Re: [Xen-devel] [PATCH v3] x86/viridian: Add Partition Reference Time enlightenment

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Matt Wilson [mailto:mswilson@xxxxxxxxx] On Behalf Of Matt Wilson
> Sent: 14 October 2014 08:54
> To: Paul Durrant
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; Keir (Xen.org); Ian Campbell; Stefano
> Stabellini; Christoph Egger; Ian Jackson; Jan Beulich
> Subject: Re: [Xen-devel] [PATCH v3] x86/viridian: Add Partition Reference
> Time enlightenment
> 
> On Mon, Oct 13, 2014 at 03:27:39PM +0100, Paul Durrant wrote:
> > The presence of the partition reference time enlightenment persuades
> newer
> > versions of Windows to prefer the TSC as their primary time source. Hence,
> > if rdtsc is not being emulated and is invariant then many vmexits (for
> > alternative time sources such as the HPET or reference counter MSR) can
> > be avoided.
> >
> > The implementation is not yet complete as no attempt is made to prevent
> > emulation of rdtsc if the enlightenment is active and guest and host
> > TSC frequencies differ. To do that requires invasive changes in the core
> > x86 time code and hence a lot more testing.
> >
> > This patch avoids the issue by disabling the enlightenment if rdtsc is
> > being emulated, causing Windows to choose another time source. This is
> > safe, but may cause a big variation in performance of guests migrated
> > between hosts of differing TSC frequency. Thus the enlightenment is not
> > enabled in the default set, but may be enabled to improve guest
> performance
> > where such migrations are not a concern.
> >
> > See section 15.4 of the Microsoft Hypervisor Top Level Functional
> > Specification v4.0a for details.
> >
> > Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
> > Cc: Keir Fraser <keir@xxxxxxx>
> > Cc: Jan Beulich <jbeulich@xxxxxxxx>
> > Cc: Ian Campbell <ian.campbell@xxxxxxxxxx>
> > Cc: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
> > Cc: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
> > Cc: Christoph Egger <chegger@xxxxxxxxx>
> > ---
> > v3:
> > - Prevent TscSequence increment loop from being exposed to a running
> >   domain.
> >
> > v2:
> > - Addressed comments from Jan:
> >   - Leave enlightenment out of default set and add a comment explaining
> >     why.
> >   - De-dup code surrounding adjustment of sequence number on resume.
> >   - Remove accidental code deletion.
> >
> >  docs/man/xl.cfg.pod.5                  |    6 ++
> >  tools/libxl/libxl_dom.c                |    3 +
> >  tools/libxl/libxl_types.idl            |    1 +
> >  xen/arch/x86/hvm/viridian.c            |  116
> +++++++++++++++++++++++++++++++-
> >  xen/include/asm-x86/hvm/viridian.h     |   35 ++++++++++
> >  xen/include/public/arch-x86/hvm/save.h |   11 +++
> >  xen/include/public/hvm/params.h        |    7 +-
> >  7 files changed, 177 insertions(+), 2 deletions(-)
> >
> > diff --git a/docs/man/xl.cfg.pod.5 b/docs/man/xl.cfg.pod.5
> > index 8bba21c..925adbb 100644
> > --- a/docs/man/xl.cfg.pod.5
> > +++ b/docs/man/xl.cfg.pod.5
> > @@ -1163,6 +1163,12 @@ This group incorporates Partition Time
> Reference Counter MSR. This
> >  enlightenment can improve performance of Windows 8 and Windows
> >  Server 2012 onwards.
> >
> > +=item B<reference_tsc>
> > +
> > +This set incorporates the Partition Reference TSC MSR. This
> > +enlightenment can improve performance of Windows 7 and Windows
> > +Server 2008 R2 onwards.
> > +
> >  =item B<defaults>
> >
> >  This is a special value that enables the default set of groups, which
> > diff --git a/tools/libxl/libxl_dom.c b/tools/libxl/libxl_dom.c
> > index bd21841..d04fc0d 100644
> > --- a/tools/libxl/libxl_dom.c
> > +++ b/tools/libxl/libxl_dom.c
> > @@ -262,6 +262,9 @@ static int hvm_set_viridian_features(libxl__gc *gc,
> uint32_t domid,
> >      if (libxl_bitmap_test(&enlightenments,
> LIBXL_VIRIDIAN_ENLIGHTENMENT_TIME_REF_COUNT))
> >          mask |= HVMPV_time_ref_count;
> >
> > +    if (libxl_bitmap_test(&enlightenments,
> LIBXL_VIRIDIAN_ENLIGHTENMENT_REFERENCE_TSC))
> > +        mask |= HVMPV_reference_tsc;
> > +
> >      if (mask != 0 &&
> >          xc_hvm_param_set(CTX->xch,
> >                           domid,
> > diff --git a/tools/libxl/libxl_types.idl b/tools/libxl/libxl_types.idl
> > index 90d152f..da75a40 100644
> > --- a/tools/libxl/libxl_types.idl
> > +++ b/tools/libxl/libxl_types.idl
> > @@ -183,6 +183,7 @@ libxl_viridian_enlightenment =
> Enumeration("viridian_enlightenment", [
> >      (0, "base"),
> >      (1, "freq"),
> >      (2, "time_ref_count"),
> > +    (3, "reference_tsc"),
> >      ])
> >
> >  #
> > diff --git a/xen/arch/x86/hvm/viridian.c b/xen/arch/x86/hvm/viridian.c
> > index 6726168..528bb8b 100644
> > --- a/xen/arch/x86/hvm/viridian.c
> > +++ b/xen/arch/x86/hvm/viridian.c
> > @@ -21,6 +21,7 @@
> >  #define VIRIDIAN_MSR_HYPERCALL                  0x40000001
> >  #define VIRIDIAN_MSR_VP_INDEX                   0x40000002
> >  #define VIRIDIAN_MSR_TIME_REF_COUNT             0x40000020
> > +#define VIRIDIAN_MSR_REFERENCE_TSC              0x40000021
> >  #define VIRIDIAN_MSR_TSC_FREQUENCY              0x40000022
> >  #define VIRIDIAN_MSR_APIC_FREQUENCY             0x40000023
> >  #define VIRIDIAN_MSR_EOI                        0x40000070
> > @@ -40,6 +41,7 @@
> >  #define CPUID3A_MSR_APIC_ACCESS    (1 << 4)
> >  #define CPUID3A_MSR_HYPERCALL      (1 << 5)
> >  #define CPUID3A_MSR_VP_INDEX       (1 << 6)
> > +#define CPUID3A_MSR_REFERENCE_TSC  (1 << 9)
> >  #define CPUID3A_MSR_FREQ           (1 << 11)
> >
> >  /* Viridian CPUID 4000004, Implementation Recommendations. */
> > @@ -95,6 +97,8 @@ int cpuid_viridian_leaves(unsigned int leaf, unsigned
> int *eax,
> >              *eax |= CPUID3A_MSR_FREQ;
> >          if ( viridian_feature_mask(d) & HVMPV_time_ref_count )
> >              *eax |= CPUID3A_MSR_TIME_REF_COUNT;
> > +        if ( viridian_feature_mask(d) & HVMPV_reference_tsc )
> > +            *eax |= CPUID3A_MSR_REFERENCE_TSC;
> >          break;
> >      case 4:
> >          /* Recommended hypercall usage. */
> > @@ -155,6 +159,17 @@ static void dump_apic_assist(const struct vcpu *v)
> >             v, aa->fields.enabled, (unsigned long)aa->fields.pfn);
> >  }
> >
> > +static void dump_reference_tsc(const struct domain *d)
> > +{
> > +    const union viridian_reference_tsc *rt;
> > +
> > +    rt = &d->arch.hvm_domain.viridian.reference_tsc;
> > +
> > +    printk(XENLOG_G_INFO "d%d: VIRIDIAN REFERENCE_TSC: enabled: %x
> pfn: %lx\n",
> > +           d->domain_id,
> > +           rt->fields.enabled, (unsigned long)rt->fields.pfn);
> > +}
> > +
> >  static void enable_hypercall_page(struct domain *d)
> >  {
> >      unsigned long gmfn = d-
> >arch.hvm_domain.viridian.hypercall_gpa.fields.pfn;
> > @@ -224,6 +239,81 @@ static void initialize_apic_assist(struct vcpu *v)
> >      put_page_and_type(page);
> >  }
> >
> > +static void update_reference_tsc(struct domain *d, bool_t initialize)
> > +{
> > +    unsigned long gmfn = d-
> >arch.hvm_domain.viridian.reference_tsc.fields.pfn;
> > +    struct page_info *page = get_page_from_gfn(d, gmfn, NULL,
> P2M_ALLOC);
> > +    HV_REFERENCE_TSC_PAGE *p;
> > +
> > +    if ( !page || !get_page_type(page, PGT_writable_page) )
> > +    {
> > +        if ( page )
> > +            put_page(page);
> > +        gdprintk(XENLOG_WARNING, "Bad GMFN %lx (MFN %lx)\n", gmfn,
> > +                 page ? page_to_mfn(page) : INVALID_MFN);
> > +        return;
> > +    }
> > +
> > +    p = __map_domain_page(page);
> > +
> > +    if ( initialize )
> > +        clear_page(p);
> > +
> > +    /*
> > +     * This enlightenment must be disabled is the host TSC is not 
> > invariant.
> > +     * However it is also disabled if vtsc is true (which means rdtsc is 
> > being
> > +     * emulated). This generally happens when guest TSC freq and host TSC
> freq
> > +     * don't match. The TscScale value could be adjusted to cope with this,
> > +     * allowing vtsc to be turned off, but support for this is not yet 
> > present
> > +     * in the hypervisor. Thus is it is possible that migrating a Windows 
> > VM
> > +     * between hosts of differing TSC frequencies may result in large
> > +     * differences in guest performance.
> > +     */
> > +    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. These two kernel
> > +         * versions are currently the only ones to make use of this
> > +         * enlightenment, so just use 0 here.
> > +         */
> > +        p->TscSequence = 0;
> > +
> > +        printk(XENLOG_G_INFO "d%d: VIRIDIAN REFERENCE_TSC:
> invalidated\n",
> > +               d->domain_id);
> > +        return;
> > +    }
> > +
> > +    /*
> > +     * The guest will calculate reference time according to the following
> > +     * formula:
> > +     *
> > +     * ReferenceTime = ((RDTSC() * TscScale) >> 64) + TscOffset
> > +     *
> > +     * Windows uses a 100ns tick, so we need a scale which is cpu
> > +     * ticks per 100ns shifted left by 64.
> > +     */
> > +    p->TscScale = ((10000ul << 32) / d->arch.tsc_khz) << 32;
> > +
> > +    if ( initialize )
> > +        p->TscSequence = 1;
> > +    else /* guest is paused */
> > +        do {
> > +            p->TscSequence++;
> > +        } while ( p->TscSequence == 0xFFFFFFFF ||
> > +                  p->TscSequence == 0 ); /* Avoid both 'invalid' values */
> 
> Why not just something like:
> 
>     p->TscSequence++;
>     /* Avoid both 'invalid' values */
>     if ( p->TscSequence == 0xFFFFFFFF || p->TscSequence == 0 )
>         p->TscSequence = 1;
> 
> If there's something wrong with this approach and you prefer your
> 'initialize' check, perhaps add some more { } since you have a
> multi-statement else.
>

True. Avoiding the loop entirely is better.

 Paul
 
> --msw
> 
> > +    unmap_domain_page(p);
> > +
> > +    put_page_and_type(page);
> > +}
> > +
> >  int wrmsr_viridian_regs(uint32_t idx, uint64_t val)
> >  {
> >      struct vcpu *v = current;
> > @@ -282,6 +372,17 @@ int wrmsr_viridian_regs(uint32_t idx, uint64_t val)
> >              initialize_apic_assist(v);
> >          break;
> >
> > +    case VIRIDIAN_MSR_REFERENCE_TSC:
> > +        if ( !(viridian_feature_mask(d) & HVMPV_reference_tsc) )
> > +            return 0;
> > +
> > +        perfc_incr(mshv_wrmsr_tsc_msr);
> > +        d->arch.hvm_domain.viridian.reference_tsc.raw = val;
> > +        dump_reference_tsc(d);
> > +        if ( d->arch.hvm_domain.viridian.reference_tsc.fields.enabled )
> > +            update_reference_tsc(d, 1);
> > +        break;
> > +
> >      default:
> >          return 0;
> >      }
> > @@ -346,6 +447,14 @@ int rdmsr_viridian_regs(uint32_t idx, uint64_t *val)
> >          *val = v->arch.hvm_vcpu.viridian.apic_assist.raw;
> >          break;
> >
> > +    case VIRIDIAN_MSR_REFERENCE_TSC:
> > +        if ( !(viridian_feature_mask(d) & HVMPV_reference_tsc) )
> > +            return 0;
> > +
> > +        perfc_incr(mshv_rdmsr_tsc_msr);
> > +        *val = d->arch.hvm_domain.viridian.reference_tsc.raw;
> > +        break;
> > +
> >      case VIRIDIAN_MSR_TIME_REF_COUNT:
> >      {
> >          uint64_t tsc;
> > @@ -452,6 +561,7 @@ static int viridian_save_domain_ctxt(struct domain
> *d, hvm_domain_context_t *h)
> >
> >      ctxt.hypercall_gpa = d->arch.hvm_domain.viridian.hypercall_gpa.raw;
> >      ctxt.guest_os_id   = d->arch.hvm_domain.viridian.guest_os_id.raw;
> > +    ctxt.reference_tsc = d->arch.hvm_domain.viridian.reference_tsc.raw;
> >
> >      return (hvm_save_entry(VIRIDIAN_DOMAIN, 0, h, &ctxt) != 0);
> >  }
> > @@ -460,11 +570,15 @@ static int viridian_load_domain_ctxt(struct
> domain *d, hvm_domain_context_t *h)
> >  {
> >      struct hvm_viridian_domain_context ctxt;
> >
> > -    if ( hvm_load_entry(VIRIDIAN_DOMAIN, h, &ctxt) != 0 )
> > +    if ( hvm_load_entry_zeroextend(VIRIDIAN_DOMAIN, h, &ctxt) != 0 )
> >          return -EINVAL;
> >
> >      d->arch.hvm_domain.viridian.hypercall_gpa.raw = ctxt.hypercall_gpa;
> >      d->arch.hvm_domain.viridian.guest_os_id.raw   = ctxt.guest_os_id;
> > +    d->arch.hvm_domain.viridian.reference_tsc.raw = ctxt.reference_tsc;
> > +
> > +    if ( d->arch.hvm_domain.viridian.reference_tsc.fields.enabled )
> > +        update_reference_tsc(d, 0);
> >
> >      return 0;
> >  }
> > diff --git a/xen/include/asm-x86/hvm/viridian.h b/xen/include/asm-
> x86/hvm/viridian.h
> > index 496da33..35a22f5 100644
> > --- a/xen/include/asm-x86/hvm/viridian.h
> > +++ b/xen/include/asm-x86/hvm/viridian.h
> > @@ -48,10 +48,35 @@ union viridian_hypercall_gpa
> >      } fields;
> >  };
> >
> > +union viridian_reference_tsc
> > +{
> > +    uint64_t raw;
> > +    struct
> > +    {
> > +        uint64_t enabled:1;
> > +        uint64_t reserved_preserved:11;
> > +        uint64_t pfn:48;
> > +    } fields;
> > +};
> > +
> > +/*
> > + * Type defintion as in Microsoft Hypervisor Top-Level Functional
> > + * Specification v4.0a, section 15.4.2.
> > + */
> > +typedef struct _HV_REFERENCE_TSC_PAGE
> > +{
> > +    uint32_t TscSequence;
> > +    uint32_t Reserved1;
> > +    uint64_t TscScale;
> > +    int64_t  TscOffset;
> > +    uint64_t Reserved2[509];
> > +} HV_REFERENCE_TSC_PAGE, *PHV_REFERENCE_TSC_PAGE;
> > +
> >  struct viridian_domain
> >  {
> >      union viridian_guest_os_id guest_os_id;
> >      union viridian_hypercall_gpa hypercall_gpa;
> > +    union viridian_reference_tsc reference_tsc;
> >  };
> >
> >  int
> > @@ -76,3 +101,13 @@ int
> >  viridian_hypercall(struct cpu_user_regs *regs);
> >
> >  #endif /* __ASM_X86_HVM_VIRIDIAN_H__ */
> > +
> > +/*
> > + * Local variables:
> > + * mode: C
> > + * c-file-style: "BSD"
> > + * c-basic-offset: 4
> > + * tab-width: 4
> > + * indent-tabs-mode: nil
> > + * End:
> > + */
> > diff --git a/xen/include/public/arch-x86/hvm/save.h
> b/xen/include/public/arch-x86/hvm/save.h
> > index 16d85a3..006924b 100644
> > --- a/xen/include/public/arch-x86/hvm/save.h
> > +++ b/xen/include/public/arch-x86/hvm/save.h
> > @@ -568,6 +568,7 @@ struct hvm_hw_cpu_xsave {
> >  struct hvm_viridian_domain_context {
> >      uint64_t hypercall_gpa;
> >      uint64_t guest_os_id;
> > +    uint64_t reference_tsc;
> >  };
> >
> >  DECLARE_HVM_SAVE_TYPE(VIRIDIAN_DOMAIN, 15, struct
> hvm_viridian_domain_context);
> > @@ -616,3 +617,13 @@ struct hvm_msr {
> >  #define HVM_SAVE_CODE_MAX 20
> >
> >  #endif /* __XEN_PUBLIC_HVM_SAVE_X86_H__ */
> > +
> > +/*
> > + * Local variables:
> > + * mode: C
> > + * c-file-style: "BSD"
> > + * c-basic-offset: 4
> > + * tab-width: 4
> > + * indent-tabs-mode: nil
> > + * End:
> > + */
> > diff --git a/xen/include/public/hvm/params.h
> b/xen/include/public/hvm/params.h
> > index 3c51072..a2d43bc 100644
> > --- a/xen/include/public/hvm/params.h
> > +++ b/xen/include/public/hvm/params.h
> > @@ -92,10 +92,15 @@
> >  #define _HVMPV_time_ref_count 2
> >  #define HVMPV_time_ref_count  (1 << _HVMPV_time_ref_count)
> >
> > +/* Enable Reference TSC Page (HV_X64_MSR_REFERENCE_TSC) */
> > +#define _HVMPV_reference_tsc 3
> > +#define HVMPV_reference_tsc  (1 << _HVMPV_reference_tsc)
> > +
> >  #define HVMPV_feature_mask \
> >     (HVMPV_base_freq | \
> >      HVMPV_no_freq | \
> > -    HVMPV_time_ref_count)
> > +    HVMPV_time_ref_count | \
> > +    HVMPV_reference_tsc)
> >
> >  #endif
> >

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