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Re: [Qemu-devel] [virtio-dev] Re: [PATCH v9 2/5] virtio-balloon: VIRTIO_

From: Wei Wang
Subject: Re: [Qemu-devel] [virtio-dev] Re: [PATCH v9 2/5] virtio-balloon: VIRTIO_BALLOON_F_BALLOON_CHUNKS
Date: Mon, 17 Apr 2017 11:35:29 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0

On 04/15/2017 05:38 AM, Michael S. Tsirkin wrote:
On Fri, Apr 14, 2017 at 04:37:52PM +0800, Wei Wang wrote:
On 04/14/2017 12:34 AM, Michael S. Tsirkin wrote:
On Thu, Apr 13, 2017 at 05:35:05PM +0800, Wei Wang wrote:

So we don't need the bitmap to talk to host, it is just
a data structure we chose to maintain lists of pages, right?
Right. bitmap is the way to gather pages to chunk.
It's only needed in the balloon page case.
For the unused page case, we don't need it, since the free
page blocks are already chunks.

OK as far as it goes but you need much better isolation for it.
Build a data structure with APIs such as _init, _cleanup, _add, _clear,
_find_first, _find_next.
Completely unrelated to pages, it just maintains bits.
Then use it here.

   static int oom_pages = OOM_VBALLOON_DEFAULT_PAGES;
   module_param(oom_pages, int, S_IRUSR | S_IWUSR);
   MODULE_PARM_DESC(oom_pages, "pages to free on OOM");
@@ -50,6 +54,10 @@ MODULE_PARM_DESC(oom_pages, "pages to free on OOM");
   static struct vfsmount *balloon_mnt;
+/* Types of pages to chunk */
Doesn't look like you are ever adding more types in this
patchset.  Pls keep code simple, generalize it later.

"#define PAGE_CHUNK_TYPE_UNUSED 1" is added in another patch.
I would say add the extra code there too. Or maybe we can avoid
adding it altogether.

I'm trying to have the two features( i.e. "balloon pages" and
"unused pages") decoupled while trying to use common functions
to deal with the commonalities. That's the reason to define
the above macro.
Without the macro, we will need to have separate functions,
for example, instead of one "add_one_chunk()", we need to
have add_one_balloon_page_chunk() and
and some of the implementations will be kind of duplicate in the
two functions.
Probably we can add it when the second feature comes to
the code.

Types of page to chunk are treated differently. Different types of page
chunks are sent to the host via different protocols.

1) PAGE_CHUNK_TYPE_BALLOON: Ballooned (i.e. inflated/deflated) pages
to chunk.  For the ballooned type, it uses the basic chunk msg format:

virtio_balloon_page_chunk_hdr +
virtio_balloon_page_chunk * MAX_PAGE_CHUNKS

2) PAGE_CHUNK_TYPE_UNUSED: unused pages to chunk. It uses this miscq msg
miscq_hdr +
virtio_balloon_page_chunk_hdr +
virtio_balloon_page_chunk * MAX_PAGE_CHUNKS

The chunk msg is actually the payload of the miscq msg.

So just combine the two message formats and then it'll all be easier?

Yes, it'll be simple with only one msg format. But the problem I see
here is that miscq hdr is something necessary for the "unused page"
usage, but not needed by the "balloon page" usage. To be more
struct virtio_balloon_miscq_hdr {
 __le16 cmd;
 __le16 flags;
'cmd' specifies  the command from the miscq (I envision that
miscq will be further used to handle other possible miscellaneous
requests either from the host or to the host), so 'cmd' is necessary
for the miscq. But the inflateq is exclusively used for inflating
pages, so adding a command to it would be redundant and look a little
bewildered there.
'flags': We currently use bit 0 of flags to indicate the completion
ofa command, this is also useful in the "unused page" usage, and not
needed by the "balloon page" usage.
+#define MAX_PAGE_CHUNKS 4096
This is an order-4 allocation. I'd make it 4095 and then it's
an order-3 one.
Sounds good, thanks.
I think it would be better to make it 4090. Leave some space for the hdr
as well.
And miscq hdr. In fact just let compiler do the math - something like:
(8 * PAGE_SIZE - sizeof(hdr)) / sizeof(chunk)
Agree, thanks.

I skimmed explanation of algorithms below but please make sure
code speaks for itself and add comments inline to document it.
Whenever you answered me inline this is where you want to
try to make code clearer and add comments.

Also, pls find ways to abstract the data structure so we don't
need to deal with its internals all over the code.


        struct scatterlist sg;
+       struct virtio_balloon_page_chunk_hdr *hdr;
+       void *buf;
        unsigned int len;
-       sg_init_one(&sg, vb->pfns, sizeof(vb->pfns[0]) * vb->num_pfns);
+       switch (type) {
+               hdr = vb->balloon_page_chunk_hdr;
+               len = 0;
+               break;
+       default:
+               dev_warn(&vb->vdev->dev, "%s: chunk %d of unknown pages\n",
+                        __func__, type);
+               return;
+       }
-       /* We should always be able to add one buffer to an empty queue. */
-       virtqueue_add_outbuf(vq, &sg, 1, vb, GFP_KERNEL);
-       virtqueue_kick(vq);
+       buf = (void *)hdr - len;
Moving back to before the header? How can this make sense?
It works fine since len is 0, so just buf = hdr.

For the unused page chunk case, it follows its own protocol:
miscq_hdr + payload(chunk msg).
  "buf = (void *)hdr - len" moves the buf pointer to the miscq_hdr, to send
the entire miscq msg.
Well just pass the correct pointer in.

OK. The miscq msg is

We can probably change the code like this:

#define CHUNK_TO_MISCQ_MSG(chunk) (chunk - sizeof(struct virtio_balloon_miscq_hdr))

switch (type) {
                msg_buf = vb->balloon_page_chunk_hdr;
                msg_len = sizeof(struct virtio_balloon_page_chunk_hdr) +
nr_chunks * sizeof(struct virtio_balloon_page_chunk_entry);
                msg_buf = CHUNK_TO_MISCQ_MSG(vb->unused_page_chunk_hdr);
                msg_len = sizeof(struct virtio_balloon_miscq_hdr) +
sizeof(struct virtio_balloon_page_chunk_hdr) +
nr_chunks * sizeof(struct virtio_balloon_page_chunk_entry);
                dev_warn(&vb->vdev->dev, "%s: chunk %d of unknown pages\n",
                         __func__, type);

Please check the patch for implementing the unused page chunk,
it will be clear. If necessary, I can put "buf = (void *)hdr - len" from
that patch.
Exactly. And all this pointer math is very messy. Please look for ways
to clean it. It's generally easy to fill structures:

struct foo *foo = kmalloc(..., sizeof(*foo) + n * sizeof(foo->a[0]));
for (i = 0; i < n; ++i)
        foo->a[i] = b;

this is the kind of code that's easy to understand and it's
obvious there are no overflows and no info leaks here.

OK, will take your suggestion:

struct virtio_balloon_page_chunk {
        struct virtio_balloon_page_chunk_hdr hdr;
        struct virtio_balloon_page_chunk_entry entries[];

+       len += sizeof(struct virtio_balloon_page_chunk_hdr);
+       len += hdr->chunks * sizeof(struct virtio_balloon_page_chunk);
+       sg_init_table(&sg, 1);
+       sg_set_buf(&sg, buf, len);
+       if (!virtqueue_add_outbuf(vq, &sg, 1, vb, GFP_KERNEL)) {
+               virtqueue_kick(vq);
+               if (busy_wait)
+                       while (!virtqueue_get_buf(vq, &len) &&
+                              !virtqueue_is_broken(vq))
+                               cpu_relax();
+               else
+                       wait_event(vb->acked, virtqueue_get_buf(vq, &len));
+               hdr->chunks = 0;
Why zero it here after device used it? Better to zero before use.
hdr->chunks tells the host how many chunks are there in the payload.
After the device use it, it is ready to zero it.
It's rather confusing. Try to pass # of chunks around
in some other way.

Not sure if this was explained clearly - we just let the chunk msg hdr
indicates the # of chunks in the payload. I think this should be a pretty
normal usage, like the network UDP hdr, which uses a length field to indicate
the packet length.

+       }
+static void add_one_chunk(struct virtio_balloon *vb, struct virtqueue *vq,
+                         int type, u64 base, u64 size)
what are the units here? Looks like it's in 4kbyte units?
what is the "unit" you referred to?
This is the function to add one chunk, base pfn and size of the chunk are
supplied to the function.

Are both size and base in bytes then?
But you do not send them to host as is, you shift them for some reason
before sending them to host.

Not in bytes actually. base is a base pfn, which is the starting address
of the continuous pfns. Size is the chunk size, which is the number of
continuous pfns.

They are shifted based on the chunk format we agreed before:

|                 Base (52 bit)        | Rsvd (12 bit) |
|                 Size (52 bit)        | Rsvd (12 bit) |

Here, the pfn will be the balloon page pfn (4KB).In this way, the host
doesn't need to know PAGE_SIZE of the guest.

+                       if (zero >= end)
+                               chunk_size = end - one;
+                       else
+                               chunk_size = zero - one;
+                       if (chunk_size)
+                               add_one_chunk(vb, vq, PAGE_CHUNK_TYPE_BALLOON,
+                                             pfn_start + one, chunk_size);
Still not so what does a bit refer to? page or 4kbytes?
I think it should be a page.
A bit in the bitmap corresponds to a pfn of a balloon page(4KB).
That's a waste on systems with large page sizes, and it does not
look like you handle that case correctly.

OK, I will change the bitmap to be PAGE_SIZE based here, instead of
BALLOON_PAGE_SIZE based. When convert them into chunks, making it based


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