Re: [PATCH 3/4] Move CONFIG_XFS handling to meson.build

[Paolo Bonzini]

On 12/10/21 09:46, Thomas Huth wrote:
> > platform_test_xfs_fd() is only used to decide whether to invoke 
> > XFS_IOC_DIOINFO; but failures of XFS_IOC_DIOINFO are ignored anyway, 
> > so we can get rid of is_xfs in BDRVRawState, too.
>
> After staring at the code for a while, I wonder why we're not simply 
> using fstat() here instead to get the st_blksize value... wouldn't that 
> be better anyway since it also works with other file system types?

The value that XFS_IOC_DIOINFO returns is the logical sector size of the 
underlying device; it should be 512 or 4096, but more likely 512.  It 
can be smaller than st_blksize, because often it will be if it is 512 
but the st_blksize is usually 4096.

If it is wrong, QEMU will do unnecessary read/modify/write operations 
for disk writes that are not 4K-aligned.

Paolo