On 09/28/15 23:45, Gabriel L. Somlo wrote: > On Mon, Sep 28, 2015 at 11:05:32PM +0200, Laszlo Ersek wrote: >> 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. > > Maybe something like this, then: > > Both <item_name> and, if applicable, the content <string> will be > passed through by QEMU without any interpretation, expansion, or > further processing. Any such processing (potentially performed by > e.g., the shell) is outside QEMU's responsibility; as such, using > plain ASCII characters is recommended. > > Let me know what you think.
Sounds good to me. Thanks for your patience. :) Laszlo