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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
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.