Re: IO on guest is 20 times slower than host

Joerg Roedel
2009-03-31T09:59:59Z

On Sun, Mar 29, 2009 at 10:10:26PM +0300, Avi Kivity wrote:
> Avi Kivity wrote:
>> Kurt Yoder wrote:
>>> slow host cpu information, core 1 of 16:
>>>
>>> processor : 0
>>> vendor_id : AuthenticAMD
>>> cpu family : 16
>>> model : 4
>>> model name : Quad-Core AMD Opteron(tm) Processor 8382
>>> stepping : 2
>>> cpu MHz : 2611.998
>>> cache size : 512 KB
>>> physical id : 0
>>> siblings : 4
>>> core id : 0
>>> cpu cores : 4
>>> apicid : 0
>>> initial apicid : 0
>>> fpu : yes
>>> fpu_exception : yes
>>> cpuid level : 5
>>> wp : yes
>>> flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr
>>> pge mca
>>> cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall mmxext fxsr_opt
>>> pdpe1gb rdtscp lm 3dnowext 3dnow constant_tsc rep_good nopl pni monitor
>>> cx16 popcnt lahf_lm cmp_legacy svm extapic cr8_legacy abm sse4a
>>> misalignsse 3dnowprefetch osvw ibs skinit wdt
>>> bogomips : 5223.97
>>> TLB size : 1024 4K pages
>>> clflush size : 64
>>> cache_alignment : 64
>>> address sizes : 48 bits physical, 48 bits virtual
>>> power management: ts ttp tm stc 100mhzsteps hwpstate
>>>
>>>
>>>
>>
>> Can you loading kvm_amd on this host with 'modprobe kvm-amd npt=0'?
>>
>
> If it helps, then the guest is messing up the cpu cache. Try the
> attached patch.
>
> --
> I have a truly marvellous patch that fixes the bug which this
> signature is too narrow to contain.
>

> diff --git a/kernel/x86/kvm/svm.c b/kernel/x86/kvm/svm.c
> index 1fcbc17..d9774e9 100644
> --- a/kernel/x86/kvm/svm.c
> +++ b/kernel/x86/kvm/svm.c
> @@ -575,7 +575,7 @@ static void init_vmcb(struct vcpu_svm *svm)
> INTERCEPT_CR3_MASK);
> control->intercept_cr_write &= ~(INTERCEPT_CR0_MASK|
> INTERCEPT_CR3_MASK);
> - save->g_pat = 0x0007040600070406ULL;
> + save->g_pat = 0x0606060606060606ULL;
> /* enable caching because the QEMU Bios doesn't enable it */
> save->cr0 = X86_CR0_ET;
> save->cr3 = 0;

Yeah, that patch makes sense. But I think we need some more work on this
because the guest may change the pat msr afterwards. Best would be a simple
shadow of the pat msr. Last question is how this will effect pci passthrough.

Joerg

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