Re: [Qemu-devel] [RFC PATCH 03/19] block: Don't use guest sector size for qemu_blockalign()

[Kevin Wolf <kwolf@redhat.com>]

Am 10.12.2013 um 04:18 hat Wenchao Xia geschrieben:
> 于 2013/12/7 1:22, Kevin Wolf 写道:
> > bs->buffer_alignment is set by the device emulation and contains the
> > logical block size of the guest device. This isn't something that the
> > block layer should know, and even less something to use for determining
> > the right alignment of buffers to be used for the host.
> > 
> > The new function bdrv_opt_mem_align() allows for hooks in a BlockDriver
> > so that it can tell the qemu block layer the optimal alignment to be
> > used so that no bounce buffer must be used in the driver.
> > 
> > This patch may change the buffer alignment from 4k to 512 for all
> > callers that used qemu_blockalign() with the top-level image format
> > BlockDriverState. The value was never propagated to other levels in the
> > tree, so in particular raw-posix never required anything else than 512.
> > 
> > While on disks with 4k sectors direct I/O requires a 4k alignment,
> > memory may still be okay when aligned to 512 byte boundaries. This is
> > what must have happened in practice, because otherwise this would
> > already have failed earlier. Therefore I don't expect regressions even
> > with this intermediate state. Later, raw-posix can implement the hook
> > and expose a different memory alignment requirement.
> > 
> > Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> > ---
> >   block.c                   | 32 +++++++++++++++++++++++++++++---
> >   include/block/block.h     |  1 +
> >   include/block/block_int.h |  4 ++++
> >   3 files changed, 34 insertions(+), 3 deletions(-)
> > 
> > diff --git a/block.c b/block.c
> > index 613201b..669793b 100644
> > --- a/block.c
> > +++ b/block.c
> > @@ -213,6 +213,31 @@ static void bdrv_io_limits_intercept(BlockDriverState *bs,
> >       qemu_co_queue_next(&bs->throttled_reqs[is_write]);
> >   }
> > 
> > +size_t bdrv_opt_mem_align(BlockDriverState *bs)
> > +{
> > +    size_t alignment;
> > +
> > +    if (!bs || !bs->drv) {
> > +        /* 4k should be on the safe side */
> > +        return 4096;
> > +    }
> > +
> > +    if (bs->drv->bdrv_opt_mem_align) {
> > +        return bs->drv->bdrv_opt_mem_align(bs);
> > +    }
> > +
> > +    if (bs->file) {
> > +        alignment = bdrv_opt_mem_align(bs->file);
> > +    } else {
> > +        alignment = 512;
> > +    }
> > +
> > +    if (bs->backing_hd) {
> > +        alignment = MAX(alignment, bdrv_opt_mem_align(bs->backing_hd));
> > +    }
> 
>   Maybe I didn't understand the commit message correctly, does this code
> intend to get MAX alignment value in a chain? For example:
> base(4096)->mid(512)->top(1024) results: 4096
> The condition to traver the backing files seems complex.

Yes, you want to align any newly allocated buffer such that no matter
what direction it takes on its way through the block layer, it will
never be misaligned. This means that you need to use the highest
alignment restriction.

Kevin