From: Sean Christopherson <sean.j.christopherson@intel.com>
To: Peter Xu <peterx@redhat.com>
Cc: "Paolo Bonzini" <pbonzini@redhat.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>,
"Vitaly Kuznetsov" <vkuznets@redhat.com>,
"Wanpeng Li" <wanpengli@tencent.com>,
"Jim Mattson" <jmattson@google.com>,
"Joerg Roedel" <joro@8bytes.org>, "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>,
linux-mips@vger.kernel.org, kvm@vger.kernel.org,
kvm-ppc@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
kvmarm@lists.cs.columbia.edu, linux-kernel@vger.kernel.org,
"Christoffer Dall" <christoffer.dall@arm.com>,
"Philippe Mathieu-Daudé" <f4bug@amsat.org>
Subject: Re: [PATCH v5 18/19] KVM: Dynamically size memslot array based on number of used slots
Date: Fri, 7 Feb 2020 07:38:29 -0800 [thread overview]
Message-ID: <20200207153829.GA2401@linux.intel.com> (raw)
In-Reply-To: <20200206221208.GI700495@xz-x1>
On Thu, Feb 06, 2020 at 05:12:08PM -0500, Peter Xu wrote:
> On Tue, Jan 21, 2020 at 02:31:56PM -0800, Sean Christopherson wrote:
> > Now that the memslot logic doesn't assume memslots are always non-NULL,
> > dynamically size the array of memslots instead of unconditionally
> > allocating memory for the maximum number of memslots.
> >
> > Note, because a to-be-deleted memslot must first be invalidated, the
> > array size cannot be immediately reduced when deleting a memslot.
> > However, consecutive deletions will realize the memory savings, i.e.
> > a second deletion will trim the entry.
> >
> > Tested-by: Christoffer Dall <christoffer.dall@arm.com>
> > Tested-by: Marc Zyngier <maz@kernel.org>
> > Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> > ---
> > include/linux/kvm_host.h | 2 +-
> > virt/kvm/kvm_main.c | 31 ++++++++++++++++++++++++++++---
> > 2 files changed, 29 insertions(+), 4 deletions(-)
> >
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index 60ddfdb69378..8bb6fb127387 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -431,11 +431,11 @@ static inline int kvm_arch_vcpu_memslots_id(struct kvm_vcpu *vcpu)
> > */
> > struct kvm_memslots {
> > u64 generation;
> > - struct kvm_memory_slot memslots[KVM_MEM_SLOTS_NUM];
> > /* The mapping table from slot id to the index in memslots[]. */
> > short id_to_index[KVM_MEM_SLOTS_NUM];
> > atomic_t lru_slot;
> > int used_slots;
> > + struct kvm_memory_slot memslots[];
>
> This patch is tested so I believe this works, however normally I need
> to do similar thing with [0] otherwise gcc might complaint. Is there
> any trick behind to make this work? Or is that because of different
> gcc versions?
array[] and array[0] have the same net affect, but array[] is given special
treatment by gcc to provide extra sanity checks, e.g. requires the field to
be the end of the struct. Last I checked, gcc also doesn't allow array[]
in unions. There are probably other restrictions.
But, it's precisely because of those restrictions that using array[] is
preferred, as it provides extra protections, e.g. if someone moved memslots
to the top of the struct it would fail to compile.
> > };
> >
> > struct kvm {
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index 9b614cf2ca20..ed392ce64e59 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -565,7 +565,7 @@ static struct kvm_memslots *kvm_alloc_memslots(void)
> > return NULL;
> >
> > for (i = 0; i < KVM_MEM_SLOTS_NUM; i++)
> > - slots->id_to_index[i] = slots->memslots[i].id = -1;
> > + slots->id_to_index[i] = -1;
> >
> > return slots;
> > }
> > @@ -1077,6 +1077,32 @@ static struct kvm_memslots *install_new_memslots(struct kvm *kvm,
> > return old_memslots;
> > }
> >
> > +/*
> > + * 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
> > + * use when invalidating a memslot prior to deleting/moving the memslot.
> > + */
> > +static struct kvm_memslots *kvm_dup_memslots(struct kvm_memslots *old,
> > + enum kvm_mr_change change)
> > +{
> > + struct kvm_memslots *slots;
> > + size_t old_size, new_size;
> > +
> > + old_size = sizeof(struct kvm_memslots) +
> > + (sizeof(struct kvm_memory_slot) * old->used_slots);
> > +
> > + if (change == KVM_MR_CREATE)
> > + new_size = old_size + sizeof(struct kvm_memory_slot);
> > + else
> > + new_size = old_size;
> > +
> > + slots = kvzalloc(new_size, GFP_KERNEL_ACCOUNT);
> > + if (likely(slots))
> > + memcpy(slots, old, old_size);
>
> (Maybe directly copy into it?)
I don't follow, are you saying do "*slots = *old"?
@new_size and @old_size are not guaranteed to be the same. More
specifically, slots->memslots and old->slots are now flexible arrays with
potentially different sizes. Doing "*slots = *old" would only copy the
standard members, a memcpy() would still be needed for @memlots.
A more effecient implementation would be:
slots = kvalloc(new_size, GFP_KERNEL_ACCOUNT);
if (likely(slots)) {
memcpy(slots, old, old_size);
if (change == KVM_MR_CREATE)
memset((void *)slots + old_size, 0, new_size - old_size);
}
to avoid unnecessarily zeroing out the entire thing. I opted for the
simpler implementation as this is not performance critical code, for most
cases @slots won't be all that large, and I wanted to be absolutely sure
any mixup would hit zeroed memory and not uninitialized memory.
>
> > +
> > + return slots;
> > +}
> > +
> > static int kvm_set_memslot(struct kvm *kvm,
> > const struct kvm_userspace_memory_region *mem,
> > struct kvm_memory_slot *old,
> > @@ -1087,10 +1113,9 @@ static int kvm_set_memslot(struct kvm *kvm,
> > struct kvm_memslots *slots;
> > int r;
> >
> > - slots = kvzalloc(sizeof(struct kvm_memslots), GFP_KERNEL_ACCOUNT);
> > + slots = kvm_dup_memslots(__kvm_memslots(kvm, as_id), change);
> > if (!slots)
> > return -ENOMEM;
> > - memcpy(slots, __kvm_memslots(kvm, as_id), sizeof(struct kvm_memslots));
> >
> > if (change == KVM_MR_DELETE || change == KVM_MR_MOVE) {
> > /*
> > --
> > 2.24.1
> >
>
> --
> Peter Xu
>
next prev parent reply other threads:[~2020-02-07 15:38 UTC|newest]
Thread overview: 70+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-21 22:31 [PATCH v5 00/19] KVM: Dynamically size memslot arrays Sean Christopherson
2020-01-21 22:31 ` [PATCH v5 01/19] KVM: x86: Allocate new rmap and large page tracking when moving memslot Sean Christopherson
2020-02-05 21:49 ` Peter Xu
2020-02-05 23:55 ` Sean Christopherson
2020-02-06 2:00 ` Peter Xu
2020-02-06 2:17 ` Sean Christopherson
2020-02-06 2:58 ` Peter Xu
2020-02-06 5:05 ` Sean Christopherson
2020-01-21 22:31 ` [PATCH v5 02/19] KVM: Reinstall old memslots if arch preparation fails Sean Christopherson
2020-02-05 22:08 ` Peter Xu
2020-01-21 22:31 ` [PATCH v5 03/19] KVM: Don't free new memslot if allocation of said memslot fails Sean Christopherson
2020-02-05 22:28 ` Peter Xu
2020-01-21 22:31 ` [PATCH v5 04/19] KVM: PPC: Move memslot memory allocation into prepare_memory_region() Sean Christopherson
2020-02-05 22:41 ` Peter Xu
2020-01-21 22:31 ` [PATCH v5 05/19] KVM: x86: Allocate memslot resources during prepare_memory_region() Sean Christopherson
2020-01-21 22:31 ` [PATCH v5 06/19] KVM: Drop kvm_arch_create_memslot() Sean Christopherson
2020-02-05 22:45 ` Peter Xu
2020-01-21 22:31 ` [PATCH v5 07/19] KVM: Explicitly free allocated-but-unused dirty bitmap Sean Christopherson
2020-01-21 22:31 ` [PATCH v5 08/19] KVM: Refactor error handling for setting memory region Sean Christopherson
2020-02-05 22:48 ` Peter Xu
2020-01-21 22:31 ` [PATCH v5 09/19] KVM: Move setting of memslot into helper routine Sean Christopherson
2020-02-06 16:26 ` Peter Xu
2020-01-21 22:31 ` [PATCH v5 10/19] KVM: Drop "const" attribute from old memslot in commit_memory_region() Sean Christopherson
2020-02-06 16:26 ` Peter Xu
2020-01-21 22:31 ` [PATCH v5 11/19] KVM: x86: Free arrays for old memslot when moving memslot's base gfn Sean Christopherson
2020-01-21 22:31 ` [PATCH v5 12/19] KVM: Move memslot deletion to helper function Sean Christopherson
2020-02-06 16:14 ` Peter Xu
2020-02-06 16:28 ` Sean Christopherson
2020-02-06 16:51 ` Peter Xu
2020-02-07 17:59 ` Sean Christopherson
2020-02-07 18:07 ` Sean Christopherson
2020-02-07 18:17 ` Peter Xu
2020-01-21 22:31 ` [PATCH v5 13/19] KVM: Simplify kvm_free_memslot() and all its descendents Sean Christopherson
2020-02-06 16:29 ` Peter Xu
2020-01-21 22:31 ` [PATCH v5 14/19] KVM: Clean up local variable usage in __kvm_set_memory_region() Sean Christopherson
2020-02-06 19:06 ` Peter Xu
2020-02-06 19:22 ` Sean Christopherson
2020-02-06 19:36 ` Peter Xu
2020-01-21 22:31 ` [PATCH v5 15/19] KVM: Provide common implementation for generic dirty log functions Sean Christopherson
2020-02-06 20:02 ` Peter Xu
2020-02-06 21:21 ` Sean Christopherson
2020-02-06 21:41 ` Peter Xu
2020-02-07 19:45 ` Sean Christopherson
2020-02-08 0:18 ` Peter Xu
2020-02-08 0:42 ` Sean Christopherson
2020-02-08 0:53 ` Peter Xu
2020-02-08 1:29 ` Sean Christopherson
2020-02-17 15:39 ` Vitaly Kuznetsov
2020-02-18 17:10 ` Sean Christopherson
2020-02-17 15:35 ` Vitaly Kuznetsov
2020-02-06 21:24 ` Peter Xu
2020-01-21 22:31 ` [PATCH v5 16/19] KVM: Ensure validity of memslot with respect to kvm_get_dirty_log() Sean Christopherson
2020-01-21 22:31 ` [PATCH v5 17/19] KVM: Terminate memslot walks via used_slots Sean Christopherson
2020-02-06 21:09 ` Peter Xu
2020-02-07 18:33 ` Sean Christopherson
2020-02-07 20:39 ` Peter Xu
2020-02-07 21:10 ` Sean Christopherson
2020-02-07 21:46 ` Peter Xu
2020-02-07 22:03 ` Sean Christopherson
2020-02-07 22:24 ` Peter Xu
2020-01-21 22:31 ` [PATCH v5 18/19] KVM: Dynamically size memslot array based on number of used slots Sean Christopherson
2020-02-06 22:12 ` Peter Xu
2020-02-07 15:38 ` Sean Christopherson [this message]
2020-02-07 16:05 ` Peter Xu
2020-02-07 16:15 ` Sean Christopherson
2020-02-07 16:37 ` Peter Xu
2020-02-07 16:47 ` Sean Christopherson
2020-01-21 22:31 ` [PATCH v5 19/19] KVM: selftests: Add test for KVM_SET_USER_MEMORY_REGION Sean Christopherson
2020-02-06 22:30 ` Peter Xu
2020-02-06 23:11 ` Sean Christopherson
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