On 03/16/16 19:15, Gabriel L. Somlo wrote: > On Wed, 16 Mar 2016 at 18:50:57 +0200, Michael S. Tsirkin wrote: >> On Wed, Mar 16, 2016 at 05:29:45PM +0100, Markus Armbruster wrote: >>> "Michael S. Tsirkin" <m...@redhat.com> writes: >>> >>>> Allowing arbitary file names on command line is setting us up for >>>> failure: future guests will look for a specific QEMU-specified name and >>>> will get confused finding a user file there. >>>> >>>> We do warn but people are conditioned to ignore warnings by now, >>>> so at best that will help users debug problem, not avoid it. >>>> >>>> Disable this by default, so distros don't get to deal with it, >>>> but leave an option for developers to configure this in, >>>> with scary warnings so people only do it if they know >>>> what they are doing. >>>> >>>> Signed-off-by: Michael S. Tsirkin <m...@redhat.com> >>> >>> I'm having a hard time to see the point. >> >> Frankly, I am having a hard time to see the point of exposing fw cfg to >> users at all. It was designed as an internal interface between QEMU PC >> hardware and firmware. As a PC maintainer, I do not like it that users >> get to poke at PC internals. >> >> So it is yet another way to pass binaries to Linux guests. Don't we >> have enough of these? But Gerd likes it for some reason, and merged it. >> OK. > > As the author of the feature, I feel I should jump back in and clarify: > > It's a way to pass arbitrary blobs to any type of guest, with two > important properties: 1. asynchronous, and 2. out-of-band. When I > started looking, all existing methods involved either having the host > start polling for the guest to bring up qga and be ready to accept an > out-of-band connection (i.e., *not* asynchronous), or have the guest > mount some special cdrom or floppy image prepared by the host (i.e., > *not* out of band). > > fw_cfg is both asynchronous and out-of-band, so it appeared to be the > perfect choice. > >> But please find a way to make sure it does not conflict with its current >> usage in PC. Asking that all files have an "opt/" prefix is one way >> but only if it is enforced. > > Enforcing the "opt/" prefix was clearly on the table when I submitted > the feature (and totally acceptable for my own needs). At the time, however, > most of the advice I received on the list was to leave it as a warning > only (i.e., "mechanism, not policy"), especially since other respondents > expressed interest in passing in non-"/opt" blobs for easier development > and debugging of alternative firmware (such as OVMF, iirc). > > Having a mis-use of this feature become "institutionalized" over time was > seen as a low/negligible risk at the time. Do we have any new reasons > to worry about it ?
OVMF uses this feature for a few flags. They are all called "opt/ovmf/...". I followed the advice in "docs/specs/fw_cfg.txt" (which shouldn't be surprising since I seem to have reviewed every patch for that file): > NOTE: Users *SHOULD* choose item names beginning with the prefix "opt/" > when using the "-fw_cfg" command line option, to avoid conflicting with > item names used internally by QEMU. For instance: > > -fw_cfg name=opt/my_item_name,file=./my_blob.bin > > Similarly, QEMU developers *SHOULD NOT* use item names prefixed with > "opt/" when inserting items programmatically, e.g. via fw_cfg_add_file(). It says "should", not "must". I liked (and like) the "mechanism, not policy" thing. Letting developers pass in whatever they want, for development / debugging / testing purposes, is a plus to me. Thanks Laszlo
