Re: [PATCH 4/4] KVM: MMU: lower the aduit frequency

Avi Kivity
2010-08-30T06:59:06Z

On 08/30/2010 05:16 AM, Xiao Guangrong wrote:
> On 08/29/2010 05:19 PM, Avi Kivity wrote:
>> On 08/28/2010 03:03 PM, Xiao Guangrong wrote:
>>> The audit is very high overhead, so we need lower the frequency to
>>> assure the guest running
>>>
>>>
>>> */
>>>
>>> #include<linux/debugfs.h>
>>> +#include<linux/ratelimit.h>
>>>
>>> static struct dentry *debugfs_file;
>>> static bool mmu_debug;
>>> @@ -233,6 +234,11 @@ static void audit_vcpu_spte(struct kvm_vcpu *vcpu)
>>>
>>> static void kvm_mmu_audit(void *ignore, struct kvm_vcpu *vcpu, const
>>> char *msg)
>>> {
>>> + static DEFINE_RATELIMIT_STATE(ratelimit_state, 5 * HZ, 10);
>>> +
>>> + if (!__ratelimit(&ratelimit_state))
>>> + return;
>>> +
>>> audit_msg = msg;
>>> audit_all_active_sps(vcpu->kvm);
>>> audit_vcpu_spte(vcpu);
>> This means we see a bug long after it happened, so we can't correlate it
>> to the cause.
>>
>> It's fine as an option (even the default) but I'd like to be able to
>> audit after every operation. Perhaps a partial audit that only looks at
>> the gfns and vaddrs that were affected in the last operation?
>>
> Audit checks all the active shadow pages and all vcpu's page table, so the
> overload is very high :-)
>
> During my test, if enable the aduit, the guest mostly hung, it means the guest
> not do anything.
> (Host: Intel(R) Xeon(R) X3430 @ 2.40GHz * 4 + 4G memory
> GUest: x2VCPU + 1G memory
> )

You're right, I remember that from the last time I used audit many years
ago.

> I'll set the 'ratelimit' as a module parameter, then if the user's machine is
> fast enough, the ratelimit can be disabled.

It's only useful in very special cases - low memory and a very fast
reproducer. I think we can live without the parameter, if someone has
this special case they can hack the code.

--
error compiling committee.c: too many arguments to function