| Message ID | 20191220212103.4192-1-liuwe@microsoft.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | Clean up viridian code | expand |
> -----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
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;
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(-)