On 21/05/19 19:22, Rik van Riel wrote: > The code using KVM_REQUEST_MASK uses a pattern reminiscent of a bitmask: > > set_bit(req & KVM_REQUEST_MASK, &vcpu->requests); > > However, the first argument passed to set_bit, test_bit, and clear_bit > is a bit number, not a bitmask. That means the current definition would > allow users of kvm_make_request to overflow the vcpu.requests bitmask, > and is confusing to developers examining the code. This is true, but the meaning of the masking is that bits above 7 define extra things to do when sending a request (wait for acknowledge, kick the recipient CPU). The fact that the "request number" field is 8 bits rather than 5 or 6 is just an implementation detail. If you change it to BITS_PER_LONG-1, the obvious way to read the code would be that requests 0, 64, 128 are all valid and map to the same request. Paolo > Redefine KVM_REQUEST_MASK to reflect the number of bits that actually > fit inside an unsigned long, and add a comment explaining set_bit and > friends take bit numbers, not a bitmask.
