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[Bug 1745247] Re: [Hyper-V] x86/hyperv: Stop suppressing X86_FEATURE_PCID

To: group.of.nepali.translators@xxxxxxxxxxxxxxxxxxx
From: Brad Figg <brad.figg@xxxxxxxxxxxxx>
Date: Fri, 26 Jan 2018 22:27:59 -0000
Reply-to: Bug 1745247 <1745247@xxxxxxxxxxxxxxxxxx>
Sender: bounces@xxxxxxxxxxxxx
** Also affects: linux-lts-vivid (Ubuntu Artful)
   Importance: Undecided
       Status: New

** Also affects: linux-lts-xenial (Ubuntu Artful)
   Importance: Undecided
       Status: New

** Also affects: linux-azure (Ubuntu Artful)
   Importance: Undecided
       Status: New

** Also affects: linux-azure-edge (Ubuntu Artful)
   Importance: Undecided
       Status: New

** Also affects: linux-lts-vivid (Ubuntu Trusty)
   Importance: Undecided
       Status: New

** Also affects: linux-lts-xenial (Ubuntu Trusty)
   Importance: Undecided
       Status: New

** Also affects: linux-azure (Ubuntu Trusty)
   Importance: Undecided
       Status: New

** Also affects: linux-azure-edge (Ubuntu Trusty)
   Importance: Undecided
       Status: New

** Also affects: linux-lts-vivid (Ubuntu Xenial)
   Importance: Undecided
       Status: New

** Also affects: linux-lts-xenial (Ubuntu Xenial)
   Importance: Undecided
       Status: New

** Also affects: linux-azure (Ubuntu Xenial)
   Importance: Undecided
       Status: New

** Also affects: linux-azure-edge (Ubuntu Xenial)
   Importance: Undecided
       Status: New

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https://bugs.launchpad.net/bugs/1745247

Title:
  [Hyper-V] x86/hyperv: Stop suppressing X86_FEATURE_PCID

Status in Linux:
  Confirmed
Status in linux package in Ubuntu:
  New
Status in linux-azure package in Ubuntu:
  New
Status in linux-azure-edge package in Ubuntu:
  New
Status in linux source package in Xenial:
  New
Status in linux-azure source package in Xenial:
  New
Status in linux-azure-edge source package in Xenial:
  New

Bug description:
  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

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