Re: [Xen-devel] [PATCH] svm: Do not intercept RDTCS(P) when TSC scaling is supported by hardware

["Huang2, Wei" <Wei.Huang2@xxxxxxx>]

Hi Keir,

This patch is a bug fix for 23437:d7c755c25bb9 than new feature. It slipped my 
hand and Boris fixed it for me. The same applies to Xen-4.1.

===== Changeset 23437 =====

HVM/SVM: enable tsc scaling ratio for SVM

Future AMD CPUs support TSC scaling. It allows guests to have a
different TSC frequency from host system using this formula: guest_tsc
= host_tsc * tsc_ratio + vmcb_offset. The tsc_ratio is a 64bit MSR
contains a fixed-point number in 8.32 format (8 bits for integer part
and 32bits for fractional part). For instance 0x00000003_80000000
means tsc_ratio=3.5.

This patch enables TSC scaling ratio for SVM. With it, guest VMs don't
need take #VMEXIT to calculate a translated TSC value when it is
running under TSC emulation mode. This can substancially reduce the
rdtsc overhead.


-Wei

-----Original Message-----
From: Keir Fraser [mailto:keir.xen@xxxxxxxxx] On Behalf Of Keir Fraser
Sent: Friday, April 20, 2012 3:15 AM
To: Ostrovsky, Boris; JBeulich@xxxxxxxx; Dan Magenheimer
Cc: Huang2, Wei; xen-devel@xxxxxxxxxxxxxxxxxxx
Subject: Re: [PATCH] svm: Do not intercept RDTCS(P) when TSC scaling is 
supported by hardware

On 20/04/2012 09:05, "Keir Fraser" <keir.xen@xxxxxxxxx> wrote:

> On 20/04/2012 03:21, "Boris Ostrovsky" <boris.ostrovsky@xxxxxxx> wrote:
> 
>> # HG changeset patch
>> # User Boris Ostrovsky <boris.ostrovsky@xxxxxxx>
>> # Date 1334875170 14400
>> # Node ID 55bf11ebce87ceb73fb2c372dcef170ec0bb4a18
>> # Parent  7c777cb8f705411b77c551f34ba88bdc09e38ab8
>> svm: Do not intercept RDTCS(P) when TSC scaling is supported by hardware
>> 
>> When running in TSC_MODE_ALWAYS_EMULATE mode on processors that support
>> TSC scaling we don't need to intercept RDTSC/RDTSCP instructions.
>> 
>> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxx>
>> Acked-by: Wei Huang <wei.huang2@xxxxxxx>
>> Tested-by: Wei Huang <wei.huang2@xxxxxxx>
> 
> Worth an ack/nack from Dan M I'd say. He'll probably have some comment about
> possible cross-CPU TSC skew.

Oh, and apart from that, we're also in feature freeze for 4.2, and this
isn't a bug fix. Similarly, it's not really a candidate for the stable 4.1
branch either, at any time.

 -- Keir

>  -- Keir
> 
>> diff -r 7c777cb8f705 -r 55bf11ebce87 xen/arch/x86/hvm/svm/svm.c
>> --- a/xen/arch/x86/hvm/svm/svm.c Wed Apr 18 16:49:55 2012 +0100
>> +++ b/xen/arch/x86/hvm/svm/svm.c Thu Apr 19 18:39:30 2012 -0400
>> @@ -724,12 +724,18 @@ static void svm_set_rdtsc_exiting(struct
>>  {
>>      struct vmcb_struct *vmcb = v->arch.hvm_svm.vmcb;
>>      u32 general1_intercepts = vmcb_get_general1_intercepts(vmcb);
>> +    u32 general2_intercepts = vmcb_get_general2_intercepts(vmcb);
>>  
>>      general1_intercepts &= ~GENERAL1_INTERCEPT_RDTSC;
>> -    if ( enable )
>> +    general2_intercepts &= ~GENERAL2_INTERCEPT_RDTSCP;
>> +
>> +    if ( enable && !cpu_has_tsc_ratio ) {
>>          general1_intercepts |= GENERAL1_INTERCEPT_RDTSC;
>> +        general2_intercepts |= GENERAL2_INTERCEPT_RDTSCP;
>> +    }
>>  
>>      vmcb_set_general1_intercepts(vmcb, general1_intercepts);
>> +    vmcb_set_general2_intercepts(vmcb, general2_intercepts);
>>  }
>>  
>>  static unsigned int svm_get_insn_bytes(struct vcpu *v, uint8_t *buf)
>> 
> 
> 





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