Hi Marc,

On Tue, May 11, 2021 at 11:13 AM Marc Zyngier [off-list ref] wrote:

On 2021-05-11 10:07, Geert Uytterhoeven wrote:

quoted

On Tue, Mar 30, 2021 at 4:56 PM Marc Zyngier [off-list ref] wrote:

quoted

From: Jianyong Wu <redacted>

Currently, there is no mechanism to keep time sync between guest and
host
in arm/arm64 virtualization environment. Time in guest will drift
compared
with host after boot up as they may both use third party time sources
to correct their time respectively. The time deviation will be in
order
of milliseconds. But in some scenarios,like in cloud environment, we
ask
for higher time precision.

kvm ptp clock, which chooses the host clock source as a reference
clock to sync time between guest and host, has been adopted by x86
which takes the time sync order from milliseconds to nanoseconds.

This patch enables kvm ptp clock for arm/arm64 and improves clock sync
precision
significantly.

quoted

--- a/drivers/ptp/Kconfig
+++ b/drivers/ptp/Kconfig

@@ -108,7 +108,7 @@ config PTP_1588_CLOCK_PCH
 config PTP_1588_CLOCK_KVM
        tristate "KVM virtual PTP clock"
        depends on PTP_1588_CLOCK
-       depends on KVM_GUEST && X86
+       depends on (KVM_GUEST && X86) || (HAVE_ARM_SMCCC_DISCOVERY &&

ARM_ARCH_TIMER)

Why does this not depend on KVM_GUEST on ARM?
I.e. shouldn't the dependency be:

    KVM_GUEST && (X86 || (HAVE_ARM_SMCCC_DISCOVERY && ARM_ARCH_TIMER))

?

arm/arm64 do not select KVM_GUEST. Any kernel can be used for a guest,
and KVM/arm64 doesn't know about this configuration symbol.

OK.

Does PTP_1588_CLOCK_KVM need to default to yes?
Perhaps only on X86, to maintain the status quo?

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds