Hi.
Pavel Machek wrote:
+ andl $(VM_MASK | (4 << 8) | 3), %eax + cmpl $((4 << 8) | 3), %eax + je ldt_ss # returning to user-space with LDT SSAll common linux apps use same %ss, no? Perhaps it would be more efficient to just check if %ss == 0x7b, and proceed directly to restore_nocheck if not?
Such an optimization will cost three more instructions, one of which is a "taken" jump. It seems like the "taken" jump on a fast path is not good, while now it is only 5 instructions with a not-taken jump. I am not sure here, but I think the current solution is better (depends on how bad the "taken" jump is, and how bad it is to have the three extra insns for that optimization purpose).
Or perhaps we could only enable this code after application loads custom ldt?
The good thing here is that the code actually does what you say, i.e. it jumps to ldt_ss only when the app has loaded the custom ldt and loaded that selector to %ss. The way it is implemented, is probably different from what you mean, I assume you mean the new per-thread flag? But I don't see how it can be more optimal, i.e. you propose to check whether or not the app altered the ldt (which can just be the old glibc I think), while the current solution is to also check whether it was loaded to %ss (so the glibc case is avoided, IIRC glibc used to load %gs with LDT selector). I.e. since right now we jump to ldt_ss only when the %ss is loaded with an LDT selector, I think the extra checks are not needed.
- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
