Re: [PATCH 12/13] vmx: Add a call-back to apply TSC scaling ratio to hardware

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 29/09/15 02:07, Haozhong Zhang wrote:
> On Mon, Sep 28, 2015 at 12:02:08PM -0400, Boris Ostrovsky wrote:
>> On 09/28/2015 03:13 AM, Haozhong Zhang wrote:
>>> This patch adds a new call-back setup_tsc_scaling in struct
>>> hvm_function_table to apply the TSC scaling ratio to hardware. For VMX,
>>> it writes the TSC scaling ratio to VMCS field TSC_MULTIPLIER.
>>>
>>> Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
>>> ---
>>>  xen/arch/x86/hvm/hvm.c        | 1 +
>>>  xen/arch/x86/hvm/svm/svm.c    | 5 +++++
>>>  xen/arch/x86/hvm/vmx/vmx.c    | 8 ++++++++
>>>  xen/include/asm-x86/hvm/hvm.h | 3 +++
>>>  4 files changed, 17 insertions(+)
>>>
>>> diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
>>> index 3522d20..2d8a148 100644
>>> --- a/xen/arch/x86/hvm/hvm.c
>>> +++ b/xen/arch/x86/hvm/hvm.c
>>> @@ -376,6 +376,7 @@ void hvm_setup_tsc_scaling(struct vcpu *v)
>>>      }
>>>      v->arch.tsc_scaling_ratio = ratio;
>>> +    hvm_funcs.setup_tsc_scaling(v);
>>>  }
>>>  void hvm_set_guest_tsc_fixed(struct vcpu *v, u64 guest_tsc, u64 at_tsc)
>>> diff --git a/xen/arch/x86/hvm/svm/svm.c b/xen/arch/x86/hvm/svm/svm.c
>>> index 73bc863..d890c1f 100644
>>> --- a/xen/arch/x86/hvm/svm/svm.c
>>> +++ b/xen/arch/x86/hvm/svm/svm.c
>>> @@ -2236,6 +2236,10 @@ static void svm_invlpg_intercept(unsigned long vaddr)
>>>      svm_asid_g_invlpg(curr, vaddr);
>>>  }
>>> +static void svm_setup_tsc_scaling(struct vcpu *v)
>>> +{
>>> +}
>>> +
>> Should this be wrmsrl(MSR_AMD64_TSC_RATIO, v->arch.tsc_scaling_ratio) ?
>>
>> -boris
>>
> MSR_AMD64_TSC_RATIO is set in svm_ctxt_switch_to() before entering guest.
>
> For VMX, the ratio is set to a VMCS field TSC_MULTIPLIER and it's not
> necessary to set it every time entering guest. Therefore, I introduce
> the call-back setup_tsc_scaling() to do this. For SVM, as the ratio is
> set every time entering guest, I leave the SVM version of setup_tsc_scaling()
> empty.

VT-x has a per-VMCS scale, while SVM has a per-core MSR to adjust the
scale.  These do require different modification algorithms.

However, if there is any chance that any part of the system can update
the ratio while an SVM VCPU is in context (which appears to be the
case), then MSR_AMD64_TSC_RATIO needs updating synchronously, or it will
be deferred until the next full context switch which could be an
arbitrary time into the future.  This appears to be a latent bug in the
SVM side.

~Andrew