x86/hyperv: Stop suppressing X86_FEATURE_PCID
authorVitaly Kuznetsov <vkuznets@redhat.com>
Wed, 24 Jan 2018 10:36:29 +0000 (11:36 +0100)
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Thu, 26 Apr 2018 09:02:11 +0000 (11:02 +0200)
commit519a7119527c26984e6804abc422808681d1a117
treed22fb19a934e167d4d7cce72c96c722d738336a8
parent9a1dda252663c930822c933232736cd05a8cb800
x86/hyperv: Stop suppressing X86_FEATURE_PCID

[ Upstream commit 617ab45c9a8900e64a78b43696c02598b8cad68b ]

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.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: David Zhang <dazhan@microsoft.com>
Cc: Stephen Hemminger <sthemmin@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: "Michael Kelley (EOSG)" <Michael.H.Kelley@microsoft.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: devel@linuxdriverproject.org
Cc: "K. Y. Srinivasan" <kys@microsoft.com>
Cc: Aditya Bhandari <adityabh@microsoft.com>
Link: https://lkml.kernel.org/r/20180124103629.29980-1-vkuznets@redhat.com
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
arch/x86/hyperv/mmu.c