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xen-devel
Re: [Xen-devel] about blkfront.c
I got the point. It's my misunderstanding. I guess fsect and lsect really
denotes the first and last sector number WITHIN ONE PAGE. That will make
more sense.
Thanks again for your kind explanation.
Xin
Thanks a lot for your explanation.
On Tue, 28 Sep 2004, Keir Fraser wrote:
>
> The extent has to fit within an aligned page. 'fsect' and 'lsect'
> denote the first and last sectors, inclusive, within a page.
>
> Since we count from zero, the minimum permissible value for fsect is 0
> and for lsect is 7. Hence the assertion.
>
> -- Keir
>
>
> > If lsect stands for the number of sector to be transferred, your
> > explanation is reasonable, but the following two lines give me an
> > impression that lsect is the index of the last sector to be transferred:
> >
> > ---------------------------------
> > fsect = (buffer_ma & ~PAGE_MASK) >> 9;
> > lsect = fsect + nr_sectors - 1;
> > -----------------------------------
> >
> > The sector index does not have to be less than 8, right? Am I wrong in
> > understanding the codes?
> >
> > Thanks for your help.
> >
> > Xin
> >
> > On Tue, 28 Sep 2004, Ian Pratt wrote:
> >
> > > > Sorry for the dumb question,
> > > >
> > > > I noticed that in the function blkfront.c: blkif_queue_request(...),
> > > > there
> > > > are these two lines of codes:
> > > >
> > > > ----------------------------
> > > > if ( lsect > 7 )
> > > > BUG();
> > > > ----------------------------
> > >
> > > Disk sectors are 512 bytes, pages 4096 bytes, hence you can have
> > > runs of up to 8 sectors in a page. At the point that
> > > queue_request is called we know that the request will fit
> > > entirely within a page. It's the job of this function to merge
> > > requests that are to sequential areas of disk into in a single
> > > scatter gather list.
> > >
> > > Ian
> > >
> >
> >
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