On 09/28/15 22:56, Laszlo Ersek wrote: > On 09/28/15 22:00, Eric Blake wrote: >> On 09/28/2015 01:51 PM, Gabriel L. Somlo wrote: >>> On Mon, Sep 28, 2015 at 01:46:33PM -0600, Eric Blake wrote: >>>> On 09/28/2015 07:30 AM, Laszlo Ersek wrote: >>>> >>>>>> >>>>>> +Small enough items may be provided directly as strings on the command >>>>>> +line, using the syntax: >>>>>> + >>>>>> + -fw_cfg [name=]<item_name>,content=<string> >>>>>> + >>>>> >>>>> Please consider spelling out that these blobs will NOT be NUL-terminated >>>>> when viewed on the guest. (It kinda follows from all the other fw_cfg >>>>> things, but once we leave host-side files for qemu command line strings, >>>>> it might become non-obvious to users.) >>>> >>>> Or else GUARANTEE that it will be NUL-terminated (and the only way to >>>> get blobs that are not NUL terminated is to use files rather than >>>> content=). >>> >>> I went with the first suggestion (leave out the trailing '\0' from the >>> blob payload, and say so in docs/specs/fw_cfg.txt) in v2 of the patch. >>> >>> Do you feel strongly about including the \0 ? Otherwise, we're already >>> there :) >> >> I don't know what users are more likely to want to push through. A >> trailing NUL implies a binary file (as text files cannot contain \0), >> but even without a trailing NUL, a file is not a text file (per the >> POSIX definition) unless it is either empty or ends in a newline. Use >> of content=... is unlikely to have users remembering to place a newline >> there if examples don't suggest it. And I don't know if guests are >> expecting text data from these blobs, or are okay with binary blobs. > > fw_cfg blobs are considered binary, unless a specific selector key or > fw_cfg file name makes different arrangements. (Described in QEMU docs, > or elsewhere.) See more below. > >> That's a long-winded way of stating that omitting the NUL is fine by me, >> as long as you document it, and as long as you are catering to the most >> common user usage of the feature. > > The main consumer of the -fw_cfg switch is guest firmware (and, perhaps > soon, the guest kernel too); the idea is to pass down firmware config > items without QEMU being aware of their actual meaning. Therefore I'd > like to see as little smarts as possible in QEMU wrt. the content passed > down with -fw_cfg. > >> >> Either that, or it's my way of dreaming about alternative 3: guarantee a >> trailing newline (rather than NUL), so that 'content="abc"' on the >> command line results in the 4-byte blob "abc\n" in the guest. >> > > Please don't :) > > The current client code in OVMF (in effect for two specific fw_cfg file > names) recognizes the following content pattern: > > [0nN1yY](\n|\r\n)? > > E.g., QEMU may pass in a simple host-side file as an fw_cfg entry on a > Windows host too. If you edited that file with "notepad.exe", it'll have > \r\n, or maybe no line terminator at all. Other (really binary) blobs > (passed in with file=...) may have embedded \0 characters. > > I think such flexibility is best left to the firmware, or else should be > restricted in specifications living outside of QEMU, and QEMU should be > dumb and transparent here, in accordance with the original goal of this > feature. > > Re: policy vs. mechanism, the opt/ prefix is also strongly recommended > (for the names), but we don't enforce it.
... This made me think of the following language that Gabriel added in v2 (at my request, and to my acceptance): > Both <item_name> and, if applicable, the content <string> are assumed > to consist exclusively of plain ASCII characters. Now I think that this could be improved. I think we should say "should consist" rather than "are assumed to consist", because neither the QEMU nor the firmware(s) "assume" anything in general here -- that would be policy --, we just want to help the user avoid shooting himself in the foot (and reporting a bug), lest he pass non-ASCII UTF-8 on the command line, and the firmware do surprising things. Maybe I should even retract my request for spelling out ASCII... That's really not a requirement, just a high-level recommendation for humans who develop guest code for this interface, to save their sanity. Thanks Laszlo