Re: [PATCH 01/13] x86/time.c: Use system time to calculate elapsed_nsec in tsc_get_info()

["Jan Beulich" <JBeulich@suse.com>]

>>> On 14.10.15 at 04:45, <haozhong.zhang@intel.com> wrote:
> However, patch 2 is still necessary. The existing tsc_get_info() uses
> the host TSC frequency as the guest TSC frequency for a domain in
> TSC_MODE_DEFAULT, which could cause errors in the following example:
>  - A domain d using TSC_MODE_DEFAULT is created on host A, then
>    migrated to host B, and finally migrated to host C.
>  - The host TSC frequencies of three hosts are f_a, f_b and f_c
>    respectively and f_a != f_b and f_b != f_c.
>  - Both host B and host C support TSC scaling (either VMX TSC scaling
>    or SVM TSC ratio).
> 
> In above example w/o patch 2,
>  1. Initially, d->arch.tsc_khz == f_a.
>  
>  2. In the first migration, tsc_get_info() on host A passes f_a as the
>     guest TSC frequency to tsc_set_info() on host B, so that after the
>     migration it's still that d->arch.tsc_khz == f_a. As TSC scaling
>     takes effect, guest programs can still observe TSC in frequency f_a.
>     So far so good.
>     
>  3. However, in the second migration, f_b (!= f_a) is passed as the
>     guest TSC frequency to tsc_set_info() on host C so that after the
>     migration d->arch.tsc_khz is not f_a any more.

Hmm, yes, looks like you're right. But I don't think the current use of
cpu_khz should be blindly replaced - instead this should be done only
when cpu_has_tsc_ratio (mirroring what tsc_set_info() does). Unless
of course it can be proven that d->arch.tsc_khz == cpu_khz in all
relevant cases without use of TSC ratio (i.e. the use of cpu_khz here
just served as kind of a shorthand).

Jan