When the FLUSH_ALL_VIRTUAL_ADDRESS_SPACES flag (0x2) is not specified, the flush hypercall matches the AddressSpace parameter with the base address of the page table root on each processor (the base address is page-aligned in long/32-bit mode, 32-byte aligned in PAE mode). If there is a match, it flushes the specified VAs, for all PCIDs.
The flush hypercall is allowed to flush more than the above, however. We will update the TLFS with this behavior. -----Original Message----- From: Andy Lutomirski [mailto:l...@kernel.org] Sent: Wednesday, January 24, 2018 8:28 AM To: David Zhang <daz...@microsoft.com>; Haiyang Zhang <haiya...@microsoft.com>; KY Srinivasan <k...@microsoft.com>; LKML <linux-kernel@vger.kernel.org>; H. Peter Anvin <h...@zytor.com>; Andrew Lutomirski <l...@kernel.org>; Vitaly Kuznetsov <vkuzn...@redhat.com>; Thomas Gleixner <t...@linutronix.de>; Michael Kelley (EOSG) <michael.h.kel...@microsoft.com>; Aditya Bhandari <adity...@microsoft.com>; Ingo Molnar <mi...@kernel.org>; Stephen Hemminger <sthem...@microsoft.com> Cc: linux-tip-comm...@vger.kernel.org Subject: Re: [tip:x86/hyperv] x86/hyperv: Stop suppressing X86_FEATURE_PCID On Wed, Jan 24, 2018 at 4:48 AM, tip-bot for Vitaly Kuznetsov <tip...@zytor.com> wrote: > Commit-ID: 04651dd978a8749e59065df14b970a127f219ac2 > Gitweb: > https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgit.kernel.org%2Ftip%2F04651dd978a8749e59065df14b970a127f219ac2&data=02%7C01%7Cdazhan%40microsoft.com%7C59a83800e5b84e1bb97008d56347705b%7Cee3303d7fb734b0c8589bcd847f1c277%7C1%7C0%7C636524080843155861&sdata=qA5eG6L9MieOSWL8SCBQaWcYJtN7IT75rO0I1PdoZ%2FE%3D&reserved=0 > Author: Vitaly Kuznetsov <vkuzn...@redhat.com> > AuthorDate: Wed, 24 Jan 2018 11:36:29 +0100 > Committer: Thomas Gleixner <t...@linutronix.de> > CommitDate: Wed, 24 Jan 2018 13:44:57 +0100 > > x86/hyperv: Stop suppressing X86_FEATURE_PCID > > 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. So what, exactly, does the flush hypercall do? --Andy
