Re:Linux page table isolation is not needed on AMD processors
by pm215 on Tuesday January 02, 2018 @11:48AM
(#16053088)
Linux gettimeofday these days is implemented in the 'vdso', which is code provided by the kernel that runs solely in userspace. So it's not a syscall in the 'privilege level switch by executing insn that takes an exception' sense and shouldn't be affected by the syscall-entry/exit path becoming more expensive.
Re:Linux page table isolation is not needed on AMD processors
by tankenmate on Tuesday January 02, 2018 @12:40PM
(#16053552)
Does this also apply to vsyscall's being emulated? Will this mean that older static binaries will no longer run? or just suffer a penalty as well?
Re:Linux page table isolation is not needed on AMD processors
by wolf550e on Tuesday January 02, 2018 @01:14PM
(#16053866)
Linux kernel has a compatibility guarantee to the user space visible API, so static binaries will continue to run. If the static binary is so old it does not know about vdso and uses regular syscall to query time, it will be slowed down.
Re:Linux page table isolation is not needed on AMD processors
by amluto on Tuesday January 02, 2018 @09:59PM
(#16057931)
Emulated vsyscalls were already very slow and will be even slower on a patched kernel. They'll still work, though.
Re:Linux page table isolation is not needed on AMD processors
by tankenmate on Tuesday January 02, 2018 @05:30PM
(#16056462)
This topic got me to look at the vDSO for my own machine so I wrote a short Perl program to dump your kernel's vDSO.
Re:Linux page table isolation is not needed on AMD processors
by tankenmate on Wednesday January 03, 2018 @09:49AM
(#16061370)
I added a quick repo to github if you want to clone or send pull requests for architectures apart from 64bit LE.
Another syscall I think that might cause issues is gettimeofday(), that particular call has been optimised to the nth degree, and lots of user programs spam the crap out of it (mostly necessarily), especially networking and streaming programs. It would be interesting to see how much of an overhead percentagewise page table isolation will cost, and its effects on low end media devices, et al.