From patchwork Tue Jun 1 12:22:48 2010 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Glauber Costa X-Patchwork-Id: 103522 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by demeter.kernel.org (8.14.3/8.14.3) with ESMTP id o51CVZlK007970 for ; Tue, 1 Jun 2010 12:31:35 GMT Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756069Ab0FAMbH (ORCPT ); Tue, 1 Jun 2010 08:31:07 -0400 Received: from mx1.redhat.com ([209.132.183.28]:14327 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755889Ab0FAMbE (ORCPT ); Tue, 1 Jun 2010 08:31:04 -0400 Received: from int-mx03.intmail.prod.int.phx2.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.16]) by mx1.redhat.com (8.13.8/8.13.8) with ESMTP id o51CV3Bq013580 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=OK); Tue, 1 Jun 2010 08:31:04 -0400 Received: from localhost.localdomain (virtlab1.virt.bos.redhat.com [10.16.72.21]) by int-mx03.intmail.prod.int.phx2.redhat.com (8.13.8/8.13.8) with ESMTP id o51CV1WA001828; Tue, 1 Jun 2010 08:31:01 -0400 From: Glauber Costa To: kvm@vger.kernel.org Cc: linux-kernel@vger.kernel.org, avi@redhat.com, zamsden@redhat.com Subject: [PATCH v4] Add Documentation/kvm/msr.txt Date: Tue, 1 Jun 2010 08:22:48 -0400 Message-Id: <1275394968-16168-1-git-send-email-glommer@redhat.com> X-Scanned-By: MIMEDefang 2.67 on 10.5.11.16 Sender: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org X-Greylist: IP, sender and recipient auto-whitelisted, not delayed by milter-greylist-4.2.3 (demeter.kernel.org [140.211.167.41]); Tue, 01 Jun 2010 12:31:36 +0000 (UTC) diff --git a/Documentation/kvm/msr.txt b/Documentation/kvm/msr.txt new file mode 100644 index 0000000..a8502e3 --- /dev/null +++ b/Documentation/kvm/msr.txt @@ -0,0 +1,153 @@ +KVM-specific MSRs. +Glauber Costa , Red Hat Inc, 2010 +===================================================== + +KVM makes use of some custom MSRs to service some requests. +At present, this facility is only used by kvmclock. + +Custom MSRs have a range reserved for them, that goes from +0x4b564d00 to 0x4b564dff. There are MSRs outside this area, +but they are deprecated and their use is discouraged. + +Custom MSR list +-------- + +The current supported Custom MSR list is: + +MSR_KVM_WALL_CLOCK_NEW: 0x4b564d00 + + data: 4-byte alignement physical address of a memory area which must be + in guest RAM. This memory is expected to hold a copy of the following + structure: + + struct pvclock_wall_clock { + u32 version; + u32 sec; + u32 nsec; + } __attribute__((__packed__)); + + whose data will be filled in by the hypervisor. The hypervisor is only + guaranteed to update this data at the moment of MSR write. + Users that want to reliably query this information more than once have + to write more than once to this MSR. Fields have the following meanings: + + version: guest has to check version before and after grabbing + time information and check that they are both equal and even. + An odd version indicates an in-progress update. + + sec: number of seconds for wallclock. + + nsec: number of nanoseconds for wallclock. + + Note that although MSRs are per-CPU entities, the effect of this + particular MSR is global. + + Availability of this MSR must be checked via bit 3 in 0x4000001 cpuid + leaf prior to usage. + +MSR_KVM_SYSTEM_TIME_NEW: 0x4b564d01 + + data: 4-byte aligned physical address of a memory area which must be in + guest RAM, plus an enable bit in bit 0. This memory is expected to hold + a copy of the following structure: + + struct pvclock_vcpu_time_info { + u32 version; + u32 pad0; + u64 tsc_timestamp; + u64 system_time; + u32 tsc_to_system_mul; + s8 tsc_shift; + u8 flags; + u8 pad[2]; + } __attribute__((__packed__)); /* 32 bytes */ + + whose data will be filled in by the hypervisor periodically. Only one + write, or registration, is needed for each VCPU. The interval between + updates of this structure is arbitrary and implementation-dependent. + The hypervisor may update this structure at any time it sees fit until + anything with bit0 == 0 is written to it. + + Fields have the following meanings: + + version: guest has to check version before and after grabbing + time information and check that they are both equal and even. + An odd version indicates an in-progress update. + + tsc_timestamp: the tsc value at the current VCPU at the time + of the update of this structure. Guests can subtract this value + from current tsc to derive a notion of elapsed time since the + structure update. + + system_time: a host notion of monotonic time, including sleep + time at the time this structure was last updated. Unit is + nanoseconds. + + tsc_to_system_mul: a function of the tsc frequency. One has + to multiply any tsc-related quantity by this value to get + a value in nanoseconds, besides dividing by 2^tsc_shift + + tsc_shift: cycle to nanosecond divider, as a power of two, to + allow for shift rights. One has to shift right any tsc-related + quantity by this value to get a value in nanoseconds, besides + multiplying by tsc_to_system_mul. + + With this information, guests can derive per-CPU time by + doing: + + time = (current_tsc - tsc_timestamp) + time = (time * tsc_to_system_mul) >> tsc_shift + time = time + system_time + + flags: bits in this field indicate extended capabilities + coordinated between the guest and the hypervisor. Availability + of specific flags has to be checked in 0x40000001 cpuid leaf. + Current flags are: + + flag bit | cpuid bit | meaning + ------------------------------------------------------------- + | | time measures taken across + 0 | 24 | multiple cpus are guaranteed to + | | be monotonic + ------------------------------------------------------------- + + Availability of this MSR must be checked via bit 3 in 0x4000001 cpuid + leaf prior to usage. + + +MSR_KVM_WALL_CLOCK: 0x11 + + data and functioning: same as MSR_KVM_WALL_CLOCK_NEW. Use that instead. + + This MSR falls outside the reserved KVM range and may be removed in the + future. Its usage is deprecated. + + Availability of this MSR must be checked via bit 0 in 0x4000001 cpuid + leaf prior to usage. + +MSR_KVM_SYSTEM_TIME: 0x12 + + data and functioning: same as MSR_KVM_SYSTEM_TIME_NEW. Use that instead. + + This MSR falls outside the reserved KVM range and may be removed in the + future. Its usage is deprecated. + + Availability of this MSR must be checked via bit 0 in 0x4000001 cpuid + leaf prior to usage. + + The suggested algorithm for detecting kvmclock presence is then: + + if (!kvm_para_available()) /* refer to cpuid.txt */ + return NON_PRESENT; + + flags = cpuid_eax(0x40000001); + if (flags & 3) { + msr_kvm_system_time = MSR_KVM_SYSTEM_TIME_NEW; + msr_kvm_wall_clock = MSR_KVM_WALL_CLOCK_NEW; + return PRESENT; + } else if (flags & 0) { + msr_kvm_system_time = MSR_KVM_SYSTEM_TIME; + msr_kvm_wall_clock = MSR_KVM_WALL_CLOCK; + return PRESENT; + } else + return NON_PRESENT;