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Re: [Xen-devel] [PATCH v3 1/1] x86/HVM: Use fixed TSC value when saving or restoring domain



On 04/10/2014 02:55 AM, Jan Beulich wrote:

--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -255,16 +255,20 @@ int hvm_set_guest_pat(struct vcpu *v, u64 guest_pat)
      return 1;
  }
-void hvm_set_guest_tsc(struct vcpu *v, u64 guest_tsc)
+void hvm_set_guest_tsc_fixed(struct vcpu *v, u64 guest_tsc, u64 at_tsc)
What I now started wondering is (namely with the struct
hvm_function_table pointer also gaining the extra argument): Is
this ever being called with a zero at_tsc when
v->domain->arch.chkpt_tsc is non-zero? If not, rather than passing
around the value I guess the function could simply read it itself. And
yes, this is meant only for the "set" version, I'm relatively convinced
that the "get" ones would (or could easily become) different.

I couldn't convince myself that v->domain->arch.chkpt_tsc is always the same as at_tsc. I think it is (and that's why arch_hvm_save_done() was added) but I was worried that there may be some other path to hvm_set_guest_tsc().

The other reason was for symmetry with the the "get" counterpart.



--- a/xen/include/asm-x86/domain.h
+++ b/xen/include/asm-x86/domain.h
@@ -308,6 +308,8 @@ struct arch_domain
                                  (possibly other cases in the future */
      uint64_t vtsc_kerncount; /* for hvm, counts all vtsc */
      uint64_t vtsc_usercount; /* not used for hvm */
+    uint64_t chkpt_tsc;      /* TSC value that VCPUs use to calculate their
+                                tsc_offset value. Used during save/restore */
I'm sorry for not asking earlier - why does this get put in struct
arch_domain rather than struct hvm_domain?

Right. hvm_domain seems like a better place given that this is only relevant to HVM.


-boris


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