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

[Haozhong Zhang <haozhong.zhang@xxxxxxxxx>]

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.

> 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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