Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1030381Ab2EPLqf (ORCPT ); Wed, 16 May 2012 07:46:35 -0400 Received: from mx1.redhat.com ([209.132.183.28]:56268 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1030304Ab2EPLqb (ORCPT ); Wed, 16 May 2012 07:46:31 -0400 Date: Wed, 16 May 2012 14:46:23 +0300 From: "Michael S. Tsirkin" To: x86@kernel.org, kvm@vger.kernel.org Cc: Ingo Molnar , "H. Peter Anvin" , Avi Kivity , Marcelo Tosatti , gleb@redhat.com, Linus Torvalds , linux-kernel@vger.kernel.org Subject: [PATCHv4 4/5] kvm: eoi msi documentation Message-ID: <83448488f2e7220e96e9bedce870c139c712a9ad.1337168687.git.mst@redhat.com> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Mutt-Fcc: =sent Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2502 Lines: 56 Document the new EOI MSR. Couldn't decide whether this change belongs conceptually on guest or host side, so a separate patch. Signed-off-by: Michael S. Tsirkin --- Documentation/virtual/kvm/msr.txt | 32 ++++++++++++++++++++++++++++++++ 1 files changed, 32 insertions(+), 0 deletions(-) diff --git a/Documentation/virtual/kvm/msr.txt b/Documentation/virtual/kvm/msr.txt index 5031780..3481f1b 100644 --- a/Documentation/virtual/kvm/msr.txt +++ b/Documentation/virtual/kvm/msr.txt @@ -219,3 +219,35 @@ MSR_KVM_STEAL_TIME: 0x4b564d03 steal: the amount of time in which this vCPU did not run, in nanoseconds. Time during which the vcpu is idle, will not be reported as steal time. + +MSR_KVM_EOI_EN: 0x4b564d04 + data: Bit 0 is 1 when PV end of interrupt is enabled on the vcpu; 0 + when disabled. When enabled, bits 63-1 hold 2-byte aligned physical address + of a 2 byte memory area which must be in guest RAM and must be zeroed. + + The first, least significant bit of 2 byte memory location will be + written to by the hypervisor, typically at the time of interrupt + injection. Value of 1 means that guest can skip writing EOI to the apic + (using MSR or MMIO write); instead, it is sufficient to signal + EOI by clearing the bit in guest memory - this location will + later be polled by the hypervisor. + Value of 0 means that the EOI write is required. + + It is always safe for the guest to ignore the optimization and perform + the APIC EOI write anyway. + + Hypervisor is guaranteed to only modify this least + significant bit while in the current VCPU context, this means that + guest does not need to use either lock prefix or memory ordering + primitives to synchronise with the hypervisor. + + However, hypervisor can set and clear this memory bit at any time: + therefore to make sure hypervisor does not interrupt the + guest and clear the least significant bit in the memory area + in the window between guest testing it to detect + whether it can skip EOI apic write and between guest + clearing it to signal EOI to the hypervisor, + guest must both read the least sgnificant bit in the memory area and + clear it using a single CPU instruction, such as test and clear, or + compare and exchange. + -- MST -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/