Re: [Qemu-devel] [PATCH 2/5] block: Honor flags during bdrv_aligned_preadv()

[Eric Blake <eblake@redhat.com>]

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On 06/07/2016 06:12 AM, Kevin Wolf wrote:
> Am 03.06.2016 um 19:03 hat Eric Blake geschrieben:
>> Not that we pass any flags during reads yet, but we may want to
>> support BDRV_REQ_FUA on reads in the future.  So don't throw
>> away the input flags.
>>
>> Signed-off-by: Eric Blake <eblake@redhat.com>
> 
> Do we want to pass flags to bdrv_co_do_copy_on_readv(), too? I guess we
> would use them for the preadv call there, but continue to use 0 as the
> pwritev flags.

What about BDRV_REQ_MAY_UNMAP if bdrv_co_do_copy_on_readv() detects a
block of zeroes?

This just got a lot trickier, so I think my short-term solution is to
just assert(!flags) and add a comment that whoever implements flags on
read has to solve the issue at that time, so that I'm not stalling the
rest of this series on thinking about things that don't matter yet.

> 
> We may also want to introduce a .supported_read_flags, but perhaps this
> is not something to do in this patch.

Yeah, I think we'll want that, at the point where we start worrying
about read flags.

> And we don't even use
> .supported_write_flags for drivers that have a native .bdrv_co_pwritev.
> Of course, that's even less related to this patch. I guess I should send
> a fix for that.

Thanks.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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