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

[Eric Blake <eblake@redhat.com>]

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