From: Jan Beulich <jbeulich@suse.com>
To: "Roger Pau Monné" <roger.pau@citrix.com>
Cc: Igor Druzhinin <igor.druzhinin@citrix.com>,
andrew.cooper3@citrix.com, wl@xen.org,
xen-devel@lists.xenproject.org
Subject: Re: [Xen-devel] [PATCH] x86/time: update TSC stamp on restore from deep C-state
Date: Wed, 15 Jan 2020 12:40:27 +0100 [thread overview]
Message-ID: <f9b7c06e-54df-bfb2-44d5-b3ef38f2a725@suse.com> (raw)
In-Reply-To: <20200115094703.GH11756@Air-de-Roger>
On 15.01.2020 10:47, Roger Pau Monné wrote:
> On Tue, Jan 14, 2020 at 07:36:21PM +0000, Igor Druzhinin wrote:
>> --- a/xen/arch/x86/time.c
>> +++ b/xen/arch/x86/time.c
>> @@ -955,10 +955,16 @@ u64 stime2tsc(s_time_t stime)
>>
>> void cstate_restore_tsc(void)
>> {
>> + struct cpu_time *t = &this_cpu(cpu_time);
>> +
>> if ( boot_cpu_has(X86_FEATURE_NONSTOP_TSC) )
>> return;
>>
>> - write_tsc(stime2tsc(read_platform_stime(NULL)));
>> + t->stamp.master_stime = read_platform_stime(NULL);
>> + t->stamp.local_tsc = stime2tsc(t->stamp.master_stime);
>> + t->stamp.local_stime = t->stamp.master_stime;
>> +
>> + write_tsc(t->stamp.local_tsc);
>
> In order to avoid the TSC write (and the likely associated vmexit),
> could you instead do:
>
> t->stamp.local_stime = t->stamp.master_stime = read_platform_stime(NULL);
> t->stamp.local_tsc = rdtsc_ordered();
>
> I think it should achieve the same as it syncs the local TSC stamp and
> times, would avoid the TSC write and slightly simplifies the logic.
Wouldn't this result in guests possibly observing the TSC moving
backwards?
Jan
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next prev parent reply other threads:[~2020-01-15 11:40 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-14 19:36 [Xen-devel] [PATCH] x86/time: update TSC stamp on restore from deep C-state Igor Druzhinin
2020-01-15 9:47 ` Roger Pau Monné
2020-01-15 11:40 ` Jan Beulich [this message]
2020-01-15 11:53 ` Roger Pau Monné
2020-01-15 12:49 ` Jan Beulich
2020-01-15 13:44 ` Roger Pau Monné
2020-01-15 16:21 ` Jan Beulich
2020-01-16 9:33 ` Roger Pau Monné
2020-01-16 9:38 ` Jan Beulich
2020-01-16 12:09 ` Igor Druzhinin
2020-01-16 12:25 ` Roger Pau Monné
2020-01-15 12:25 ` Igor Druzhinin
2020-01-15 12:31 ` Igor Druzhinin
2020-01-15 12:41 ` Jan Beulich
2020-01-15 12:36 ` Igor Druzhinin
2020-01-15 13:23 ` Roger Pau Monné
2020-01-15 14:45 ` Igor Druzhinin
2020-01-15 11:32 ` Jan Beulich
2020-01-15 12:28 ` Igor Druzhinin
2020-01-15 12:39 ` Jan Beulich
2020-01-15 12:47 ` Igor Druzhinin
2020-01-15 12:54 ` Jan Beulich
2020-01-15 14:55 ` Andrew Cooper
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