Hi Alexandre, > I'm trying to understand how the TLS is setup for the root server on > x64 (to access for example the IPC buffer) > but I'm failing to connect the pieces and how the server is correctly > setting the FS register.
There is detailed documentation for the TLS change on the page for RFC- 3. https://sel4.atlassian.net/browse/RFC-3 In essence, the new TLS model stores the address of the IPC buffer in a thread-local variable which must be initialised by your process or threading runtime along with general TLS intialisation. https://github.com/seL4/seL4/blob/master/libsel4/include/sel4/functions.h#L28 To assist with this there is a simple sel4runtime library which provides a minimal C-runtime and tools for initialising TLS. https://github.com/SEL4PROJ/sel4runtime On x86_64 the TLS register can be set without calling into the kernel by using the `wrfsbase` instruction or by using the `sel4runtime_set_tls_base` function from the sel4runtime library. > Now, I'm failing to find in the codebase where the ELF loader is > actually detecting the TLS segment and the according setup with the > TLS register. The ELF loader is not responsible for TLS initialisation, the C-runtime is responsible for configuring TLS of the initial thread. https://bitbucket.ts.data61.csiro.au/projects/SEL4PROJ/repos/sel4runtime/browse/src/env.c#325 > Never mind, I discovered that I in order to access a TLS variable you > need to call `seL4_TCB_SetTLSBase` on seL4_CapInitThreadTCB first in > the root server, as I'm trying to work on a bare bone usage of seL4 to > better understand it, this part was not obvious from the existing more > complex samples! In general I would recommend having the initial thread in a process initialise its own TLS via the runtime and any new thread configure its own TLS in the initialisation code before the main function call of that thread with the `sel4runtime_set_tls_base`. Each new thread will also need to use `seL4_SetIPCBuffer` to initialise the IPC buffer address in its TLS before performing any invocations or any other system calls. Cheers, Curtis Millar Systems Engineer, Trustworthy Systems, CSIRO's Data61 curtis.mil...@data61.csiro.au Data61, CSIRO Level 3, K17 Building UNSW Gate 14, Barker St Kensington NSW 2033 Australia
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