[souprock]

An addition to "Linux 2.2 (1999)" is: introduced meltdown vulnerability. That was the then-unknown cost of software context switching.

Later, with Red Hat's 4g4g kernels that Linus rejected, the problem would go away for people who installed Red Hat's version of the OS on systems with many gigabytes of memory.

[bogomipz]

Can you elaborate? How does relying on pure TSS for context switching prevent meltdown?

What were the 4g4g kernels? Might you have any literature and/or on those?

[DSingularity]

Separate address space for kernel and user. Hardware will use TSS to switch address space as needed for syscalls.

[amluto]

Huh? Hardware TSS has nothing to do with separating address spaces. It’s just a trick for switching the address space, and it’s really quite slow on modern CPUs. In theory, a kernel could use hardware task switching to switch address spaces on user/kernel transitions by forcing a hardware task switch when this happens, but the performance impacts would be considerably worse than KPTI.

What does seem to mitigate Meltdown on some CPUs is enabling segment limits for user code. This does nothing for 64-bit code, though.

edit: not to mention that there are no hardware context switches on 64-bit kernels. AMD removed support entirely in 64-bit mode. The TSS still exists, but it’s just an awkward dumping ground for a couple of data structures.

[bogomipz]

Do you the reason why Linus rejected this idea?

[souprock]

The main reason seemed to be the relatively bad performance of the hardware task switch (loading segment registers and the page table base) that would be required for any system call.

Of course, that turns out to be the fix for meltdown, unless you have the process-context identifiers (PCID) available on Haswell chips and newer. The meltdown fix for older CPUs, such as the Pentium III and Intel Core, is roughly the same as the 4g4g kernel changes.

BTW, the 4g4g kernels were created for a different reason. The kernel needed more virtual address space for itself, and thus couldn't share with user code. This was for a time when people were trying to run 32-bit kernels on systems with 32 gigabytes of RAM.

[bogomipz]

>"Of course, that turns out to be the fix for meltdown, unless you have the process-context identifiers (PCID) available on Haswell chips and newer."

Using TSS based switching is incompatible with PCIDs? Or is it incompatible with separate address spaces for user space and kernel space?

PCIDs are process ID tags on cache lines correct?

[souprock]

TSS switching is incompatible with running in 64-bit mode. It is very slow. Doing most of the same actions (reload segment registers and the page table base) in software is also very slow. Prior to the meltdown issue, Linux system calls had avoided most reloads of the segment registers and page table base.

PCIDs are incompatible with older hardware. They are modestly slow. I think the PCID state includes the TLB.

That pretty much means the kernel must support both methods. The PCIDs is used when possible. When the hardware doesn't support PCIDs, Linux must instead reload segment registers and the page table base, either step-by-step in software or via a TSS switch.

BTW, I had to implement x86 hardware task switching for an x86 emulator. The complexity is insane. See my "Who is Hiring?" post if that sounds fun for you.