Re: [Qemu-block] [Qemu-devel] [PATCH RFC 1/2] virtio-blk: add DISCARD and WRITE ZEROES features

[Stefan Hajnoczi]

On Fri, Jan 25, 2019 at 05:18:13PM +0100, Stefano Garzarella wrote:
> On Fri, Jan 25, 2019 at 02:58:56PM +0000, Stefan Hajnoczi wrote:
> > On Thu, Jan 24, 2019 at 06:23:22PM +0100, Stefano Garzarella wrote:
> > > +            virtio_error(vdev, "virtio-blk discard/wzeroes header too 
> > > short");
> > > +            return -1;
> > > +        }
> > > +
> > > +        sector = virtio_ldq_p(VIRTIO_DEVICE(req->dev), &dwz_hdr.sector);
> > > +        bytes = virtio_ldl_p(VIRTIO_DEVICE(req->dev),
> > > +                             &dwz_hdr.num_sectors) << BDRV_SECTOR_BITS;
> > 
> > Please handle num_sectors << BDRV_SECTOR_BITS overflow so that we don't
> > need to worry about what happens with an overflowed value later on.
> > 
> 
> Should I also check if num_sectors is less than "max_discard_sectors" or
> "max_write_zeroes_sectors"?

Yes, anything that virtio_blk_sect_range_ok() doesn't check needs to be
checked manually in this patch.  I can't see a reason for allowing
requests through that exceed the limit.

> Do you think make sense flush the MultiReqBuffer before call
> blk_aio_pdiscard() or blk_aio_pwrite_zeroes()?

I don't think that is necessary since virtio-blk doesn't guarantee
ordering (the legacy barrier feature isn't supported by QEMU).

Even the MultiReqBuffer is flushed, -drive aio=threads (the default
setting) could end up submitting discard/write zeroes before "earlier"
requests - it depends on host kernel thread scheduler decisions.

Stefan

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