Re: [PATCH v11 13/17] Add mp(mediate passthru) device.

Michael S. Tsirkin
2010-09-28T13:13:30Z

On Mon, Sep 27, 2010 at 10:23:33PM +0100, Ben Hutchings wrote:
> > +/* The main function to transform the guest user space address
> > + * to host kernel address via get_user_pages(). Thus the hardware
> > + * can do DMA directly to the external buffer address.
> > + */
> > +static struct page_info *alloc_page_info(struct page_ctor *ctor,
> > + struct kiocb *iocb, struct iovec *iov,
> > + int count, struct frag *frags,
> > + int npages, int total)
> > +{
> > + int rc;
> > + int i, j, n = 0;
> > + int len;
> > + unsigned long base, lock_limit;
> > + struct page_info *info = NULL;
> > +
> > + lock_limit = current->signal->rlim[RLIMIT_MEMLOCK].rlim_cur;
> > + lock_limit >>= PAGE_SHIFT;
> > +
> > + if (ctor->lock_pages + count > lock_limit && npages) {
> > + printk(KERN_INFO "exceed the locked memory rlimit.");
> > + return NULL;
> > + }
>
> What if the process is locking pages with mlock() as well? Doesn't this
> allow it to lock twice as many pages as it should be able to?

No, since locked_vm is incremented by both mp and mlock.
Or at least, that's the idea :)
In any case, twice as many would not be a big deal: admin can control
which processes can do this by controlling access to the device.

> > + info = kmem_cache_alloc(ext_page_info_cache, GFP_KERNEL);
> > +
> > + if (!info)
> > + return NULL;
> > + info->skb = NULL;
> > + info->next = info->prev = NULL;
> > +
> > + for (i = j = 0; i < count; i++) {
> > + base = (unsigned long)iov[i].iov_base;
> > + len = iov[i].iov_len;
> > +
> > + if (!len)
> > + continue;
> > + n = ((base & ~PAGE_MASK) + len + ~PAGE_MASK) >> PAGE_SHIFT;
> > +
> > + rc = get_user_pages_fast(base, n, npages ? 1 : 0,
> > + &info->pages[j]);
> > + if (rc != n)
> > + goto failed;
> > +
> > + while (n--) {
> > + frags[j].offset = base & ~PAGE_MASK;
> > + frags[j].size = min_t(int, len,
> > + PAGE_SIZE - frags[j].offset);
> > + len -= frags[j].size;
> > + base += frags[j].size;
> > + j++;
> > + }
> > + }
> > +
> > +#ifdef CONFIG_HIGHMEM
> > + if (npages && !(dev->features & NETIF_F_HIGHDMA)) {
> > + for (i = 0; i < j; i++) {
> > + if (PageHighMem(info->pages[i]))
> > + goto failed;
> > + }
> > + }
> > +#endif
>
> Shouldn't you try to allocate lowmem pages explicitly, rather than
> failing at this point?

We don't allocate pages, we lock given pages. Once this is
integrated in macvtap presumably we'll fall back on data copy
for such devices.

...

> > + skb_reserve(skb, NET_IP_ALIGN);
> > + skb_put(skb, len);
> > +
> > + if (skb_copy_datagram_from_iovec(skb, 0, iov, 0, len)) {
> > + kfree_skb(skb);
> > + return -EAGAIN;
> > + }
> > +
> > + skb->protocol = eth_type_trans(skb, mp->dev);
>
> Why are you calling eth_type_trans() on transmit?

So that GSO can work. BTW macvtap does:

skb_set_network_header(skb, ETH_HLEN);
skb_reset_mac_header(skb);
skb->protocol = eth_hdr(skb)->h_proto;

and I think this is broken for vlans. Arnd?

--
MST