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Re: [Qemu-devel] [PATCH v7 kernel 3/5] virtio-balloon: implementation of
From: |
Matthew Wilcox |
Subject: |
Re: [Qemu-devel] [PATCH v7 kernel 3/5] virtio-balloon: implementation of VIRTIO_BALLOON_F_CHUNK_TRANSFER |
Date: |
Fri, 10 Mar 2017 09:11:44 -0800 |
User-agent: |
Mutt/1.7.1 (2016-10-04) |
On Fri, Mar 10, 2017 at 05:58:28PM +0200, Michael S. Tsirkin wrote:
> One of the issues of current balloon is the 4k page size
> assumption. For example if you free a huge page you
> have to split it up and pass 4k chunks to host.
> Quite often host can't free these 4k chunks at all (e.g.
> when it's using huge tlb fs).
> It's even sillier for architectures with base page size >4k.
I completely agree with you that we should be able to pass a hugepage
as a single chunk. Also we shouldn't assume that host and guest have
the same page size. I think we can come up with a scheme that actually
lets us encode that into a 64-bit word, something like this:
bit 0 clear => bits 1-11 encode a page count, bits 12-63 encode a PFN, page
size 4k.
bit 0 set, bit 1 clear => bits 2-12 encode a page count, bits 13-63 encode a
PFN, page size 8k
bits 0+1 set, bit 2 clear => bits 3-13 for page count, bits 14-63 for PFN, page
size 16k.
bits 0-2 set, bit 3 clear => bits 4-14 for page count, bits 15-63 for PFN, page
size 32k
bits 0-3 set, bit 4 clear => bits 5-15 for page count, bits 16-63 for PFN, page
size 64k
That means we can always pass 2048 pages (of whatever page size) in a single
chunk. And
we support arbitrary power of two page sizes. I suggest something like this:
u64 page_to_chunk(struct page *page)
{
u64 chunk = page_to_pfn(page) << PAGE_SHIFT;
chunk |= (1UL << compound_order(page)) - 1;
}
(note this is a single page of order N, so we leave the page count bits
set to 0, meaning one page).
> Two things to consider:
> - host should pass its base page size to guest
> this can be a separate patch and for now we can fall back on 12 bit if not
> there
With this encoding scheme, I don't think we need to do this? As long as
it's *at least* 12 bit, then we're fine.
> - guest should pass full huge pages to host
> this should be done correctly to avoid breaking up huge pages
> I would say yes let's use a single format but drop the "normal chunk"
> and always use the extended one.
> Also, size is in units of 4k, right? Please document that low 12 bit
> are reserved, they will be handy as e.g. flags.
What per-chunk flags are you thinking would be useful?
- [Qemu-devel] [PATCH v7 kernel 4/5] virtio-balloon: define flags and head for host request vq, (continued)
- [Qemu-devel] [PATCH v7 kernel 4/5] virtio-balloon: define flags and head for host request vq, Wei Wang, 2017/03/03
- [Qemu-devel] [PATCH v7 kernel 3/5] virtio-balloon: implementation of VIRTIO_BALLOON_F_CHUNK_TRANSFER, Wei Wang, 2017/03/03
- Re: [Qemu-devel] [PATCH v7 kernel 3/5] virtio-balloon: implementation of VIRTIO_BALLOON_F_CHUNK_TRANSFER, Matthew Wilcox, 2017/03/09
- Re: [Qemu-devel] [PATCH v7 kernel 3/5] virtio-balloon: implementation of VIRTIO_BALLOON_F_CHUNK_TRANSFER, Michael S. Tsirkin, 2017/03/10
- Re: [Qemu-devel] [PATCH v7 kernel 3/5] virtio-balloon: implementation of VIRTIO_BALLOON_F_CHUNK_TRANSFER, Matthew Wilcox, 2017/03/10
- Re: [Qemu-devel] [PATCH v7 kernel 3/5] virtio-balloon: implementation of VIRTIO_BALLOON_F_CHUNK_TRANSFER, Michael S. Tsirkin, 2017/03/11
- Re: [Qemu-devel] [PATCH v7 kernel 3/5] virtio-balloon: implementation of VIRTIO_BALLOON_F_CHUNK_TRANSFER, Wei Wang, 2017/03/11
- Re: [Qemu-devel] [PATCH v7 kernel 3/5] virtio-balloon: implementation of VIRTIO_BALLOON_F_CHUNK_TRANSFER, Matthew Wilcox, 2017/03/11
- Re: [Qemu-devel] [PATCH v7 kernel 3/5] virtio-balloon: implementation of VIRTIO_BALLOON_F_CHUNK_TRANSFER, Wang, Wei W, 2017/03/11
- Re: [Qemu-devel] [PATCH v7 kernel 3/5] virtio-balloon: implementation of VIRTIO_BALLOON_F_CHUNK_TRANSFER, Michael S. Tsirkin, 2017/03/11
- Re: [Qemu-devel] [PATCH v7 kernel 3/5] virtio-balloon: implementation of VIRTIO_BALLOON_F_CHUNK_TRANSFER, Wang, Wei W, 2017/03/13