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Re: [Xen-devel] [PATCH v4 04/14] x86/rtc: replace paravirt rtc check with platform legacy quirk

On 04/08/2016 02:29 AM, Luis R. Rodriguez wrote:
On Thu, Apr 7, 2016 at 10:18 PM, Juergen Gross <jgross@xxxxxxxx> wrote:
On 08/04/16 02:32, Luis R. Rodriguez wrote:
On Thu, Apr 07, 2016 at 08:55:54AM -0400, Boris Ostrovsky wrote:
On 04/06/2016 08:06 PM, Luis R. Rodriguez wrote:
We have 4 types of x86 platforms that disable RTC:

   * Intel MID
   * Lguest - uses paravirt
   * Xen dom-U - uses paravirt
   * x86 on legacy systems annotated with an ACPI legacy flag

We can consolidate all of these into a platform specific legacy
quirk set early in boot through i386_start_kernel() and through
x86_64_start_reservations(). This deals with the RTC quirks which
we can rely on through the hardware subarch, the ACPI check can
be dealt with separately.

v2: split the subarch check from the ACPI check, clarify
     on the ACPI change commit log why ordering works

Suggested-by: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: Luis R. Rodriguez <mcgrof@xxxxxxxxxx>
<-- snip -->

diff --git a/arch/x86/kernel/platform-quirks.c 
new file mode 100644
index 000000000000..1b114ac5996f
--- /dev/null
+++ b/arch/x86/kernel/platform-quirks.c
@@ -0,0 +1,18 @@
+#include <linux/kernel.h>
+#include <linux/init.h>
+#include <asm/setup.h>
+#include <asm/bios_ebda.h>
+void __init x86_early_init_platform_quirks(void)
+   x86_platform.legacy.rtc = 1;
+   switch (boot_params.hdr.hardware_subarch) {
+   case X86_SUBARCH_XEN:
+   case X86_SUBARCH_LGUEST:
+           x86_platform.legacy.rtc = 0;
+           break;
+   }
What about Xen dom0 (aka initial domain)?
Indeed, thanks for catching this, the hunk below removes the re-enablement of
the the RTC for dom0:

--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -1192,7 +1192,6 @@ static const struct pv_info xen_info __initconst = {
  #ifdef CONFIG_X86_64
     .extra_user_64bit_cs = FLAT_USER_CS64,
-   .features = 0,
     .name = "Xen",
@@ -1525,8 +1524,6 @@ asmlinkage __visible void __init xen_start_kernel(void)
     /* Install Xen paravirt ops */
     pv_info = xen_info;
-   if (xen_initial_domain())
-           pv_info.features |= PV_SUPPORTED_RTC;
     pv_init_ops = xen_init_ops;
     if (!xen_pvh_domain()) {
             pv_cpu_ops = xen_cpu_ops;
This should then break dom0 unless of course you have the respective next
patch applied and that disabled the RTC due to an ACPI setting on your
platform. Juergen, can you check to see if that was the case for your
testing platform on dom0 ?
Are you sure it would break?
No, suspected that it should though.

Wouldn't it just fall back to another
clock source, e.g. hpet?
I suppose so.

I looked into my test system: seems as if add_rtc_cmos() is returning
before the .legacy.rtc test.
OK thanks...

It works because the clock must have been discovered by ACPI prior to add_rtc_cmos() call. It's PNP0b00 object, I believe. The rest of the routine is to handle the case when RTC is not found in ACPI tables for whatever reasons (I think).

That's why we added paravirt_has(RTC) --- dom0 should be able to handle such cases, just like bare metal.


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