From: Peter Xu <peterx@redhat.com>
To: Sean Christopherson <sean.j.christopherson@intel.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 11:05:46 -0500 [thread overview]
Message-ID: <20200207160546.GA707371@xz-x1> (raw)
In-Reply-To: <20200207153829.GA2401@linux.intel.com>
On Fri, Feb 07, 2020 at 07:38:29AM -0800, Sean Christopherson wrote:
> 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.
However...
xz-x1:tmp $ cat a.c
struct a {
int s[];
};
int main(void) { }
xz-x1:tmp $ make a
cc a.c -o a
a.c:2:9: error: flexible array member in a struct with no named members
2 | int s[];
| ^
make: *** [<builtin>: a] Error 1
My gcc version is 9.2.1 20190827 (Red Hat 9.2.1-1) (GCC).
>
> > > };
> > >
> > > 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.
I made a silly mistake on reading "slots" as "old". Ignore my
comment, sorry! And please take my R-b for this patch too:
Reviewed-by: Peter Xu <peterx@redhat.com>
--
Peter Xu
next prev parent reply other threads:[~2020-02-07 16:05 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
2020-02-07 16:05 ` Peter Xu [this message]
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20200207160546.GA707371@xz-x1 \
--to=peterx@redhat.com \
--cc=borntraeger@de.ibm.com \
--cc=christoffer.dall@arm.com \
--cc=cohuck@redhat.com \
--cc=david@redhat.com \
--cc=f4bug@amsat.org \
--cc=frankja@linux.ibm.com \
--cc=james.morse@arm.com \
--cc=jmattson@google.com \
--cc=joro@8bytes.org \
--cc=julien.thierry.kdev@gmail.com \
--cc=kvm-ppc@vger.kernel.org \
--cc=kvm@vger.kernel.org \
--cc=kvmarm@lists.cs.columbia.edu \
--cc=linux-arm-kernel@lists.infradead.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mips@vger.kernel.org \
--cc=maz@kernel.org \
--cc=paulus@ozlabs.org \
--cc=pbonzini@redhat.com \
--cc=sean.j.christopherson@intel.com \
--cc=suzuki.poulose@arm.com \
--cc=vkuznets@redhat.com \
--cc=wanpengli@tencent.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).