Messages in this thread | | | Date | Mon, 25 May 2020 10:17:18 +0100 | From | Marc Zyngier <> | Subject | Re: [RFC PATCH v12 10/11] arm64: add mechanism to let user choose which counter to return |
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On 2020-05-24 03:11, Richard Cochran wrote: > On Fri, May 22, 2020 at 04:37:23PM +0800, Jianyong Wu wrote: >> In general, vm inside will use virtual counter compered with host use >> phyical counter. But in some special scenarios, like nested >> virtualization, phyical counter maybe used by vm. A interface added in >> ptp_kvm driver to offer a mechanism to let user choose which counter >> should be return from host. > > Sounds like you have two time sources, one for normal guest, and one > for nested. Why not simply offer the correct one to user space > automatically? If that cannot be done, then just offer two PHC > devices with descriptive names.
There is no such thing as a distinction between nested or non-nested. Both counters are available to the guest at all times, and said guest can choose whichever it wants to use. So the hypervisor (KVM) has to support both counters as a reference.
For a Linux guest, we always know which reference we're using (the virtual counter). So it is pointless to expose the choice to userspace at all.
> >> diff --git a/drivers/ptp/ptp_chardev.c b/drivers/ptp/ptp_chardev.c >> index fef72f29f3c8..8b0a7b328bcd 100644 >> --- a/drivers/ptp/ptp_chardev.c >> +++ b/drivers/ptp/ptp_chardev.c >> @@ -123,6 +123,9 @@ long ptp_ioctl(struct posix_clock *pc, unsigned >> int cmd, unsigned long arg) >> struct timespec64 ts; >> int enable, err = 0; >> >> +#ifdef CONFIG_ARM64 >> + static long flag; > > static? This is not going to fly. > >> + * In most cases, we just need virtual counter from host and >> + * there is limited scenario using this to get physical counter >> + * in guest. >> + * Be careful to use this as there is no way to set it back >> + * unless you reinstall the module. > > How on earth is the user supposed to know this? > > From your description, this "flag" really should be a module > parameter.
Not even that. If anything, the driver can obtain full knowledge of which counter is in use without any help. And the hard truth is that it is *always* the virtual counter as far as Linux is concerned.
M. -- Jazz is not dead. It just smells funny...
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