[PATCH v3 0/3] KVM: LAPIC: Rework lapic timer to behave more like real-hardware

[Wanpeng Li]

The issue is reported in xen community.

Anthony PERARD pointed out:

https://www.mail-archive.com/xen-devel@xxxxxxxxxxxxx/msg117283.html#

 | When developing PVH for OVMF, I've used the lapic timer. It turns out that the
 | way it is used by OVMF did not work with Xen [1]. I tried to find out how
 | real-hw behave, and write a XTF tests [2]. And this patch series tries to fix
 | the behavior of the vlapic timer.
 | 
 | 
 | The OVMF driver for the APIC timer initialize the timer like this:
 | 	write to TMICT (initial counter)
 | 	write to TMDCR (divide configuration)
 | 	enable the timer (this may change timer mode from one-shot to periodic)
 | It turns out that TMICT is set to 0 on the last step, but OVMF expect the timer
 | to run.
 | 
 | Here is some description of the APIC timer, base on observation as well as read
 | of the Intel SDM. The description is also patch of patch description
 | (reworded).
 | 
 | Maybe a way of thinking how the APIC timer is evaluated, is to think of how
 | hardward will do it. There is a counter TMCCT which always keeps counting down.
 | 
 | Setting TMICT also set TMCCT, nothing else matter.
 | Setting LVTT does not change anything right away.
 | Setting TMDCR does not change much.
 | 
 | Now TMCCT keeps counting down, by a value related to TMDCR.
 | Once, TMCCT reach 0, it is only at this time that LVTT is taken into account.
 | Is there an interrupt to deliver? Should the timer restart counting from the
 | value in TMICT?
 | 
 | In the Intel SDM, there is the word "disarm" of the timer used. I guess the
 | easier way to disarm the APIC timer (when in periodic or one-shot) is to set
 | TMICT to 0. But if we take TSC-Deadline mode out of the picture, there is
 | nothing in the manual that say that the timer is disarm or stopped when
 | changing timer mode (there is only two modes left, period and one-shot).
 | 
 | As for the TSC-deadline timer mode, observation shown that changing to it (or
 | from it) does reset and disarm both timers, so effectively TMICT and the
 | tscdeadline are set to 0.
 | 
 | [1] https://lists.xenproject.org/archives/html/xen-devel/2016-12/msg00959.html
 | [2] v1: 
 | https://lists.xenproject.org/archives/html/xen-devel/2017-03/msg02533.html
 |     v2: look for "[XTF PATCH V2 0/3] Testing vlapic timer"


v2 -> v3:
 * move the write 0 to APIC_TMICT logic to apic_update_lvtt()
 * skip hrtimer_cancel() in apic_update_lvtt() when either from one-shot mode to 
   periodic or vice versa.

v1 -> v2:
 * add cover-letter and collect recent lapic patches to one patchset


Wanpeng Li (3):
  KVM: LAPIC: Fix lapic timer mode transition
  KVM: LAPIC: Keep timer running when switching between one-shot and periodic mode
  KVM: LAPIC: Apply change to TDCR right away to the timer

 arch/x86/include/asm/apicdef.h |  1 +
 arch/x86/kvm/lapic.c           | 63 +++++++++++++++++++++++++++++++-----------
 2 files changed, 48 insertions(+), 16 deletions(-)

-- 
2.7.4