Re: [PATCH] KVM x86: Handle hypercalls for assigned PCI devices

Avi Kivity
2008-04-29T22:48:38Z

Amit Shah wrote:
>
>>> + if (is_error_page(host_page)) {
>>> + printk(KERN_INFO "%s: gfn %p not valid\n",
>>> + __func__, (void *)page_gfn);
>>> + r = -1;
>>>
>> r = -1 is not really informative. Better use some meaningful error.
>>
>
> The error's going to the guest. The guest, as we know, has already done a
> successful DMA allocation. Something went wrong in the hypercall, and we
> don't know why (bad page). Any kind of error here isn't going to be
> intelligible to the guest anyway. It's mostly a host thing if we ever hit
> this.
>
>

If the guest is not able to handle it, why bother returning an error?
Better to kill it.

But in any case, -1 is not a good error number.

>>> + if (find_pci_pt_dev(&vcpu->kvm->arch.pci_pt_dev_head,
>>> + &pci_pt_info, 0, KVM_PT_SOURCE_ASSIGN))
>>> + r++; /* We have assigned the device */
>>> +
>>> + kunmap(host_page);
>>>
>> better use atomic mappings here.
>>
>
> We can't use atomic mappings for guest pages. They can be swapped out.
>

kmap()ed pages can't be swapped out either. The atomic in kmap_atomic()
only refers to the context in which the pages are used. Atmoic kmaps
are greatly preferable to the nonatomic ones.

--
Any sufficiently difficult bug is indistinguishable from a feature.

-------------------------------------------------------------------------
This SF.net email is sponsored by the 2008 JavaOne(SM) Conference
Don't miss this year's exciting event. There's still time to save $100.
Use priority code J8TL2D2.