Re: [Qemu-devel] [PATCH/RFC] block: Ensure that block size constraints are considered

[Christian Borntraeger <borntraeger@de.ibm.com>]

On 21/11/12 17:03, Paolo Bonzini wrote:
> Il 21/11/2012 10:15, Kevin Wolf ha scritto:
>>>> +    if ((bs->open_flags & BDRV_O_NOCACHE)) {
>>>> +        bs->file->buffer_alignment = align;
>>>> +    }
>> Any reason to restrict this to BDRV_O_NOCACHE?
>>
>> There have been patches to change the BDRV_O_NOCACHE flag from the
>> monitor, in which case bdrv_set_buffer_alignment() wouldn't be called
>> anew and O_DIRECT requests start to fail again.
>>
> 
> bdrv_set_buffer_alignment() is completely broken.  It should set host
> alignment, but in fact it is passed the guest alignment.
> 
> In practice, we only support logical_block_size matching the host's or
> bigger (which is unsafe due to torn writes, but works).


For other reasons (partition table format) we want to have host block
size == guest block size on s390 anyway - so it would not really matter for
us.
But I certainly agree that it makes more sense to use the host block size
for the alignment checks.



> So I suggest that we just look at writes outside the device models, and
> "fix" them to always read a multiple of 4k.

Wouldnt that cause performance regressions for block devices with 512 byte 
block size, because we read more than necessary. Wouldnt that also require
read/update/write combinations for valid 512 byte writes?

Christian