Re: [PATCH v3 02/13] x86/time.c: Use correct guest TSC frequency in tsc_get_info()

From: Haozhong Zhang <haozhong.zhang@intel.com>
To: Jan Beulich <JBeulich@suse.com>
Cc: Kevin Tian <kevin.tian@intel.com>, Keir Fraser <keir@xen.org>,
	Jun Nakajima <jun.nakajima@intel.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	xen-devel@lists.xen.org,
	Aravind Gopalakrishnan <Aravind.Gopalakrishnan@amd.com>,
	Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>
Subject: Re: [PATCH v3 02/13] x86/time.c: Use correct guest TSC frequency in tsc_get_info()
Date: Fri, 8 Jan 2016 17:12:25 +0800	[thread overview]
Message-ID: <20160108091224.GD3828@hz-desktop.sh.intel.com> (raw)
In-Reply-To: <568F89D902000078000C4B3E@prv-mh.provo.novell.com>

On 01/08/16 02:05, Jan Beulich wrote:
> >>> On 04.01.16 at 18:48, <boris.ostrovsky@oracle.com> wrote:
> > On 12/30/2015 10:03 PM, Haozhong Zhang wrote:
> >> When the TSC mode of a HVM container is TSC_MODE_DEFAULT or
> >> TSC_MODE_PVRDTSCP and no TSC emulation is used, the existing
> >> tsc_get_info() uses the host TSC frequency (cpu_khz) as the guest TSC
> >> frequency. However, tsc_set_info() may set the guest TSC frequency to a
> >> value different than the host. In order to keep consistent to
> >> tsc_set_info(), this patch makes tsc_get_info() use the value set by
> >> tsc_set_info() as the guest TSC frequency.
> >>
> >> Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
> >> ---
> >> Changes in v3:
> >>   (addressing Boris Ostrovsky's comments)
> >>   * Use this_cpu(cpu_time).tsc_scale for both scaling and non-scaling cases.
> >>
> >>   xen/arch/x86/time.c | 9 ++++++---
> >>   1 file changed, 6 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c
> >> index 0059b6a..d83f068 100644
> >> --- a/xen/arch/x86/time.c
> >> +++ b/xen/arch/x86/time.c
> >> @@ -1749,6 +1749,9 @@ void tsc_get_info(struct domain *d, uint32_t *tsc_mode,
> >>                     uint64_t *elapsed_nsec, uint32_t *gtsc_khz,
> >>                     uint32_t *incarnation)
> >>   {
> >> +    bool_t enable_tsc_scaling = has_hvm_container_domain(d) &&
> >> +                                cpu_has_tsc_ratio;
> >> +
> > 
> > Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
> > 
> > although I still think that having !d->arch.vtsc would be better, even 
> > if not strictly required.
> 
> Since Haozhong agreed, and for symmetry with patch 1 I'm going to
> commit with this added, plus ...
> 
> >> @@ -1780,9 +1783,9 @@ void tsc_get_info(struct domain *d, uint32_t *tsc_mode,
> >>           else
> >>           {
> >>               tsc = rdtsc();
> >> -            *elapsed_nsec = scale_delta(tsc, &d->arch.vtsc_to_ns) -
> >> +            *elapsed_nsec = scale_delta(tsc, &this_cpu(cpu_time).tsc_scale) -
> >>                               d->arch.vtsc_offset;
> >> -            *gtsc_khz = 0; /* ignored by tsc_set_info */
> >> +            *gtsc_khz = enable_tsc_scaling ? d->arch.tsc_khz : 0;
> 
> ... with the comment here retained.
>
And it's better to refine the comment like
/* ignored by tsc_set_info if TSC scaling disabled */

Haozhong