Re: [Qemu-devel] [RFC PATCH] vl.c: add -semihosting-config "arg" sub-argument

[Liviu Ionescu]

> On 01 Apr 2015, at 16:18, Leon Alrae <address@hidden> wrote:
> 
> ... This simple patch adds "arg" sub-argument which in my opinion is flexible
> enough to satisfy semi-hosting interfaces which allocate buffers at runtime
> (so it is possible to pass any number of strings of any length) as well as the
> ones which have limited buffer for a single input string.

I'm not sure I understand this.

the ARM semihosting interface (see below for the specs) uses a single buffer, 
usually of 80 bytes for embedded applications, that must be entirely allocated 
before the call, and in this buffer a single null terminated string is 
returned, assuming the buffer is large enough, otherwise -1 is returned.

what the semihosting debugger (qemu in our case) must do, is to get this string 
from somewhere, preferably from the command line, and pass it to the debugged 
application.


to me, the natural implementation is to use a single string.

can you explain what are the advantages of handling multiple strings that must 
be later concatenated with spaces separators, to compose the full command line?

in other words, why is it better to define something like 

  "gcm --gtest_output=xml:gcm.xml --gtest_filter=FooTest.* --gtest_repeat=10" 

as 

  -semihosting-config 
target=native,arg="gcm",arg="--gtest_output=xml:gcm.xml",arg="--gtest_filter=FooTest.*",arg="--gtest_repeat=10"

instead of

  -semihosting-config target=native,cmdline="gcm --gtest_output=xml:gcm.xml 
--gtest_filter=FooTest.* --gtest_repeat=10"

(the example if from the Google Test infrastructure).

please note that the command line is generally passed to the test from an upper 
layer, and, in your case, it needs to be parsed, split into substrings, and 
these substrings be surrounded by ,arg="%s".

for my use case, this solution is not convenient at all.


regards,

Liviu



SYS_GET_CMDLINE (0x15)
Returns the command line used to call the executable, that is, argc and argv.

Entry
-----
On entry, R1 points to a two-word data block to be used for returning the 
command string and its length:

word 1
        a pointer to a buffer of at least the size specified in word two

word 2
        the length of the buffer in bytes.

Return
------
On exit:

Register R1 points to a two-word data block:
        
word 1
        a pointer to null-terminated string of the command line
word 2
        the length of the string.

The debug agent might impose limits on the maximum length of the string that 
can be transferred. However, the agent must be able to transfer a command line 
of at least 80 bytes. 

Register R0 contains an error code:
        • 0 if the call is successful
        • -1 if the call is not successful, for example, because of a 
communications error.