On Mon, Oct 05, 2020 at 07:57:05PM -0700, Sean Christopherson wrote: > On Sat, Oct 03, 2020 at 07:50:56AM +0300, Jarkko Sakkinen wrote: > > + __u16 exception_vector; > > + __u16 exception_error_code; > > + __u64 exception_addr; > > + __u8 reserved[24]; > > I also think it's a waste of space to bother with multiple reserved fields. > 24 bytes isn't so much that it guarantees we'll never run into problems in > the future. But I care far less about this than I do about exit_reason. For me the real problem is that there has not been "no brainer" basis for any size, so a one cache line worth of data is just something that makes sense, because would neither make much sense to have less. I'll throw an argument to have it a bit bigger amount of reserved space for future use. First, there is always some amount of unknown unknowns when it comes to run-time structures, given the evolution of microarchitectures. So yes, some more "state" might be needed in the future. Secondly, this is a bigger problem for the vDSO than it is for ioctl's because we can have only one. With ioctl's, in the absolute worst case, we can have a second version of the same ioctl. At least 256 bytes would be probably a good number, if we want to increase it. The reserved space zero validation that I implemented to this version probably does not add much to the overhead anyway. I'm not sure why care about one struct field more than making sure that the run-time structure can stand time. /Jarkko
