linux-kernel - RE: [RESEND PATCH v3 kernel 2/7] virtio-balloon: define new feature bit and page bitmap head

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Date:   Tue, 25 Oct 2016 01:21:21 +0000
From:   "Li, Liang Z" <liang.z.li@...el.com>
To:     "Hansen, Dave" <dave.hansen@...el.com>,
        "mst@...hat.com" <mst@...hat.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "virtio-dev@...ts.oasis-open.org" <virtio-dev@...ts.oasis-open.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "qemu-devel@...gnu.org" <qemu-devel@...gnu.org>,
        "quintela@...hat.com" <quintela@...hat.com>,
        "dgilbert@...hat.com" <dgilbert@...hat.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "cornelia.huck@...ibm.com" <cornelia.huck@...ibm.com>,
        "amit.shah@...hat.com" <amit.shah@...hat.com>
Subject: RE: [RESEND PATCH v3 kernel 2/7] virtio-balloon: define new feature
 bit and page bitmap head

> On 10/20/2016 11:24 PM, Liang Li wrote:
> > Add a new feature which supports sending the page information with a
> > bitmap. The current implementation uses PFNs array, which is not very
> > efficient. Using bitmap can improve the performance of
> > inflating/deflating significantly
> 
> Why is it not efficient?  How is using a bitmap more efficient?  What kinds of
> cases is the bitmap inefficient?
> 
> > The page bitmap header will used to tell the host some information
> > about the page bitmap. e.g. the page size, page bitmap length and
> > start pfn.
> 
> Why did you choose to add these features to the structure?  What benefits
> do they add?
> 
> Could you describe your solution a bit here, and describe its strengths and
> weaknesses?
> 

Will elaborate the solution in V4.

> >  /* Size of a PFN in the balloon interface. */  #define
> > VIRTIO_BALLOON_PFN_SHIFT 12 @@ -82,4 +83,22 @@ struct
> > virtio_balloon_stat {
> >  	__virtio64 val;
> >  } __attribute__((packed));
> >
> > +/* Page bitmap header structure */
> > +struct balloon_bmap_hdr {
> > +	/* Used to distinguish different request */
> > +	__virtio16 cmd;
> > +	/* Shift width of page in the bitmap */
> > +	__virtio16 page_shift;
> > +	/* flag used to identify different status */
> > +	__virtio16 flag;
> > +	/* Reserved */
> > +	__virtio16 reserved;
> > +	/* ID of the request */
> > +	__virtio64 req_id;
> > +	/* The pfn of 0 bit in the bitmap */
> > +	__virtio64 start_pfn;
> > +	/* The length of the bitmap, in bytes */
> > +	__virtio64 bmap_len;
> > +};
> 
> FWIW this is totally unreadable.  Please do something like this:
> 
> > +struct balloon_bmap_hdr {
> > +	__virtio16 cmd; 	/* Used to distinguish different ...
> > +	__virtio16 page_shift; 	/* Shift width of page in the bitmap */
> > +	__virtio16 flag; 	/* flag used to identify different...
> > +	__virtio16 reserved;	/* Reserved */
> > +	__virtio64 req_id;	/* ID of the request */
> > +	__virtio64 start_pfn;	/* The pfn of 0 bit in the bitmap */
> > +	__virtio64 bmap_len;	/* The length of the bitmap, in bytes */
> > +};
> 
> and please make an effort to add useful comments.  "/* Reserved */"
> seems like a waste of bytes to me.

OK. Maybe 'padding' is better than 'reserved' .

Thanks for your comments!

Liang