Re: [Qemu-devel] [PATCH v2 05/13] block: Switch to 64-bit bl.max_transfer

[Eric Blake]

On 11/16/18 9:32 AM, Kevin Wolf wrote:
> Am 15.11.2018 um 17:28 hat Eric Blake geschrieben:
> > On 11/15/18 9:45 AM, Kevin Wolf wrote:
> > > Am 15.11.2018 um 03:03 hat Eric Blake geschrieben:
> > > > This change has no semantic impact: all drivers either leave the
> > > > value at 0 (no inherent 32-bit limit is still translated into
> > > > fragmentation below 2G; see the previous patch for that audit), or
> > > > set it to a value less than 2G.  However, switching to a larger
> > > > type and enforcing the 2G cap at the block layer makes it easier
> > > > to audit specific drivers for their robustness to larger sizing,
> > > > by letting them specify a value larger than INT_MAX if they have
> > > > been audited to be 64-bit clean.

> > > > +
> > > > +    /* Clamp max_transfer to 2G */
> > > > +    if (bs->bl.max_transfer > UINT32_MAX) {
> > >
> > > UINT32_MAX is 4G, not 2G.
> > >
> > > Would it make more sense to make BDRV_REQUEST_MAX_BYTES the maximum
> > > anyway?
> >
> > D'oh.  Yes, that's what I intended, possibly by spelling it INT_MAX (the
> > fact that the 'if' goes away in patch 13 is not an excuse for sloppy coding
> > in the meantime).
>
> INT_MAX is not a different spelling of BDRV_REQUEST_MAX_BYTES. The
> latter is slightly lower (0x7ffffe00).

Ah, but:

> +    if (bs->bl.max_transfer > UINT32_MAX) {
> +        bs->bl.max_transfer = QEMU_ALIGN_DOWN(BDRV_REQUEST_MAX_BYTES,
> +                                              MAX(bs->bl.opt_transfer,
> +                                                  bs->bl.request_alignment));
> +    }

QEMU_ALIGN_DOWN() will change INT_MAX to BDRV_REQUEST_MAX_BYTES if 
bs->bl.request_alignment is 512 and opt_transfer is 0; and if either 
alignment number is larger than 512, max_transfer is capped even lower 
regardless of whether I was aligning INT_MAX or BDRV_REQUEST_MAX_BYTES 
down to an alignment boundary.

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org