4 ms·

With spectre/meltdown mitigations increasing the cost of syscalls I imagine this becomes quite significant. Somewhat orthogonally, does anyone happen to know i

With spectre/meltdown mitigations increasing the cost of syscalls I imagine this becomes quite significant. Somewhat orthogonally, does anyone happen to know if the hardware fixes eliminating the need for software mitigations still increase syscall cost substantially? I imagine if you have to inhibit speculation across context boundaries the cost is still higher than a pre-spectre world, even if done in hardware...

nightfly 8y ago
Last time I looked, the hardware mitigations were almost exactly as expensive as the software ones.
- wtallis 8y ago
  I'm not sure if there are even any true hardware mitigations actually shipping yet. I think the closest we have are products that ship with stock microcode implementing the same changes that aftermarket microcode updates applied to existing products.
  - ac29 8y ago
    Some hardware fixes are already in place: https://www.anandtech.com/show/13301/spectre-and-meltdown-in-hardware-intel-clarifies-whiskey-lake-and-amber-lake https://www.anandtech.com/show/13301/spectre-and-meltdown-in...
- saagarjha 8y ago
  That's because they effectively ossified the software fix in hardware, right? At some point we might be able to come up with a real way to fix the issue.
- bonzini 8y ago
  For most of them (not Spectre v1) hardware fixes are easy, just adding some extra bits to cache or BTB tags. Problem is it's whack-a-mole.
caf 8y ago
The Meltdown workaround - separate kernel and userspace page tables with %cr3 switched on kernel entry and exit - is responsible for the bulk of the syscall cost increase. That one is entirely fixable in hardware without a significant cost penalty (after all, AMD wasn't affected by that one). The various kinds of Spectre are harder (and are wider than just the kernel, things like Javascript VMs are also affected), but the cost there is more diffuse.

nightfly 8y ago

Last time I looked, the hardware mitigations were almost exactly as expensive as the software ones.

wtallis 8y ago
I'm not sure if there are even any true hardware mitigations actually shipping yet. I think the closest we have are products that ship with stock microcode implementing the same changes that aftermarket microcode updates applied to existing products.
- ac29 8y ago
  Some hardware fixes are already in place: https://www.anandtech.com/show/13301/spectre-and-meltdown-in-hardware-intel-clarifies-whiskey-lake-and-amber-lake https://www.anandtech.com/show/13301/spectre-and-meltdown-in...
saagarjha 8y ago
That's because they effectively ossified the software fix in hardware, right? At some point we might be able to come up with a real way to fix the issue.
bonzini 8y ago
For most of them (not Spectre v1) hardware fixes are easy, just adding some extra bits to cache or BTB tags. Problem is it's whack-a-mole.

caf 8y ago

The Meltdown workaround - separate kernel and userspace page tables with %cr3 switched on kernel entry and exit - is responsible for the bulk of the syscall cost increase. That one is entirely fixable in hardware without a significant cost penalty (after all, AMD wasn't affected by that one). The various kinds of Spectre are harder (and are wider than just the kernel, things like Javascript VMs are also affected), but the cost there is more diffuse.