Re: [PATCH v4 3/4] arm64: arch_timer: Work around QorIQ Erratum A-008585

On Thu, 8 Sep 2016 20:08:56 -0500
Scott Wood [off-list ref] wrote:

On Fri, 2016-08-26 at 13:40 +0100, Marc Zyngier wrote:

quoted

On Thu, 7 Jul 2016 02:46:11 -0500
Scott Wood [off-list ref] wrote:

(+Mark)
  

quoted

 static __always_inline
 u32 arch_timer_reg_read_cp15(int access, enum arch_timer_reg reg)
 {

@@ -66,19 +102,19 @@ u32 arch_timer_reg_read_cp15(int access, enum

arch_timer_reg reg)
 	if (access == ARCH_TIMER_PHYS_ACCESS) {
 		switch (reg) {
 		case ARCH_TIMER_REG_CTRL:
-			asm volatile("mrs %0,  cntp_ctl_el0" : "=r"
(val));
+			asm volatile("mrs %0, cntp_ctl_el0" : "=r"
(val));  

Spurious change?
  

quoted

 			break;
 		case ARCH_TIMER_REG_TVAL:
-			asm volatile("mrs %0, cntp_tval_el0" : "=r"
(val));
+			val = _arch_timer_get_ptval();
 			break;
 		}
 	} else if (access == ARCH_TIMER_VIRT_ACCESS) {
 		switch (reg) {
 		case ARCH_TIMER_REG_CTRL:
-			asm volatile("mrs %0,  cntv_ctl_el0" : "=r"
(val));
+			asm volatile("mrs %0, cntv_ctl_el0" : "=r"
(val));  

Here too?  

No, it's not spurious.

I answered this in http://lists.infradead.org/pipermail/linux-arm-kernel/2016-
June/438310.html

The extra spacing seemed to be an attempt to get things to line up between the
CTRL and TVAL asm statements.  When the TVAL case was converted to a function
call, there was nothing for the above to line up with, so I moved it back to
normal spacing.

quoted

I'm still worried that this series doesn't address Xen or KVM guests
that need to be made aware of the broken timers.

At the very least, I'd like a kernel command line option that'd let the
user reliably run its VMs. You can do something along the lines of
46fd5c6b, and have a command line argument like
"clocksource.arm_arch_timer.fsl-a008585=1", which would enable the
workaround.  

OK, I'll respin with a command line argument to use for now.  Mike Caraman has
said he plans to do a better solution for KVM -- Mike, have you had a chance
to look at this?

If there is a plan, we'd all like to hear about it, specially if this
involves a userspace ABI (which is likely).

Thanks,

	M.
-- 
Without deviation from the norm, progress is not possible.
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