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
