu);
> @@ -5491,7 +5501,12 @@ void kvm_mmu_init_vm(struct kvm *kvm)
> struct kvm_page_track_notifier_node *node = &kvm->arch.mmu_sp_tracker;
>
> if (!kvm_mmu_init_tdp_mmu(kvm))
> - activate_shadow_mmu(kvm);
> + /*
> + * No memslots can have been allocated at this point.
> + * activate_shadow_mmu won't actually need to allocate
> + * rmaps, so it cannot fail.
> + */
> + WARN_ON(activate_shadow_mmu(kvm));
This is where I really don't like calling the full flow. VM init is already
special, I don't see any harm in open coding the setting of the flag. This also
provides a good place to document that the smp_store/load business is unnecessary
since there can't be users.
> node->track_write = kvm_mmu_pte_write;
> node->track_flush_slot = kvm_mmu_invalidate_zap_pages_in_memslot;
> -static int kvm_alloc_memslot_metadata(struct kvm_memory_slot *slot,
> +int alloc_memslots_rmaps(struct kvm *kvm, struct kvm_memslots *slots)
> +{
> + struct kvm_memory_slot *slot;
> + int r = 0;
> +
> + kvm_for_each_memslot(slot, slots) {
> + r = alloc_memslot_rmap(kvm, slot, slot->npages);
> + if (r)
> + break;
> + }
> + return r;
> +}
Just open code this in the caller, it's literally one line of code and the
indentation isn't bad.
> +
> +int alloc_all_memslots_rmaps(struct kvm *kvm)
> +{
> + struct kvm_memslots *slots;
> + int r = 0;
> + int i;
> +
> + mutex_lock(&kvm->slots_arch_lock);
> + kvm->arch.alloc_memslot_rmaps = true;
> +
> + for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) {
> + slots = __kvm_memslots(kvm, i);
> + r = alloc_memslots_rmaps(kvm, slots);
> + if (r)
It'd be easier just to destroy the rmaps on failure and then do:
if (kvm->arch.needs_memslots_rmaps)
return;
mutex_lock(&kvm->slots_arch_lock);
for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) {
kvm_for_each_memslot(slot, __kvm_memslots(kvm, i)) {
r = alloc_memslot_rmap(kvm, slot, slot->npages);
break;
}
}
if (!r)
smp_store_release(kvm->arch.needs_memslots_rmaps, true);
else
kvm_free_rmaps(kvm);
mutex_unlock(&kvm->slots_arch_lock);
and make alloc_memslot_rmap() a pure allocator (no checks on whether it should
actually do allocations), i.e. push the check to the memslot flow:
static int kvm_alloc_memslot_metadata(struct kvm *kvm,
struct kvm_memory_slot *slot,
unsigned long npages)
{
int i;
int r;
/*
* Clear out the previous array pointers for the KVM_MR_MOVE case. The
* old arrays will be freed by __kvm_set_memory_region() if installing
* the new memslot is successful.
*/
memset(&slot->arch, 0, sizeof(slot->arch));
if (kvm->arch.needs_memslots_rmaps) {
r = alloc_memslot_rmap(kvm, slot, npages);
if (r)
return r;
}
With that, there's no need for the separate shadow_mmu_active flag, and you can
do s/activate_shadow_mmu/kvm_activate_rmaps or so.
> + break;
> + }
> + mutex_unlock(&kvm->slots_arch_lock);
> + return r;
> +}
> +
> +static int kvm_alloc_memslot_metadata(struct kvm *kvm,
> + struct kvm_memory_slot *slot,
> unsigned long npages)
> {
> int i;
> @@ -10881,7 +10927,7 @@ static int kvm_alloc_memslot_metadata(struct kvm_memory_slot *slot,
> */
> memset(&slot->arch, 0, sizeof(slot->arch));
>
> - r = alloc_memslot_rmap(slot, npages);
> + r = alloc_memslot_rmap(kvm, slot, npages);
> if (r)
> return r;
>
> @@ -10954,7 +11000,7 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
> enum kvm_mr_change change)
> {
> if (change == KVM_MR_CREATE || change == KVM_MR_MOVE)
> - return kvm_alloc_memslot_metadata(memslot,
> + return kvm_alloc_memslot_metadata(kvm, memslot,
> mem->memory_size >> PAGE_SHIFT);
> return 0;
> }
> --
> 2.31.1.527.g47e6f16901-goog
>
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