From: Sean Christopherson <seanjc@google.com>
To: Anish Moorthy <amoorthy@google.com>
Cc: Oliver Upton <oliver.upton@linux.dev>,
Marc Zyngier <maz@kernel.org>,
Paolo Bonzini <pbonzini@redhat.com>,
James Houghton <jthoughton@google.com>,
Ben Gardon <bgardon@google.com>,
David Matlack <dmatlack@google.com>,
Ricardo Koller <ricarkol@google.com>,
Chao Peng <chao.p.peng@linux.intel.com>,
Axel Rasmussen <axelrasmussen@google.com>,
kvm@vger.kernel.org, kvmarm@lists.linux.dev, peterx@redhat.com
Subject: Re: [PATCH 5/8] kvm: Add cap/kvm_run field for memory fault exits
Date: Fri, 17 Feb 2023 12:47:33 -0800 [thread overview]
Message-ID: <Y+/n5X8+fGitI8FE@google.com> (raw)
In-Reply-To: <CAF7b7mqeXcHdFHewX3enn-vxf6y7CUWjXjB3TXithZ_PnzVLQQ@mail.gmail.com>
On Fri, Feb 17, 2023, Anish Moorthy wrote:
> On Thu, Feb 16, 2023 at 1:38 PM Sean Christopherson <seanjc@google.com> wrote:
> > Ensuring that vCPUs "see" the new value and not corrupting memory are two very
> > different things. Making the flag an atomic, wrapping with a rwlock, etc... do
> > nothing to ensure vCPUs observe the new value. And for non-crazy usage of bools,
> > they're not even necessary to avoid memory corruption...
>
> Oh, that's news to me- I've learned to treat any unprotected concurrent accesses
> to memory as undefined behavior: guess there's always more to learn.
To expand a bit, undefined and non-deterministic are two different things. In
this case, the behavior is not deterministic, but it _is_ defined. Readers will
either see %true or %false, but they will not see %beagle.
And more importantly _KVM_ doesn't care whether or not the behavior is deterministic,
that's userspace's responsibility. E.g. if KVM were endangered by the flag changing
then some form of locking absolutely would be needed, but that's not the case here.
next prev parent reply other threads:[~2023-02-17 20:47 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-02-15 1:16 [PATCH 0/8] Add memory fault exits to avoid slow GUP Anish Moorthy
2023-02-15 1:16 ` [PATCH 1/8] selftests/kvm: Fix bug in how demand_paging_test calculates paging rate Anish Moorthy
2023-02-15 7:27 ` Oliver Upton
2023-02-15 16:44 ` Sean Christopherson
2023-02-15 18:05 ` Anish Moorthy
2023-02-15 1:16 ` [PATCH 2/8] selftests/kvm: Allow many vcpus per UFFD in demand paging test Anish Moorthy
2023-02-15 1:16 ` [PATCH 3/8] selftests/kvm: Switch demand paging uffd readers to epoll Anish Moorthy
2023-02-15 1:16 ` [PATCH 4/8] kvm: Allow hva_pfn_fast to resolve read-only faults Anish Moorthy
2023-02-15 9:01 ` Oliver Upton
2023-02-15 17:03 ` Sean Christopherson
2023-02-15 18:19 ` Anish Moorthy
2023-02-15 1:16 ` [PATCH 5/8] kvm: Add cap/kvm_run field for memory fault exits Anish Moorthy
2023-02-15 8:41 ` Marc Zyngier
2023-02-15 17:07 ` Sean Christopherson
2023-02-16 18:53 ` Anish Moorthy
2023-02-16 21:38 ` Sean Christopherson
2023-02-17 19:14 ` Anish Moorthy
2023-02-17 20:33 ` Sean Christopherson
2023-02-23 1:16 ` Anish Moorthy
2023-02-23 20:55 ` Sean Christopherson
2023-02-23 23:03 ` Anish Moorthy
2023-02-24 0:01 ` Sean Christopherson
2023-02-17 20:47 ` Sean Christopherson [this message]
2023-02-15 8:59 ` Oliver Upton
2023-02-15 1:16 ` [PATCH 6/8] kvm/x86: Add mem fault exit on EPT violations Anish Moorthy
2023-02-15 17:23 ` Sean Christopherson
2023-02-16 22:55 ` Peter Xu
2023-02-23 0:35 ` Anish Moorthy
2023-02-23 20:11 ` Sean Christopherson
2023-02-15 1:16 ` [PATCH 7/8] kvm/arm64: Implement KVM_CAP_MEM_FAULT_NOWAIT for arm64 Anish Moorthy
2023-02-15 18:24 ` Oliver Upton
2023-02-15 23:28 ` Anish Moorthy
2023-02-15 23:37 ` Oliver Upton
2023-02-15 1:16 ` [PATCH 8/8] selftests/kvm: Handle mem fault exits in demand paging test Anish Moorthy
2023-02-15 1:46 ` [PATCH 0/8] Add memory fault exits to avoid slow GUP James Houghton
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