From: "Maciej S. Szmigiero" <maciej.szmigiero@oracle.com>
To: Sean Christopherson <seanjc@google.com>
Cc: James Morse <james.morse@arm.com>,
Alexandru Elisei <alexandru.elisei@arm.com>,
Suzuki K Poulose <suzuki.poulose@arm.com>,
Atish Patra <atish.patra@wdc.com>,
David Hildenbrand <david@redhat.com>,
Cornelia Huck <cohuck@redhat.com>,
Claudio Imbrenda <imbrenda@linux.ibm.com>,
Vitaly Kuznetsov <vkuznets@redhat.com>,
Wanpeng Li <wanpengli@tencent.com>,
Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
linux-arm-kernel@lists.infradead.org,
kvmarm@lists.cs.columbia.edu, linux-mips@vger.kernel.org,
kvm@vger.kernel.org, kvm-ppc@vger.kernel.org,
kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org,
linux-kernel@vger.kernel.org, Ben Gardon <bgardon@google.com>,
Marc Zyngier <maz@kernel.org>,
Huacai Chen <chenhuacai@kernel.org>,
Aleksandar Markovic <aleksandar.qemu.devel@gmail.com>,
Paul Mackerras <paulus@ozlabs.org>,
Anup Patel <anup.patel@wdc.com>,
Paul Walmsley <paul.walmsley@sifive.com>,
Palmer Dabbelt <palmer@dabbelt.com>,
Albert Ou <aou@eecs.berkeley.edu>,
Christian Borntraeger <borntraeger@de.ibm.com>,
Janosch Frank <frankja@linux.ibm.com>,
Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH v5.5 26/30] KVM: Keep memslots in tree-based structures instead of array-based ones
Date: Sat, 13 Nov 2021 16:22:48 +0100 [thread overview]
Message-ID: <d1c648e4-5536-111d-a7bf-3644ac68c9f5@oracle.com> (raw)
In-Reply-To: <YY26dxv2kM3m2H7Z@google.com>
On 12.11.2021 01:51, Sean Christopherson wrote:
> On Fri, Nov 12, 2021, Maciej S. Szmigiero wrote:
>> On 04.11.2021 01:25, Sean Christopherson wrote:
>>> - /*
>>> - * Remove the old memslot from the hash list and interval tree, copying
>>> - * the node data would corrupt the structures.
>>> - */
>>> + int as_id = kvm_memslots_get_as_id(old, new);
>>> + struct kvm_memslots *slots = kvm_get_inactive_memslots(kvm, as_id);
>>> + int idx = slots->node_idx;
>>> +
>>> if (old) {
>>> - hash_del(&old->id_node);
>>> - interval_tree_remove(&old->hva_node, &slots->hva_tree);
>>> + hash_del(&old->id_node[idx]);
>>> + interval_tree_remove(&old->hva_node[idx], &slots->hva_tree);
>>> - if (!new)
>>> + if ((long)old == atomic_long_read(&slots->last_used_slot))
>>> + atomic_long_set(&slots->last_used_slot, (long)new);
>>
>> Open-coding cmpxchg() is way less readable than a direct call.
>
> Doh, I meant to call this out and/or add a comment.
>
> My objection to cmpxchg() is that it implies atomicity is required (the kernel's
> version adds the lock), which is very much not the case. So this isn't strictly
> an open-coded version of cmpxchg().
>
>> The open-coded version also compiles on x86 to multiple instructions with
>> a branch, instead of just a single instruction.
>
> Yeah. The lock can't be contended, so that part of cmpxchg is a non-issue. But
> that's also why I don't love using cmpxchg.
>
> I don't have a strong preference, I just got briefly confused by the atomicity part.
We can simply add a comment there to explain that the atomicity isn't actually
strictly required here - will do that.
>>> +static void kvm_invalidate_memslot(struct kvm *kvm,
>>> + struct kvm_memory_slot *old,
>>> + struct kvm_memory_slot *working_slot)
>>> +{
>>> + /*
>>> + * Mark the current slot INVALID. As with all memslot modifications,
>>> + * this must be done on an unreachable slot to avoid modifying the
>>> + * current slot in the active tree.
>>> + */
>>> + kvm_copy_memslot(working_slot, old);
>>> + working_slot->flags |= KVM_MEMSLOT_INVALID;
>>> + kvm_replace_memslot(kvm, old, working_slot);
>>> +
>>> + /*
>>> + * Activate the slot that is now marked INVALID, but don't propagate
>>> + * the slot to the now inactive slots. The slot is either going to be
>>> + * deleted or recreated as a new slot.
>>> + */
>>> + kvm_swap_active_memslots(kvm, old->as_id);
>>> +
>>> + /*
>>> + * From this point no new shadow pages pointing to a deleted, or moved,
>>> + * memslot will be created. Validation of sp->gfn happens in:
>>> + * - gfn_to_hva (kvm_read_guest, gfn_to_pfn)
>>> + * - kvm_is_visible_gfn (mmu_check_root)
>>> + */
>>> + kvm_arch_flush_shadow_memslot(kvm, old);
>>
>> This should flush the currently active slot (that is, "working_slot",
>> not "old") to not introduce a behavior change with respect to the existing
>> code.
>>
>> That's also what the previous version of this patch set did.
>
> Eww. I would much prefer to "fix" the existing code in a prep patch. It shouldn't
> matter, but arch code really should not get passed an INVALID slot.
>
I will add a separate patch that switches that kvm_arch_flush_shadow_memslot()
call to use a valid (old) memslot instead.
It is actually simpler to do it *after* the main patch series to not add
more dead code that next patches remove anyway.
Thanks,
Maciej
next prev parent reply other threads:[~2021-11-13 15:24 UTC|newest]
Thread overview: 66+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-11-04 0:25 [PATCH v5.5 00/30] KVM: Scalable memslots implementation Sean Christopherson
2021-11-04 0:25 ` [PATCH v5.5 01/30] KVM: Ensure local memslot copies operate on up-to-date arch-specific data Sean Christopherson
2021-11-04 21:27 ` Ben Gardon
2021-11-04 22:41 ` Sean Christopherson
2021-11-09 0:37 ` Maciej S. Szmigiero
2021-11-09 1:17 ` Sean Christopherson
2021-11-04 0:25 ` [PATCH v5.5 02/30] KVM: Disallow user memslot with size that exceeds "unsigned long" Sean Christopherson
2021-11-09 0:38 ` Maciej S. Szmigiero
2021-11-04 0:25 ` [PATCH v5.5 03/30] KVM: Require total number of memslot pages to fit in an unsigned long Sean Christopherson
2021-11-09 0:38 ` Maciej S. Szmigiero
2021-11-04 0:25 ` [PATCH v5.5 04/30] KVM: Open code kvm_delete_memslot() into its only caller Sean Christopherson
2021-11-09 0:38 ` Maciej S. Szmigiero
2021-11-04 0:25 ` [PATCH v5.5 05/30] KVM: Resync only arch fields when slots_arch_lock gets reacquired Sean Christopherson
2021-11-09 0:38 ` Maciej S. Szmigiero
2021-11-04 0:25 ` [PATCH v5.5 06/30] KVM: Use "new" memslot's address space ID instead of dedicated param Sean Christopherson
2021-11-09 0:39 ` Maciej S. Szmigiero
2021-11-04 0:25 ` [PATCH v5.5 07/30] KVM: Let/force architectures to deal with arch specific memslot data Sean Christopherson
2021-11-09 0:39 ` Maciej S. Szmigiero
2021-11-09 1:13 ` Sean Christopherson
2021-11-04 0:25 ` [PATCH v5.5 08/30] KVM: arm64: Use "new" memslot instead of userspace memory region Sean Christopherson
2021-11-09 6:36 ` Reiji Watanabe
2021-11-04 0:25 ` [PATCH v5.5 09/30] KVM: MIPS: Drop pr_debug from memslot commit to avoid using "mem" Sean Christopherson
2021-11-04 0:25 ` [PATCH v5.5 10/30] KVM: PPC: Avoid referencing userspace memory region in memslot updates Sean Christopherson
2021-11-04 0:25 ` [PATCH v5.5 11/30] KVM: s390: Use "new" memslot instead of userspace memory region Sean Christopherson
2021-11-04 0:25 ` [PATCH v5.5 12/30] KVM: x86: " Sean Christopherson
2021-11-09 0:40 ` Maciej S. Szmigiero
2021-11-04 0:25 ` [PATCH v5.5 13/30] KVM: RISC-V: " Sean Christopherson
2021-11-04 0:25 ` [PATCH v5.5 14/30] KVM: Stop passing kvm_userspace_memory_region to arch memslot hooks Sean Christopherson
2021-11-09 0:40 ` Maciej S. Szmigiero
2021-11-04 0:25 ` [PATCH v5.5 15/30] KVM: Use prepare/commit hooks to handle generic memslot metadata updates Sean Christopherson
2021-11-09 0:40 ` Maciej S. Szmigiero
2021-11-04 0:25 ` [PATCH v5.5 16/30] KVM: x86: Don't assume old/new memslots are non-NULL at memslot commit Sean Christopherson
2021-11-09 0:40 ` Maciej S. Szmigiero
2021-11-04 0:25 ` [PATCH v5.5 17/30] KVM: s390: Skip gfn/size sanity checks on memslot DELETE or FLAGS_ONLY Sean Christopherson
2021-11-04 0:25 ` [PATCH v5.5 18/30] KVM: Don't make a full copy of the old memslot in __kvm_set_memory_region() Sean Christopherson
2021-11-09 0:41 ` Maciej S. Szmigiero
2021-11-04 0:25 ` [PATCH v5.5 19/30] KVM: x86: Don't call kvm_mmu_change_mmu_pages() if the count hasn't changed Sean Christopherson
2021-11-04 0:25 ` [PATCH v5.5 20/30] KVM: x86: Use nr_memslot_pages to avoid traversing the memslots array Sean Christopherson
2021-11-09 0:41 ` Maciej S. Szmigiero
2021-11-09 1:34 ` Sean Christopherson
2021-11-09 16:29 ` Maciej S. Szmigiero
2021-11-04 0:25 ` [PATCH v5.5 21/30] KVM: Integrate gfn_to_memslot_approx() into search_memslots() Sean Christopherson
2021-11-04 0:25 ` [PATCH v5.5 22/30] KVM: Move WARN on invalid memslot index to update_memslots() Sean Christopherson
2021-11-04 0:25 ` [PATCH v5.5 23/30] KVM: Resolve memslot ID via a hash table instead of via a static array Sean Christopherson
2021-11-11 23:51 ` Maciej S. Szmigiero
2021-11-12 1:03 ` Sean Christopherson
2021-11-04 0:25 ` [PATCH v5.5 24/30] KVM: Use interval tree to do fast hva lookup in memslots Sean Christopherson
2021-11-11 23:52 ` Maciej S. Szmigiero
2021-11-12 1:05 ` Sean Christopherson
2021-11-04 0:25 ` [PATCH v5.5 25/30] KVM: s390: Introduce kvm_s390_get_gfn_end() Sean Christopherson
2021-11-04 0:25 ` [PATCH v5.5 26/30] KVM: Keep memslots in tree-based structures instead of array-based ones Sean Christopherson
2021-11-11 23:52 ` Maciej S. Szmigiero
2021-11-12 0:51 ` Sean Christopherson
2021-11-13 15:22 ` Maciej S. Szmigiero [this message]
2021-11-04 0:25 ` [PATCH v5.5 27/30] KVM: Optimize gfn lookup in kvm_zap_gfn_range() Sean Christopherson
2021-11-04 0:25 ` [PATCH v5.5 28/30] KVM: Optimize overlapping memslots check Sean Christopherson
2021-11-04 0:25 ` [PATCH v5.5 29/30] KVM: Wait 'til the bitter end to initialize the "new" memslot Sean Christopherson
2021-11-11 23:52 ` Maciej S. Szmigiero
2021-11-04 0:25 ` [PATCH v5.5 30/30] KVM: Dynamically allocate "new" memslots from the get-go Sean Christopherson
2021-11-11 23:53 ` Maciej S. Szmigiero
2021-11-12 1:32 ` Sean Christopherson
2021-11-09 0:43 ` [PATCH v5.5 00/30] KVM: Scalable memslots implementation Maciej S. Szmigiero
2021-11-09 1:21 ` Sean Christopherson
2021-11-11 23:53 ` Maciej S. Szmigiero
2021-11-23 14:42 ` Maciej S. Szmigiero
2021-11-26 12:33 ` Paolo Bonzini
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