Re: [Xen-devel] [PATCH 5/4] x86/viridian: drop a wrong invalid value from reference TSC implementation

From: "Durrant, Paul" <pdurrant@amazon.com>
To: Wei Liu <wl@xen.org>,
	Xen Development List <xen-devel@lists.xenproject.org>
Cc: "Wei Liu" <liuwe@microsoft.com>, "Paul Durrant" <paul@xen.org>,
	"Andrew Cooper" <andrew.cooper3@citrix.com>,
	"Michael Kelley" <mikelley@microsoft.com>,
	"Jan Beulich" <jbeulich@suse.com>,
	"Roger Pau Monné" <roger.pau@citrix.com>
Subject: Re: [Xen-devel] [PATCH 5/4] x86/viridian: drop a wrong invalid value from reference TSC implementation
Date: Sat, 21 Dec 2019 12:06:59 +0000	[thread overview]
Message-ID: <affb8b6f87f744ec865101401d4f9131@EX13D32EUC003.ant.amazon.com> (raw)
In-Reply-To: <20191220212103.4192-1-liuwe@microsoft.com>

> -----Original Message-----
> From: Wei Liu <wei.liu.xen@gmail.com> On Behalf Of Wei Liu
> Sent: 20 December 2019 21:21
> To: Xen Development List <xen-devel@lists.xenproject.org>
> Cc: Durrant, Paul <pdurrant@amazon.com>; Michael Kelley
> <mikelley@microsoft.com>; Wei Liu <liuwe@microsoft.com>; Paul Durrant
> <paul@xen.org>; Jan Beulich <jbeulich@suse.com>; Andrew Cooper
> <andrew.cooper3@citrix.com>; Wei Liu <wl@xen.org>; Roger Pau Monné
> <roger.pau@citrix.com>
> Subject: [PATCH 5/4] x86/viridian: drop a wrong invalid value from
> reference TSC implementation
> 
> The only invalid value mentioned in Hyper-V TLFS 5.0c is 0. Michael
> Kelley confirmed that 0xFFFFFFFF was never used [0].
> 
> [0] https://lists.xen.org/archives/html/xen-devel/2019-12/msg01564.html
> 
> Signed-off-by: Wei Liu <liuwe@microsoft.com>
> ---
>  xen/arch/x86/hvm/viridian/time.c | 13 ++++---------
>  1 file changed, 4 insertions(+), 9 deletions(-)
> 
> diff --git a/xen/arch/x86/hvm/viridian/time.c
> b/xen/arch/x86/hvm/viridian/time.c
> index 6b2d745f3a..b8280a1a60 100644
> --- a/xen/arch/x86/hvm/viridian/time.c
> +++ b/xen/arch/x86/hvm/viridian/time.c
> @@ -45,14 +45,9 @@ static void update_reference_tsc(const struct domain
> *d, bool initialize)
>      if ( !host_tsc_is_safe() || d->arch.vtsc )
>      {
>          /*
> -         * The specification states that valid values of TscSequence
> range
> -         * from 0 to 0xFFFFFFFE. The value 0xFFFFFFFF is used to indicate
> -         * this mechanism is no longer a reliable source of time and that
> -         * the VM should fall back to a different source.
> -         *
> -         * Server 2012 (6.2 kernel) and 2012 R2 (6.3 kernel) actually
> -         * violate the spec. and rely on a value of 0 to indicate that
> this
> -         * enlightenment should no longer be used.
> +         * The value 0 is used to indicate this mechanism is no longer a
> +         * reliable source of time and that the VM should fall back to a
> +         * different source.
>           */
>          p->tsc_sequence = 0;
> 
> @@ -77,7 +72,7 @@ static void update_reference_tsc(const struct domain *d,
> bool initialize)
>      smp_wmb();
> 
>      seq = p->tsc_sequence + 1;
> -    if ( seq == 0xFFFFFFFF || seq == 0 ) /* Avoid both 'invalid' values
> */
> +    if ( seq == 0 ) /* Avoid 'invalid' value */
>          seq = 1;
> 
>      p->tsc_sequence = seq;

Now that we're just dealing with 0, I think the code would be neater as:

seq = p->tsc_sequence + 1;
p->tsc_sequence = seq ? seq : 1; /* Avoid 'invalid' value 0 */

  Paul

> --
> 2.20.1

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