On 2/14/2018 10:11 AM, David Woodhouse wrote: > > > On Wed, 2018-02-14 at 10:07 -0600, Tom Lendacky wrote: >> Shouldn't these writes to the MSR be just for the IBRS bit? The spec >> also defines the STIBP bit for this MSR, and if that bit had been set by >> BIOS for example, these writes will clear it. And who knows what future >> bits may be defined and how they'll be used. > > We don't use STIBP. If one day we do decide to set it in userspace for Right, I understand the kernel doesn't use STIBP, that's why I mentioned BIOS as an example. > "sensitive" processes, if we're done having the debate about what those > are, then that seems unlikely to conflict what what this code is doing > anyway, as we would presumably *clear* it again on the way back into > the kernel. > > I certainly don't want to add a read/modify/write cycle here just to Right, definitely to be avoided. Maybe the value could be tracked in a per-cpu variable so you never have to read it before the write. Just change the bit in question and write. Not sure that's really feasible though. > cope with some hypothetical future use case for STIBP, when there would > be better ways to cope. Just putting it out there, no worries. Thanks, Tom >
