On Thu, May 12, 2022 at 4:35 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote: > > On Thu, May 12 2022 at 15:10, H. J. Lu wrote: > > On Thu, May 12, 2022 at 2:51 PM Dave Hansen <dave.hansen@xxxxxxxxx> wrote: > >> On 5/12/22 12:39, Thomas Gleixner wrote: > >> >> It's OK for a debugging build that runs on one kind of hardware. But, > >> >> if we want LAM-using binaries to be portable, we have to do something > >> >> different. > >> >> > >> >> One of the stated reasons for adding LAM hardware is that folks want to > >> >> use sanitizers outside of debugging environments. To me, that means > >> >> that LAM is something that the same binary might run with or without. > >> > On/off yes, but is there an actual use case where such a mechanism would > >> > at start time dynamically chose the number of bits? > >> > >> I'd love to hear from folks doing the userspace side of this. Will > >> userspace be saying: "Give me all the bits you can!". Or, will it > >> really just be looking for 6 bits only, and it doesn't care whether it > >> gets 6 or 15, it will use only 6? > >> > >> Do the sanitizers have more overhead with more bits? Or *less* overhead > >> because they can store more metadata in the pointers? > >> > >> Will anyone care about the difference about potentially missing 1/64 > >> issues with U57 versus 1/32768 with U48? > > > > The only LAM usage I know so far is LAM_U57 in HWASAN. > > That's at least a halfways useful answer. > > > An application can ask for LAM_U48 or LAM_U57. But the decision should > > be made by application. > > It can ask for whatever, but the decision whether it's granted is made > by the kernel for obvious reasons. > > > When an application asks for LAM_U57, I expect it will store tags in > > upper 6 bits, even if the kernel enables LAM_U48. > > The kernel does not enable LAM_U48 when the application only wants to > have LAM_U57, because that would restrict the address space of the > application to 47 bits on 5-level capable system for no reason. > > So what are you trying to tell me? > I am expecting applications to ask for LAM_U48 or LAM_U57, not just LAM. -- H.J.
