Public bug reported: Anywhere where Meltdown fixes have gone in have likely also picked up upstream work on TLB flushes. This is an important performance change.
x86/hyperv: Stop suppressing X86_FEATURE_PCIDx86/hyperv When hypercall-based TLB flush was enabled for Hyper-V guests PCID feature was deliberately suppressed as a precaution: back then PCID was never exposed to Hyper-V guests and it wasn't clear what will happen if some day it becomes available. The day came and PCID/INVPCID features are already exposed on certain Hyper-V hosts. >From TLFS (as of 5.0b) it is unclear how TLB flush hypercalls combine with PCID. In particular the usage of PCID is per-cpu based: the same mm gets different CR3 values on different CPUs. If the hypercall does exact matching this will fail. However, this is not the case. David Zhang explains: "In practice, the AddressSpace argument is ignored on any VM that supports PCIDs. Architecturally, the AddressSpace argument must match the CR3 with PCID bits stripped out (i.e., the low 12 bits of AddressSpace should be 0 in long mode). The flush hypercalls flush all PCIDs for the specified AddressSpace." With this, PCID can be enabled. Upstream commit link: https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/commit/?id=04651dd978a8749e59065df14b970a127f219ac2 ** Affects: linux-lts-vivid (Ubuntu) Importance: Undecided Status: New -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1745247 Title: [Hyper-V] x86/hyperv: Stop suppressing X86_FEATURE_PCID To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/linux-lts-vivid/+bug/1745247/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
