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



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

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