Re: [Qemu-devel] [PATCH v7] pflash: Require backend size to match device, improve errors

[Kevin Wolf <kwolf@redhat.com>]

Am 08.03.2019 um 15:29 hat Markus Armbruster geschrieben:
> Kevin Wolf <kwolf@redhat.com> writes:
> 
> > Am 08.03.2019 um 13:28 hat Markus Armbruster geschrieben:
> >> Laszlo Ersek <lersek@redhat.com> writes:
> >> > This one has got to be one of the longest bike-shedding sessions! :)
> >> >
> >> > I'm fine with this patch, but I could suggest two improvements.
> >> >
> >> > (1) When blk_getlength() fails, we could format the negative error code
> >> > returned by it into the error message.
> >> 
> >> I can do that.
> >
> > By using error_setg_errno(), I assume. Not throwing away error details
> > is always good.
> >
> >> > (2) We could extract the common code to a new function in
> >> > "hw/block/block.c". (It says "Common code for block device models" on
> >> > the tin.)
> >> 
> >> There's so much common code in these two files even before this patch...
> >
> > My understanding is that hw/block/block.c contains code that is
> > potentially useful to all kinds of block devices, not random code that
> > two specific similar devices happen to share.
> >
> > If we want to deduplicate some code in the flash devices, without any
> > expectation that other devices will use it at some point, I'd rather
> > create a new source file hw/block/pflash_common.c or something like
> > that.
> 
> Yes.
> 
> The helper I came up with (appended) isn't really specific to flash
> devices.  Would it be okay for hw/block/block.c even though only the two
> flash devices use it for now?

Hm, it feels more like a helper for devices that can't decide whether
they want to be a block device or not. Or that actually don't want to be
a block device, but use a BlockBackend anyway. Reading in the whole
image isn't something that a normal block device would do.

But yes, it doesn't have flash-specific knowledge, even though I hope
that it's functionality that will remain very specific to these two
devices.

So it's your call, I don't have a strong opinion either way.

> 
> bool blk_check_size_and_read_all(BlockBackend *blk, void *buf, hwaddr size,
>                                  Error **errp)
> {
>     int64_t blk_len;
>     int ret;
> 
>     blk_len = blk_getlength(blk);
>     if (blk_len < 0) {
>         error_setg_errno(errp, -blk_len,
>                          "can't get size of block backend '%s'",
>                          blk_name(blk));
>         return false;
>     }
>     if (blk_len != size) {
>         error_setg(errp, "device requires %" PRIu64 " bytes, "
>                    "block backend '%s' provides %" PRIu64 " bytes",
>                    size, blk_name(blk), blk_len);

Should size use HWADDR_PRIu?

I'm not sure if printing the BlockBackend name is a good idea because
hopefully one day the BlockBackend will be anonymous even for the flash
devices.

>         return false;
>     }
> 
>     /* TODO for @size > BDRV_REQUEST_MAX_BYTES, we'd need to loop */
>     assert(size <= BDRV_REQUEST_MAX_BYTES);

I don't think we'd ever want to read in more than 2 GB into a memory
buffer. Before we even get close to this point, the devices should be
reworked to be more like an actual block device and read only what is
actually accessed.

>     ret = blk_pread(blk, 0, buf, size);
>     if (ret < 0) {
>         error_setg_errno(errp, -ret, "can't read block backend '%s'",
>                          blk_name(blk));
>         return false;
>     }
>     return true;
> }

Kevin