Re: [PATCH 3/5] KVM: Add paravirt MMU write support

[Anthony Liguori <anthony-rdkfGonbjUSkNkDKm+mE6A@public.gmane.org>]

Avi Kivity wrote:
> Anthony Liguori wrote:
>> +static int kvm_hypercall_mmu_write(struct kvm_vcpu *vcpu, gva_t addr,
>> +                   unsigned long size, unsigned long a0,
>> +                   unsigned long a1)
>> +{
>> +    gpa_t gpa = vcpu->mmu.gva_to_gpa(vcpu, addr);
>> +    u64 value;
>> +
>> +    if (gpa == UNMAPPED_GVA)
>> +        return -EFAULT;
>> +    if (size == 1) {
>> +        if (!emulator_write_phys(vcpu, gpa, &a0, sizeof(a0)))
>> +            return -EFAULT;
>> +    } else if (size == 2) {
>> +        value = (u64)a1 << 32 | a0;
>> +        if (!emulator_write_phys(vcpu, gpa, &value, sizeof(value)))
>> +            return -EFAULT;
>> +    } else
>> +        return -E2BIG;
>> +
>> +    return 0;
>> +}
>
> Hypercalls should return kvm-specific error codes (defined in 
> include/linux/kvm_para.h), not Linux error codes, as they could be 
> used in operating systems which have different values for E2BIG and 
> friends.

If Linux's errnos are stable, we could just use them and let a non-Linux 
guest define a set of KVM_E2BIG, etc.?  It just seemed pretty ugly to 
add a bunch of these.

>> +static void kvm_pte_clear(struct mm_struct *mm,
>> +              unsigned long addr, pte_t *ptep)
>> +{
>> +    pte_t pte = {0};
>>   
>
> Surely there's a nice macro for creating a pte from an int?

Probably :-)

> Any performance measurement?

Yes, surprisingly enough.  COW faults in virtbench drop by a significant 
amount.  I'll repost each patch with virtbench results.  I suspect that 
the vmmcall path is much faster than a page fault.

Regards,

Anthony Liguori


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