From: Paolo Bonzini <pbonzini@redhat.com>
To: Ben Gardon <bgardon@google.com>,
linux-kernel@vger.kernel.org, kvm@vger.kernel.org
Cc: Peter Xu <peterx@redhat.com>,
Sean Christopherson <seanjc@google.com>,
Peter Shier <pshier@google.com>,
Yulei Zhang <yulei.kernel@gmail.com>,
Wanpeng Li <kernellwp@gmail.com>,
Xiao Guangrong <xiaoguangrong.eric@gmail.com>,
Kai Huang <kai.huang@intel.com>,
Keqian Zhu <zhukeqian1@huawei.com>
Subject: Re: [PATCH v3 7/8] KVM: x86/mmu: Protect rmaps independently with SRCU
Date: Fri, 7 May 2021 10:42:19 +0200 [thread overview]
Message-ID: <e2e73709-f247-1a60-4835-f3fad37ab736@redhat.com> (raw)
In-Reply-To: <20210506184241.618958-8-bgardon@google.com>
On 06/05/21 20:42, Ben Gardon wrote:
> In preparation for lazily allocating the rmaps when the TDP MMU is in
> use, protect the rmaps with SRCU. Unfortunately, this requires
> propagating a pointer to struct kvm around to several functions.
Thinking more about it, this is not needed because all reads of the rmap
array are guarded by the load-acquire of kvm->arch.memslots_have_rmaps.
That is, the pattern is always
if (!load-acquire(memslot_have_rmaps))
return;
... = __gfn_to_rmap(...)
slots->arch.rmap[x] = ...
store-release(memslot_have_rmaps, true)
where the load-acquire/store-release have the same role that
srcu_dereference/rcu_assign_pointer had before this patch.
We also know that any read that misses the check has the potential for a
NULL pointer dereference, so it *has* to be like that.
That said, srcu_dereference has zero cost unless debugging options are
enabled, and it *is* true that the rmap can disappear if kvm->srcu is
not held, so I lean towards keeping this change and just changing the
commit message like this:
---------
Currently, rmaps are always allocated and published together with a new
memslot, so the srcu_dereference for the memslots array already ensures
that the memory pointed to by slots->arch.rmap is zero at the time
slots->arch.rmap. However, they still need to be accessed in an SRCU
read-side critical section, as the whole memslot can be deleted outside
SRCU.
--------
Thanks,
Paolo
>
> Suggested-by: Paolo Bonzini<pbonzini@redhat.com>
> Signed-off-by: Ben Gardon<bgardon@google.com>
> ---
> arch/x86/kvm/mmu/mmu.c | 57 +++++++++++++++++++++++++-----------------
> arch/x86/kvm/x86.c | 6 ++---
> 2 files changed, 37 insertions(+), 26 deletions(-)
next prev parent reply other threads:[~2021-05-07 8:42 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-06 18:42 [PATCH v3 0/8] Lazily allocate memslot rmaps Ben Gardon
2021-05-06 18:42 ` [PATCH v3 1/8] KVM: x86/mmu: Deduplicate rmap freeing Ben Gardon
2021-05-07 7:42 ` David Hildenbrand
2021-05-06 18:42 ` [PATCH v3 2/8] KVM: x86/mmu: Factor out allocating memslot rmap Ben Gardon
2021-05-07 7:46 ` David Hildenbrand
2021-05-10 16:29 ` Ben Gardon
2021-05-06 18:42 ` [PATCH v3 3/8] KVM: mmu: Refactor memslot copy Ben Gardon
2021-05-07 7:48 ` David Hildenbrand
2021-05-06 18:42 ` [PATCH v3 4/8] KVM: mmu: Add slots_arch_lock for memslot arch fields Ben Gardon
2021-05-06 18:42 ` [PATCH v3 5/8] KVM: x86/mmu: Add a field to control memslot rmap allocation Ben Gardon
2021-05-06 23:44 ` Ben Gardon
2021-05-07 7:50 ` David Hildenbrand
2021-05-07 8:28 ` Paolo Bonzini
2021-05-10 16:14 ` Ben Gardon
2021-05-10 16:33 ` Paolo Bonzini
2021-05-10 16:37 ` Ben Gardon
2021-05-06 18:42 ` [PATCH v3 6/8] KVM: x86/mmu: Skip rmap operations if rmaps not allocated Ben Gardon
2021-05-06 23:07 ` kernel test robot
2021-05-06 18:42 ` [PATCH v3 7/8] KVM: x86/mmu: Protect rmaps independently with SRCU Ben Gardon
2021-05-06 23:58 ` kernel test robot
2021-05-07 0:56 ` kernel test robot
2021-05-07 8:42 ` Paolo Bonzini [this message]
2021-05-10 17:45 ` Sean Christopherson
2021-05-10 17:53 ` Paolo Bonzini
2021-05-10 18:28 ` Sean Christopherson
2021-05-11 16:22 ` Ben Gardon
2021-05-11 16:45 ` Paolo Bonzini
2021-05-06 18:42 ` [PATCH v3 8/8] KVM: x86/mmu: Lazily allocate memslot rmaps Ben Gardon
2021-05-07 1:10 ` kernel test robot
2021-05-07 8:28 ` Paolo Bonzini
2021-05-07 7:40 ` [PATCH v3 0/8] " David Hildenbrand
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