On Fri, May 15, 2020 at 01:53:32PM +0100, Szabolcs Nagy wrote: > The 05/15/2020 13:13, Catalin Marinas wrote: > > On Fri, May 15, 2020 at 01:04:33PM +0100, Szabolcs Nagy wrote: > > > The 05/15/2020 12:27, Catalin Marinas wrote: > > > > Thanks Szabolcs. While we are at this, no-one so far asked for the > > > > GCR_EL1.RRND to be exposed to user (and this implies RGSR_EL1.SEED). > > > > Since RRND=1 guarantees a distribution "no worse" than that of RRND=0, I > > > > thought there isn't much point in exposing this configuration to the > > > > user. The only advantage of RRND=0 I see is that the kernel can change > > > > > > it seems RRND=1 is the impl specific algorithm. > > > > Yes, that's the implementation specific algorithm which shouldn't be > > worse than the standard one. > > > > > > the seed randomly but, with only 4 bits per tag, it really doesn't > > > > matter much. > > > > > > > > Anyway, mentioning it here in case anyone is surprised later about the > > > > lack of RRND configurability. > > > > > > i'm not familiar with how irg works. > > > > It generates a random tag based on some algorithm. > > > > > is the seed per process state that's set up at process startup in some > > > way? or shared (and thus effectively irg is non-deterministic in > > > userspace)? > > > > The seed is only relevant if the standard algorithm is used (RRND=0). > > i wanted to understand if we can get deterministic > irg behaviour in user space (which may be useful > for debugging to get reproducible tag failures). > > i guess if no control is exposed that means non- > deterministic irg. i think this is fine. Hmmm, I guess this might eventually be wanted. But it's probably OK not to have it to begin with. Things like CRIU restores won't be reproducible unless the seeds can be saved/restored. Doesn't seem essential from day 1 though. Cheers ---Dave
