Re: [PATCH v5 07/13] KVM: Just resync arch fields when slots_arch_lock gets reacquired

From: Sean Christopherson <seanjc@google.com>
To: "Maciej S. Szmigiero" <mail@maciej.szmigiero.name>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>,
	Igor Mammedov <imammedo@redhat.com>,
	Marc Zyngier <maz@kernel.org>, James Morse <james.morse@arm.com>,
	Julien Thierry <julien.thierry.kdev@gmail.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Huacai Chen <chenhuacai@kernel.org>,
	Aleksandar Markovic <aleksandar.qemu.devel@gmail.com>,
	Paul Mackerras <paulus@ozlabs.org>,
	Christian Borntraeger <borntraeger@de.ibm.com>,
	Janosch Frank <frankja@linux.ibm.com>,
	David Hildenbrand <david@redhat.com>,
	Cornelia Huck <cohuck@redhat.com>,
	Claudio Imbrenda <imbrenda@linux.ibm.com>,
	Joerg Roedel <joro@8bytes.org>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v5 07/13] KVM: Just resync arch fields when slots_arch_lock gets reacquired
Date: Tue, 19 Oct 2021 23:55:06 +0000	[thread overview]
Message-ID: <YW9a2s8wHXzf8Xqw@google.com> (raw)
In-Reply-To: <311810ebd1111bed50d931d424297384171afc36.1632171479.git.maciej.szmigiero@oracle.com>

On Mon, Sep 20, 2021, Maciej S. Szmigiero wrote:
> From: "Maciej S. Szmigiero" <maciej.szmigiero@oracle.com>
> 
> There is no need to copy the whole memslot data after releasing
> slots_arch_lock for a moment to install temporary memslots copy in
> kvm_set_memslot() since this lock only protects the arch field of each
> memslot.
> 
> Just resync this particular field after reacquiring slots_arch_lock.

I assume this needed to avoid having a mess when introducing the r-b tree?  If so,
please call that out.  Iterating over the slots might actually be slower than the
full memcpy, i.e. as a standalone patch this may or may not be make sense.

> Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
> ---
>  virt/kvm/kvm_main.c | 17 ++++++++++++-----
>  1 file changed, 12 insertions(+), 5 deletions(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 348fae880189..48d182840060 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -1482,6 +1482,15 @@ static void kvm_copy_memslots(struct kvm_memslots *to,
>  	memcpy(to, from, kvm_memslots_size(from->used_slots));
>  }
>  
> +static void kvm_copy_memslots_arch(struct kvm_memslots *to,
> +				   struct kvm_memslots *from)
> +{
> +	int i;
> +
> +	for (i = 0; i < from->used_slots; i++)
> +		to->memslots[i].arch = from->memslots[i].arch;

This should probably be a memcpy(), I don't know what all shenanigans the compiler
can throw at us if it gets to copy a struct by value.

> +}
> +
>  /*
>   * Note, at a minimum, the current number of used slots must be allocated, even
>   * when deleting a memslot, as we need a complete duplicate of the memslots for

There's an out-of-sight comment that's now stale, can you revert to the
pre-slots_arch_lock comment?

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 48d182840060..ef3345428047 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1555,9 +1555,10 @@ static int kvm_set_memslot(struct kvm *kvm,
                slot->flags |= KVM_MEMSLOT_INVALID;

                /*
-                * We can re-use the memory from the old memslots.
-                * It will be overwritten with a copy of the new memslots
-                * after reacquiring the slots_arch_lock below.
+                * We can re-use the old memslots, the only difference from the
+                * newly installed memslots is the invalid flag, which will get
+                * dropped by update_memslots anyway.  We'll also revert to the
+                * old memslots if preparing the new memory region fails.
                 */
                slots = install_new_memslots(kvm, as_id, slots);

> @@ -1567,10 +1576,10 @@ static int kvm_set_memslot(struct kvm *kvm,
>  		/*
>  		 * The arch-specific fields of the memslots could have changed
>  		 * between releasing the slots_arch_lock in
> -		 * install_new_memslots and here, so get a fresh copy of the
> -		 * slots.
> +		 * install_new_memslots and here, so get a fresh copy of these
> +		 * fields.
>  		 */
> -		kvm_copy_memslots(slots, __kvm_memslots(kvm, as_id));
> +		kvm_copy_memslots_arch(slots, __kvm_memslots(kvm, as_id));
>  	}
>  
>  	r = kvm_arch_prepare_memory_region(kvm, old, new, mem, change);
> @@ -1587,8 +1596,6 @@ static int kvm_set_memslot(struct kvm *kvm,
>  
>  out_slots:
>  	if (change == KVM_MR_DELETE || change == KVM_MR_MOVE) {
> -		slot = id_to_memslot(slots, old->id);
> -		slot->flags &= ~KVM_MEMSLOT_INVALID;
>  		slots = install_new_memslots(kvm, as_id, slots);
>  	} else {

The braces can be dropped since both branches are now single lines.

>  		mutex_unlock(&kvm->slots_arch_lock);
