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.zh...@intel.com>
> >---
> > 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.
> 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
>
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