September 2016 - Devel - lists.sel4.systems

Questions about Memory Management in seL4
by Daniel Wang 19 May '23

19 May '23

Hi all, Sorry to bother, I have some basic questions about seL4 memory management. Can someone help me? 1. where is the kernel’s address space? I know there are basically three methods: a) separate virtual address space, e.g. through changing page tables on entry into privileged mode to access b) physical space, e.g. through disable automatic hardware translation of virtual addresses on entry into privilege mode c) privilege region in each process’s virtual address space, e.g. through page table to map kernel address into use-mode process. My question is which of this is being used by seL4 microkernel? 2. Objects like CSpace, VSpace, CNode, TCB, Endpoint, etc. are called kernel objects, but it seems all dynamically allocated in thread running/creating time. Are those objects (e.g. Figure 3.3 in seL4 manual) in kernel space or in user space? 3. I read that after the kernel boot up, it passes all resources, untyped memory to the first thread. Are those untyped memory physical memory or virtual memory in a single page? The reason I ask is I’m learning the code of the UNSW AOS project (two of the assignment projects are implementing frame and papers). I’m confused that before the implementation of frames and pages what kind of memory is passed to the the first thread? I saw int AOS project, it uses ut_table_init(), ut_steal_mem(), I wonder what kind of memory it operates on? Thanks -Dan

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Post-mutikernel approach to multiprocessor support?
by Jeff Waugh 04 Oct '16

04 Oct '16

Hi all, Is there anything that can be shared so far about seL4's post-multikernel approach to multiprocessor support? (I couldn't see anything new or upcoming on the publications pages, and figured a paper or two might land before code aimed for Q4'16.) Thanks, Jeff

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Why send-only IPC doesn't and shouldn't return a success indicator
by Gernot.Heiser＠data61.csiro.au 29 Sep '16

29 Sep '16

We have just received this comment on github, which I think merits some explanation: > There are times when IPC needs to happen, but a sending process cannot wait around for another process to be receiving. seL4_NBSend is basically the system call for this, but it never provides any feedback whether or not IPC took place. Summary: I accept that this is a valid use case, but the suggested solution is not the right one. There are two reasons why I’m saying this: 1) the current behaviour of send-only IPC is correct and cannot be changed, 2) send-only IPC is the wrong way to address this use case. Re (1): Anyone who tried to use seL4_Send() or seL4_NBSend() will likely have noticed that they are of very little use, because you don’t know whether the message was transferred. So, why can't we return at least a boolean value, indicating whether a message has been delivered or it was dropped on the floor? It’s simple: this would violate the dataflow guarantees provided by seL4. Send-only IPC is there to enable one-way information flow, and consequently can be invoked using a send-only cap. But a send-only cap must not allow any information flow from the receiver to the sender (or else all our isolation proofs would break down). Basically, a return status that can be influenced by the receiver constitutes a back channel that is not authorised by a cap, and as such is verboten. This means that send-only IPC is of very little practical use, except where you really want one-way information transfer, or you have a higher-level protocol that lets you get away with not knowing whether the message was lost. (Eg if the receiver being not ready is a protocol violation by the receiver, and it has only itself to blame for losing the message.) So, could we make send-only IPC a bit more useful by changing the semantics slightly? For example, we could have a return value that indicates successful message delivery if the send was invoked with a send+receive cap, but will always return false if the invocation used a send-only cap? In principle that would be doable without violating security requirements. The cost would be extra checking. It would also be somewhat unsystematic if the semantics of a send-only operation would depend on whether your cap has receive rights as well. It’d be arguably misleading, as you’re doing more than sending. This gets me to (2): send-only IPC is the wrong way to solve the problem. The above use case is clearly one where info flow is inherently two-way: you send something and need an acknowledgment of receipt. This means call-type IPC is the right mechanism, it does exactly that. And on top of that it is more efficient than separate send-receiver, and avoids (thanks to reply caps) the need to set up a separate reply channel. The problem, of course, is that seL4_Call() is always blocking, there is no polling variant of this syscall. Should there be an seL4_NBCall()? Arguably yes, just as the RT branch already provides a seL4 SignalRecv() syscall, which is the non-blocking equivalent of seL4_ReplyRecv(). Will it happen? Possibly, but not in mainline the near future. The reason is that to add this (fairly simple) feature to the mainline kernel will require verifying it, and our verification engineers are presently very busy delivering on contracted milestones. We’ll discuss this internally. In the meantime I’ve added some explanation of the semantics of seL4_Send()/seL4NBSend() to the FAQ on the seL4 wiki. Gernot

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quesion about TK1 Linux irq
by PX 27 Sep '16

27 Sep '16

Hi, I have questions about the irqs defined for TK1 guest linux. 1. The linux_pt_irqs[] defined in tk1_vmlinux.h lists all the IRQs for I/O devices, but why they are different from TK1 manual? Does seL4 perform some mapping for guest Linux? 2. what 's the Physical address for Ethernet card on TK1 board? Thanks Peng

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Fwd: [Fwd: quesion about TK1 Linux irq]
by Jason.Butler＠data61.csiro.au 27 Sep '16

27 Sep '16

The irq numbers are remapped in: kernel/include/plat/tk1/plat/machine.h Jason ? -------- Forwarded Message -------- From: PX <seawolf.peng(a)gmail.com> To: devel(a)sel4.systems Subject: [seL4] quesion about TK1 Linux irq Date: Mon, 26 Sep 2016 17:38:47 -0400 Hi, I have questions about the irqs defined for TK1 guest linux. 1. The linux_pt_irqs[] defined in tk1_vmlinux.h lists all the IRQs for I/O devices, but why they are different from TK1 manual? Does seL4 perform some mapping for guest Linux? 2. what 's the Physical address for Ethernet card on TK1 board? Thanks Peng _______________________________________________ Devel mailing list Devel(a)sel4.systems https://sel4.systems/lists/listinfo/devel

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Why send-only IPC doesn't and shouldn't return a success indicator
by Matt Rice 26 Sep '16

26 Sep '16

Gernot.Heiser wrote: > We have just received this comment on github, which I think merits some explanation: >> There are times when IPC needs to happen, but a sending process cannot wait around for another process to be receiving. seL4_NBSend is basically the system call for this, but it never provides any feedback whether or not IPC took place. ... >This gets me to (2): send-only IPC is the wrong way to solve the problem. > >The above use case is clearly one where info flow is inherently two-way: you send >something and need an acknowledgment of > receipt. This means call-type IPC is the right mechanism, it does exactly that. And on top of that it is more efficient than > separate send-receiver, and avoids (thanks to reply caps) the need to set up a separate reply channel. New to sel4 and the list (so sorry for stuffing up the threading) Couldn't at the expense of an extra notification send, use a notification where the receiver of the notification then should call the then do a non-blocking reply. I didn't see reply being non-blocking described in the manual so please correct me if i am wrong. This only works one party is willing to block correct, and there is no way to short circuit the call if an incoming message could be delivered to the call'ing process before the call can be completed? Although it would seem to take some amount of gymnastics to balance sending and receiving... so if really neither are willing to wait there is always notifications+shared memory?

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Questions about Capability model
by Daniel Wang 21 Sep '16

21 Sep '16

Hi all, I’m new to the capability concept of seL4. Please correct me if I’m wrong. My understand of the capability is that it is a reference to a chunk of memory that allocated for specific purpose. Say if I want to do IPC between two threads A and B, I will need to apply an chunk of memory as endpoint, which give me a capability that I can pass from thread A to thread B is that correct? Also it is ok to assume the message passing mechanism in IPC is a producer-consumer model? The reason I ask it because I’m thinking is it possible for three (or more) threads that need to communicate with each other using the same endpoint, or it has to be paired for each two of those threads to use an endpoint? Thanks -Dan

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developing a Ethernet card server for seL4
by PX 20 Sep '16

20 Sep '16

Hi, all, We want to develop a networking service on top of seL4, which works as a hypervisor, on Jetson TK1. We know that there is source code for lwip under /projects/util_libs/libethdrivers, but there is no support for TK1. We want to know how much effort needed to port the code to TK1 platform? Is it straightforward? Is there any TK1 ethernet driver source code available? Peng

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How to run seL4 as a hypervisor?
by Daniel Wang 19 Sep '16

19 Sep '16

Hi all, Is there any document/tutorial about using seL4 as a hypervisor to running Linux on top? Appreciate for your help. Thanks -Dan

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General Questions
by sixpackgurkenwasser＠use.startmail.com 15 Sep '16

15 Sep '16

Hello, I've got several questions. 1. Is there a simple way to relate a virtual address in the initial threads address space to the corresponding frame cap in the BootInfo? 2. In 8.2.2 of the manual it is stated in that "seL4 provides no way to obtain the physical address of a page from its capability". Isn't that exactly what the GetAddress method of a Page object does(10.4.14)? Cheers, Steve

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