Re: Host latency peaks due to kvm-intel

[Jan Kiszka]

Avi Kivity wrote:
> On 07/24/2009 12:41 PM, Jan Kiszka wrote:
>> I vaguely recall that someone promised to add a feature reporting
>> facility for all those nice things, modern VM-extensions may or may not
>> support (something like or even an extension of /proc/cpuinfo). What is
>> the state of this plan? Would be specifically interesting for Intel CPUs
>> as there seem to be many of them out there with restrictions for special
>> use cases - like real-time.
>>    
> 
> Newer kernels do report some vmx features (like flexpriority) in
> /proc/cpuinfo but not all.
> 

Ah, nice. Then we just need this?

------------>

From: Jan Kiszka <jan.kis...@siemens.com>
Subject: [PATCH] x86: Report VMX feature vwbinvd

Not all VMX-capable CPUs support guest exists on wbinvd execution. If
this is not supported, the instruction will run natively on behalf of
the guest. This can cause multi-millisecond latencies to the host which
is very problematic in real-time scenarios.

Report the wbinvd trapping feature along with other VMX feature flags,
calling it 'vwbinvd' ('virtual wbinvd').

Signed-off-by: Jan Kiszka <jan.kis...@siemens.com>
---

 arch/x86/include/asm/cpufeature.h |    1 +
 arch/x86/kernel/cpu/intel.c       |    4 ++++
 2 files changed, 5 insertions(+), 0 deletions(-)

diff --git a/arch/x86/include/asm/cpufeature.h 
b/arch/x86/include/asm/cpufeature.h
index 4a28d22..8647524 100644
--- a/arch/x86/include/asm/cpufeature.h
+++ b/arch/x86/include/asm/cpufeature.h
@@ -165,6 +165,7 @@
 #define X86_FEATURE_FLEXPRIORITY (8*32+ 2) /* Intel FlexPriority */
 #define X86_FEATURE_EPT         (8*32+ 3) /* Intel Extended Page Table */
 #define X86_FEATURE_VPID        (8*32+ 4) /* Intel Virtual Processor ID */
+#define X86_FEATURE_VWBINVD     (8*32+ 5) /* Guest Exiting on WBINVD */
 
 #if defined(__KERNEL__) && !defined(__ASSEMBLY__)
 
diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index 3260ab0..2d921b0 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -297,6 +297,7 @@ static void __cpuinit detect_vmx_virtcap(struct cpuinfo_x86 
*c)
 #define X86_VMX_FEATURE_PROC_CTLS2_VIRT_APIC   0x00000001
 #define X86_VMX_FEATURE_PROC_CTLS2_EPT         0x00000002
 #define X86_VMX_FEATURE_PROC_CTLS2_VPID                0x00000020
+#define X86_VMX_FEATURE_PROC_CTLS2_VWBINVD     0x00000040
 
        u32 vmx_msr_low, vmx_msr_high, msr_ctl, msr_ctl2;
 
@@ -305,6 +306,7 @@ static void __cpuinit detect_vmx_virtcap(struct cpuinfo_x86 
*c)
        clear_cpu_cap(c, X86_FEATURE_FLEXPRIORITY);
        clear_cpu_cap(c, X86_FEATURE_EPT);
        clear_cpu_cap(c, X86_FEATURE_VPID);
+       clear_cpu_cap(c, X86_FEATURE_VWBINVD);
 
        rdmsr(MSR_IA32_VMX_PROCBASED_CTLS, vmx_msr_low, vmx_msr_high);
        msr_ctl = vmx_msr_high | vmx_msr_low;
@@ -323,6 +325,8 @@ static void __cpuinit detect_vmx_virtcap(struct cpuinfo_x86 
*c)
                        set_cpu_cap(c, X86_FEATURE_EPT);
                if (msr_ctl2 & X86_VMX_FEATURE_PROC_CTLS2_VPID)
                        set_cpu_cap(c, X86_FEATURE_VPID);
+               if (msr_ctl2 & X86_VMX_FEATURE_PROC_CTLS2_VWBINVD)
+                       set_cpu_cap(c, X86_FEATURE_VWBINVD);
        }
 }
 

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