linux-kernel - Re: [RFC for Linux v4 1/2] virtio_balloon: Add VIRTIO_BALLOON_F_CONT_PAGES and inflate_cont

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Date:   Thu, 16 Jul 2020 16:25:00 +0800
From:   teawater <teawaterz@...ux.alibaba.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Hui Zhu <teawater@...il.com>, David Hildenbrand <david@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Virtualization <virtualization@...ts.linux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        qemu-devel@...gnu.org, virtio-dev@...ts.oasis-open.org
Subject: Re: [RFC for Linux v4 1/2] virtio_balloon: Add
 VIRTIO_BALLOON_F_CONT_PAGES and inflate_cont_vq



> 2020年7月16日 14:43，Michael S. Tsirkin <mst@...hat.com> 写道：
> 
> On Thu, Jul 16, 2020 at 10:41:51AM +0800, Hui Zhu wrote:
>> diff --git a/include/uapi/linux/virtio_balloon.h b/include/uapi/linux/virtio_balloon.h
>> index dc3e656..4d0151a 100644
>> --- a/include/uapi/linux/virtio_balloon.h
>> +++ b/include/uapi/linux/virtio_balloon.h
>> @@ -37,6 +37,7 @@
>> #define VIRTIO_BALLOON_F_FREE_PAGE_HINT	3 /* VQ to report free pages */
>> #define VIRTIO_BALLOON_F_PAGE_POISON	4 /* Guest is using page poisoning */
>> #define VIRTIO_BALLOON_F_REPORTING	5 /* Page reporting virtqueue */
>> +#define VIRTIO_BALLOON_F_CONT_PAGES	6 /* VQ to report continuous pages */
>> 
>> /* Size of a PFN in the balloon interface. */
>> #define VIRTIO_BALLOON_PFN_SHIFT 12
> 
> So how does the guest/host interface look like?
> Could you write up something about it?

Continuous pages are report by num_pfns and pfns in virtio_balloon too.
The function to set pfns is set_page_pfns_size in https://github.com/teawater/linux/blob/balloon_conts/drivers/virtio/virtio_balloon.c#L221

static void set_page_pfns_size(struct virtio_balloon *vb,
                               __virtio32 pfns[], struct page *page,
                               size_t size)
{
        /* Set the first pfn of the continuous pages.  */
        pfns[0] = cpu_to_virtio32(vb->vdev, page_to_balloon_pfn(page));
        /* Set the size of the continuous pages.  */
        pfns[1] = (__virtio32) size;
}

Each of continuous pages need 2 pfn.
The first pfn of the pages is set to pfns[0].  The size of the pages is set to pfns[1].

The pfn is 32 bits.
So the max order of inflate continuous pages is VIRTIO_BALLOON_INFLATE_MAX_ORDER.
#define VIRTIO_BALLOON_INFLATE_MAX_ORDER min((int) (sizeof(__virtio32) * BITS_PER_BYTE - \
                                                    1 - PAGE_SHIFT), (MAX_ORDER-1))

The max page number of deflate continuous pages is VIRTIO_BALLOON_DEFLATE_MAX_PAGES_NUM.
#define VIRTIO_BALLOON_DEFLATE_MAX_PAGES_NUM (((__virtio32)~0U) >> PAGE_SHIFT)

Best,
Hui