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

Anthony Liguori
2007-06-18T12:33:16Z

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