Re: [Xen-devel] [PATCH 11/13] vmx: Use scaled host TSC to calculate TSC offset

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 10/22/2015 01:12 PM, Haozhong Zhang wrote:
> On Thu, Oct 22, 2015 at 11:55:00AM -0400, Boris Ostrovsky wrote:
> > On 09/28/2015 03:13 AM, Haozhong Zhang wrote:
> > > If VMX TSC scaling is enabled and no TSC emulation is used,
> > > vmx_set_tsc_offset() will calculate the TSC offset by substracting the
> > > scaled host TSC from the current guest TSC.
> > >
> > > Signed-off-by: Haozhong Zhang <haozhong.zhang@xxxxxxxxx>
> > > ---
> > >   xen/arch/x86/hvm/vmx/vmx.c | 15 +++++++++++++++
> > >   1 file changed, 15 insertions(+)
> > >
> > > diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
> > > index 454440e..163974d 100644
> > > --- a/xen/arch/x86/hvm/vmx/vmx.c
> > > +++ b/xen/arch/x86/hvm/vmx/vmx.c
> > > @@ -1102,11 +1102,26 @@ static void vmx_handle_cd(struct vcpu *v, unsigned long 
> > > value)
> > >   static void vmx_set_tsc_offset(struct vcpu *v, u64 offset, u64 at_tsc)
> > >   {
> > > +    uint64_t host_tsc, guest_tsc;
> > > +    struct domain *d = v->domain;
> > > +
> > > +    guest_tsc = hvm_get_guest_tsc_fixed(v, at_tsc);
> > > +
> > > +    if ( cpu_has_vmx_tsc_scaling && !d->arch.vtsc )
> > > +    {
> > > +        host_tsc = at_tsc ? at_tsc : rdtsc();
> > > +        offset = guest_tsc - hvm_scale_tsc(v, host_tsc);
> > > +    }
> > > +
> > >       vmx_vmcs_enter(v);
> > > +    if ( !nestedhvm_enabled(d) )
> > > +        goto out;
> > > +
> > >       if ( nestedhvm_vcpu_in_guestmode(v) )
> > >           offset += nvmx_get_tsc_offset(v);
> > > +out:
> > >       __vmwrite(TSC_OFFSET, offset);
> > >       vmx_vmcs_exit(v);
> > >   }
> > This (and corresponding SVM code) looks somewhat suspect to me: if the
> > processor supports scaling we are ignoring caller-provided offset.
> Yes, if TSC scaling is available, [svm|vmx]_set_tsc_offset() do not
> trust the offset from callers and calculate a scaled version by itself
> instead.
>
> The original svm_set_tsc_offset() did so and I keep its semantics
> here.
>
> Maybe moving the scaling logic out of [svm|vmx]_set_tsc_offset() could
> make the semantics more clear.

Yes: since scaling is now architectural, offsets are (or at least can 
be) calculated correctly at HVM level and so arch-specific handlers 
should not have to do this. In case of hvm_set_guest_tsc_fixed() we 
definitely doing it twice.
-boris

> > Besides, at least when called from hvm_set_guest_tsc_fixed() --- we've
> > already taken scaling into account, that's what patch 6 does, doesn't it?
> Yes.
>
> > -boris
> >
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@xxxxxxxxxxxxx
> > http://lists.xen.org/xen-devel

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