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    SubjectRe: [PATCH v3 04/10] KVM: Implement kvm_put_guest()
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    On 22/08/2019 16:28, Sean Christopherson wrote:
    > On Wed, Aug 21, 2019 at 04:36:50PM +0100, Steven Price wrote:
    >> kvm_put_guest() is analogous to put_user() - it writes a single value to
    >> the guest physical address. The implementation is built upon put_user()
    >> and so it has the same single copy atomic properties.
    >
    > What you mean by "single copy atomic"? I.e. what guarantees does
    > put_user() provide that __copy_to_user() does not?

    Single-copy atomicity is defined by the Arm architecture[1] and I'm not
    going to try to go into the full details here, so this is a summary.

    For the sake of this feature what we care about is that the value
    written/read cannot be "torn". In other words if there is a read (in
    this case from another VCPU) that is racing with the write then the read
    will either get the old value or the new value. It cannot return a
    mixture. (This is of course assuming that the read is using a
    single-copy atomic safe method).

    __copy_to_user() is implemented as a memcpy() and as such cannot provide
    single-copy atomicity in the general case (the buffer could easily be
    bigger than the architecture can guarantee).

    put_user() on the other hand is implemented (on arm64) as an explicit
    store instruction and therefore is guaranteed by the architecture to be
    single-copy atomic (i.e. another CPU cannot see a half-written value).

    Steve

    [1] https://static.docs.arm.com/ddi0487/ea/DDI0487E_a_armv8_arm.pdf#page=110

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