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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 12/19] KVM: Move memslot deletion to helper function
Date: Fri, 7 Feb 2020 09:59:12 -0800
Message-ID: <20200207175912.GG2401@linux.intel.com> (raw)
In-Reply-To: <20200206165116.GE695333@xz-x1>

On Thu, Feb 06, 2020 at 11:51:16AM -0500, Peter Xu wrote:
> On Thu, Feb 06, 2020 at 08:28:18AM -0800, Sean Christopherson wrote:
> > On Thu, Feb 06, 2020 at 11:14:15AM -0500, Peter Xu wrote:
> > > On Tue, Jan 21, 2020 at 02:31:50PM -0800, Sean Christopherson wrote:
> > > > Move memslot deletion into its own routine so that the success path for
> > > > other memslot updates does not need to use kvm_free_memslot(), i.e. can
> > > > explicitly destroy the dirty bitmap when necessary.  This paves the way
> > > > for dropping @dont from kvm_free_memslot(), i.e. all callers now pass
> > > > NULL for @dont.
> > > > 
> > > > Add a comment above the code to make a copy of the existing memslot
> > > > prior to deletion, it is not at all obvious that the pointer will become
> > > > stale during sorting and/or installation of new memslots.
> > > 
> > > Could you help explain a bit on this explicit comment?  I can follow
> > > up with the patch itself which looks all correct to me, but I failed
> > > to catch what this extra comment wants to emphasize...
> > 
> > It's tempting to write the code like this (I know, because I did it):
> > 
> > 	if (!mem->memory_size)
> > 		return kvm_delete_memslot(kvm, mem, slot, as_id);
> > 
> > 	new = *slot;
> > 
> > Where @slot is a pointer to the memslot to be deleted.  At first, second,
> > and third glances, this seems perfectly sane.
> > 
> > The issue is that slot was pulled from struct kvm_memslots.memslots, e.g.
> > 
> > 	slot = &slots->memslots[index];
> > 
> > Note that slots->memslots holds actual "struct kvm_memory_slot" objects,
> > not pointers to slots.  When update_memslots() sorts the slots, it swaps
> > the actual slot objects, not pointers.  I.e. after update_memslots(), even
> > though @slot points at the same address, it's could be pointing at a
> > different slot.  As a result kvm_free_memslot() in kvm_delete_memslot()
> > will free the dirty page info and arch-specific points for some random
> > slot, not the intended slot, and will set npages=0 for that random slot.
> 
> Ah I see, thanks.  Another alternative is we move the "old = *slot"
> copy into kvm_delete_memslot(), which could be even clearer imo.

The copy is also needed in __kvm_set_memory_region() for the MOVE case.

> However I'm not sure whether it's a good idea to drop the test-by for
> this.  Considering that comment change should not affect it, would you
> mind enrich the comment into something like this (or anything better)?
> 
> /*
>  * Make a full copy of the old memslot, the pointer will become stale
>  * when the memslots are re-sorted by update_memslots() in
>  * kvm_delete_memslot(), while to make the kvm_free_memslot() work as
>  * expected later on, we still need the cached memory slot.
>  */

As above, it's more subtle than just the kvm_delete_memslot() case.

	/*
	 * Make a full copy of the old memslot, the pointer will become stale
	 * when the memslots are re-sorted by update_memslots() when deleting
	 * or moving a memslot, and additional modifications to the old memslot
	 * need to be made after calling update_memslots().
	 */

  reply index

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 [this message]
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
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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