Re: IO on guest is 20 times slower than host

[Avi Kivity <avi@redhat.com>]

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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.


[-- Attachment #2: svm-pat-writeback.patch --]
[-- Type: text/x-patch, Size: 507 bytes --]

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;