Re: [Qemu-devel] [PATCH v6 11/20] block: vhdx write support

[Jeff Cody]

On Tue, Oct 01, 2013 at 03:29:17PM +0200, Stefan Hajnoczi wrote:
> On Wed, Sep 25, 2013 at 05:02:56PM -0400, Jeff Cody wrote:
> > @@ -1070,7 +1070,43 @@ exit:
> >      return ret;
> >  }
> >  
> > +/*
> > + * Allocate a new payload block at the end of the file.
> > + *
> > + * Allocation will happen at 1MB alignment inside the file
> > + *
> > + * Returns the file offset start of the new payload block
> > + */
> > +static int vhdx_allocate_block(BlockDriverState *bs, BDRVVHDXState *s,
> > +                                    uint64_t *new_offset)
> > +{
> > +    *new_offset = bdrv_getlength(bs->file);
> > +
> > +    /* per the spec, the address for a block is in units of 1MB */
> > +    *new_offset = ROUND_UP(*new_offset, 1024*1024);
> > +
> > +    return bdrv_truncate(bs->file, *new_offset + s->block_size);
> > +}
> > +
> > +/*
> > + * Update the BAT tablet entry with the new file offset, and the new entry
> 
> s/tablet/table/
> 
> > + * state */
> > +static void vhdx_update_bat_table_entry(BlockDriverState *bs, 
> > BDRVVHDXState *s,
> > +                                       VHDXSectorInfo *sinfo,
> > +                                       uint64_t *bat_entry,
> > +                                       uint64_t *bat_offset, int state)
> > +{
> > +    /* The BAT entry is a uint64, with 44 bits for the file offset in 
> > units of
> > +     * 1MB, and 3 bits for the block state. */
> > +    s->bat[sinfo->bat_idx]  = ((sinfo->file_offset>>20) <<
> 
> QEMU coding style requires space around the '>>'.  Please run
> scripts/checkpatch.pl.

I missed that - but I did run checkpatch.pl, it did not catch it:

vhdx-v6/0011-block-vhdx-write-support.patch has no obvious style problems and
is ready for submission.
total: 0 errors, 0 warnings, 31 lines checked

I think the parenthesis hid it.

> 
> > +                               VHDX_BAT_FILE_OFF_BITS);
> >  
> > +    s->bat[sinfo->bat_idx] |= state & VHDX_BAT_STATE_BIT_MASK;
> > +
> > +    *bat_entry = cpu_to_le64(s->bat[sinfo->bat_idx]);
> 
> Call this argument bat_entry_le so callers know now to manipulate the
> little-endian value directly?
>

Yes, good idea.

> > +    *bat_offset = s->bat_offset + sinfo->bat_idx * sizeof(VHDXBatEntry);
> > +
> > +}
> >  
> >  /* Per the spec, on the first write of guest-visible data to the file the
> >   * data write guid must be updated in the header */
> > @@ -1087,7 +1123,117 @@ int vhdx_user_visible_write(BlockDriverState *bs, 
> > BDRVVHDXState *s)
> >  static coroutine_fn int vhdx_co_writev(BlockDriverState *bs, int64_t 
> > sector_num,
> >                                        int nb_sectors, QEMUIOVector *qiov)
> >  {
> > -    return -ENOTSUP;
> > +    int ret = -ENOTSUP;
> > +    BDRVVHDXState *s = bs->opaque;
> > +    VHDXSectorInfo sinfo;
> > +    uint64_t bytes_done = 0;
> > +    uint64_t bat_entry = 0;
> > +    uint64_t bat_entry_offset = 0;
> > +    bool bat_update;
> > +    QEMUIOVector hd_qiov;
> > +
> > +    qemu_iovec_init(&hd_qiov, qiov->niov);
> > +
> > +    qemu_co_mutex_lock(&s->lock);
> > +
> > +    ret = vhdx_user_visible_write(bs, s);
> > +    if (ret < 0) {
> > +        goto exit;
> > +    }
> > +
> > +    while (nb_sectors > 0) {
> > +        if (s->params.data_bits & VHDX_PARAMS_HAS_PARENT) {
> > +            /* not supported yet */
> > +            ret = -ENOTSUP;
> > +            goto exit;
> > +        } else {
> > +            bat_update = false;
> > +            vhdx_block_translate(s, sector_num, nb_sectors, &sinfo);
> > +
> > +            qemu_iovec_reset(&hd_qiov);
> > +            qemu_iovec_concat(&hd_qiov, qiov,  bytes_done, 
> > sinfo.bytes_avail);
> > +            /* check the payload block state */
> > +            switch (s->bat[sinfo.bat_idx] & VHDX_BAT_STATE_BIT_MASK) {
> > +            case PAYLOAD_BLOCK_ZERO:
> > +                /* in this case, we need to preserve zero writes for
> > +                 * data that is not part of this write, so we must pad
> > +                 * the rest of the buffer to zeroes */
> > +
> > +                /* if we are on a posix system with ftruncate() that 
> > extends
> > +                 * a file, then it is zero-filled for us.  On Win32, the 
> > raw
> > +                 * layer uses SetFilePointer and SetFileEnd, which does not
> > +                 * zero fill AFAIK */
> > +
> > +                /* TODO: queue another write of zero buffers if the host 
> > OS does
> > +                 * not zero-fill on file extension */
> 
> This is never addressed in the series.
>

I will address it in v7

> > +
> > +                /* fall through */
> > +            case PAYLOAD_BLOCK_NOT_PRESENT: /* fall through */
> > +            case PAYLOAD_BLOCK_UNMAPPED:    /* fall through */
> > +            case PAYLOAD_BLOCK_UNDEFINED:   /* fall through */
> > +                ret = vhdx_allocate_block(bs, s, &sinfo.file_offset);
> > +                if (ret < 0) {
> > +                    goto exit;
> > +                }
> > +                /* once we support differencing files, this may also be
> > +                 * partially present */
> > +                /* update block state to the newly specified state */
> > +                vhdx_update_bat_table_entry(bs, s, &sinfo, &bat_entry,
> > +                                            &bat_entry_offset,
> > +                                            PAYLOAD_BLOCK_FULL_PRESENT);
> 
> In the specification the constant is called PAYLOAD_BLOCK_FULLY_PRESENT.
> Is s/FULLY/FULL/ intentional and can you match the spec for consistency?
>

That was unintentional, and yes I will change it to match the spec
naming.

> > +                bat_update = true;
> > +                /* since we just allocated a block, file_offset is the
> > +                 * beginning of the payload block. It needs to be the
> > +                 * write address, which includes the offset into the block 
> > */
> > +                sinfo.file_offset += sinfo.block_offset;
> > +                /* fall through */
> > +            case PAYLOAD_BLOCK_FULL_PRESENT:
> > +                /* if the file offset address is in the header zone,
> > +                 * there is a problem */
> > +                if (sinfo.file_offset < (1024*1024)) {
> > +                    ret = -EFAULT;
> > +                    goto exit;
> > +                }
> > +                /* block exists, so we can just overwrite it */
> > +                qemu_co_mutex_unlock(&s->lock);
> > +                ret = bdrv_co_writev(bs->file,
> > +                                    sinfo.file_offset>>BDRV_SECTOR_BITS,
> > +                                    sinfo.sectors_avail, &hd_qiov);
> > +                qemu_co_mutex_lock(&s->lock);
> > +                if (ret < 0) {
> > +                    goto exit;
> > +                }
> > +                break;
> > +            case PAYLOAD_BLOCK_PARTIALLY_PRESENT:
> > +                /* we don't yet support difference files, fall through
> > +                 * to error */
> > +            default:
> > +                ret = -EIO;
> > +                goto exit;
> > +                break;
> > +            }
> > +
> > +            if (bat_update) {
> 
> Data must be stable on disk before we can expose the updated BAT entry.
> Missing bdrv_co_flush().
> 

Good point, I will add that in for v7.

Thanks,  

Jeff