kvm: x86: export TSC information to user-space
authorLuiz Capitulino <lcapitulino@redhat.com>
Fri, 16 Sep 2016 14:27:36 +0000 (10:27 -0400)
committerPaolo Bonzini <pbonzini@redhat.com>
Fri, 16 Sep 2016 14:57:48 +0000 (16:57 +0200)
commit4f5758fc67607b915c7513baf0438a35a5fc457c
treeec8b9da28d9bc78d5802fcf9e44f81fd7095f161
parent45b5939e50746b92fd4cb47c02524f79ba8fabe6
kvm: x86: export TSC information to user-space

This commit exports the following information to
user-space via the newly created per-vcpu debugfs
directory:

 - TSC offset (as a signed number)
 - TSC scaling ratio
 - TSC scaling ratio fractinal bits

The original intention of this commit was to
export only the TSC offset, but the TSC scaling
information is exported for completeness.

We need to retrieve the TSC offset from user-space
in order to support the merging of host and guest
traces in trace-cmd. Today, we use the kvm_write_tsc_offset
tracepoint, but it has a number of problems (mainly,
it requires a running VM to be rebooted, ftrace setup,
and also tracepoints are not supposed to be ABIs).

The merging of host and guest traces is explained
in more detail in this thread:

 [Qemu-devel] [RFC] host and guest kernel trace merging
 https://lists.nongnu.org/archive/html/qemu-devel/2016-03/msg00887.html

This commit creates the following files in debugfs:

/sys/kernel/debug/kvm/66828-10/vcpu0/tsc-offset
/sys/kernel/debug/kvm/66828-10/vcpu0/tsc-scaling-ratio
/sys/kernel/debug/kvm/66828-10/vcpu0/tsc-scaling-ratio-frac-bits

The last two are only created if TSC scaling is supported.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
arch/x86/kvm/debugfs.c