Re: [Qemu-block] [Qemu-devel] Disk Image Read Location

[Max Reitz]

On 2017-09-18 20:47, Derrick McKee wrote:
> Hi,
> 
> I am trying to figure out if a particular disk operation is reading from
> the same location in an image file.  I am simulating a USB disk, and during
> an operation the guest OS performs, I would like to know how many times the
> backing image file is read.  I have identified the function
> address_space_write as the function that writes the data to the guest OS
> from the image, but I am not sure what to compare with past writes.  There
> are AddressSpace*, MemoryRegion*, and hwaddr types in that function.  Can
> any of those be used to compare image read locations?  I am hoping that if,
> for example, the AddressSpace* passed into address_space_write during one
> invocation equals the AddressSpace* of another invocation means that the
> same location within the image file was read twice.  Any help would be
> appreciated.

If you're referring to images as in images of block devices, then the
block layer contains the functionality you're looking for.

bdrv_driver_preadv() from block/io.c is a function that every read
request should go through.  However, at this point it's already aligned
to (host) sectors, so maybe bdrv_co_preadv() is better suited.

But then again, because of how the block layer works you'll likely see
every read request twice there (once for the disk image's format (e.g.
qcow2 or raw) and once for the storage backend used for the image (e.g.
file for normal files or http)).  So you either correct for that or you
instead go one layer above into block/block-backend.c.

All read requests from the guest go through either blk_co_preadv() or
blk_aio_preadv(), and they should go through there only once.

Max

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