** Changed in: linux-azure-edge (Ubuntu Xenial)
Status: In Progress => Fix Committed
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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 Trusty:
Invalid
Status in linux source package in Xenial:
Invalid
Status in linux-azure source package in Xenial:
Fix Committed
Status in linux-azure-edge source package in Xenial:
Fix Committed
Status in linux source package in Artful:
Invalid
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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