virtio-blk and Userspace debugging using LLDB on the board. Which one do you think is more suitable given the short timeframe? Does the LLDB one involve that much work? Please let me know your thoughts.
@nuta So I started some work on the virtio-blk driver and went through the spec and some existing implementations (1, 2).
I noticed how they're sending three different buffers (one for header, one for data and one for device writeable status byte) and chaining them using the next field. resea's virtio_modern implementation doesn't have that but does mention descriptor chaining in the code. So can you please explain how that would work in resea?
I think I should be able to make it work like those existing ones using the legacy interface but I'm confused regarding how chaining works in the modern implementation.
@PrayagS I think implementing virtio-blk driver (for legacy virtio device) is more realistic.
You may already know, Resea's virtio_modern implementation uses Packed Virtqueue. See 2.7.6 Next Flag: Descriptor Chaining in the spec (the specification describes both legacy and modern devices in the single page and it makes a bit confusing...).
I haven't tried yet but according to the QEMU's implemenetation (in virtqueue_packed_read_next_desc()), we don't use next field in the packed virtqueue. Instead, we need to use continuous descriptors as a chain and set VIRTIO_DESC_F_NEXT to the descriptor flags (except the last one).
The current virtio-net driver does not use that simply because it's not needed. Thus, you may also need to improve also the Resea's virtio_modern to handle the descriptor chaining.
next field and just set the flags accordingly. Please let me know if I wrongly interpreted this. I'll still implement it for the legacy device first and then proceed with the modern implementation.
since we're using packed virtqueues, we don't need to set the next field and just set the flags accordingly
i'm not 100% sure but I think so.
I'll still implement it for the legacy device first and then proceed with the modern implementation.
Ah I got it wrong. It looks that Resea's virtio library abstracts modern/legacy interfaces so you won't have to care the differences.
Relevant section from the paper -
We use the following encoding for each shadow byte:
0 means that all 8 bytes of the corresponding application
memory region are addressable; k (1 ≤ k ≤ 7) means that
the first k bytes are addressible; any negative value indi-
cates that the entire 8-byte word is unaddressable. We
use different negative values to distinguish between dif-
ferent kinds of unaddressable memory (heap redzones,
stack redzones, global redzones, freed memory).
vq->num_descs number of buffers of both the same size and r/w access (either all are device-writeable for reading or all are device-readable for transmission).In the case of
virtio-blk, as I had mentioned before, the buffer containing the header needs to be device-readable. The payload buffer's r/w access depends on the operation. And the status byte needs to be device-writeable.What I did was create three packets for writing and then three for reading. I chained them using the
VIRTIO_DESC_F_NEXT flag. Most of what I did is according to the examples given in the spec (2.7.21.3 and 2.7.22).The missing headers error doesn't come up now but the device isn't responding at the read request for some reason. You can take a look at my code here.
I think it's either the way I'm handling the buffer IDs (the spec doesn't go in much detail but looking at their example, it seemed like it is important) or it is the buffer allocation process since I'm using
virtq_allocate_buffers() to allocate buffers of the same size regardless of their type.Any help would be appreciated. Especially if you can elaborate on the
id part since all the elements of a block request are referred to as elements of a buffer whose ID is stored in the last descriptor of the chain.
hw/virtio/virtio.c).
@nuta I was thinking I can take up one more task in parallel with KASAN, now that my exams are over and I have some time (and curiosity xD). Are we considering a slab allocation scheme in malloc?
A slab allocator sounds good idea :)
Hi @nuta, our 6 weeks are about to get over. We have our project presentation scheduled this Sunday evening. where we have to explain what we did and show our work (Fingers crossed haha :D). But that is just for our University and we'd love to continue working on the features we're implementing. Meantime, we'd be really grateful if you can share what you feel about our work, which we can use as a testimony in the concluding part of our presentation.
It's the first time we worked on an open-source at this scale and you've been a true guide to us in our journey! Thanks a ton :heart:
@PrayagS Good point. Currently, I'm working on the virtio-gpu driver and plan to improve the virtio library in couple of days. In the improvement, I'll try addressing the issue you described. FWIW, you can investigate why your virtio driver doesn't work by inserting some printf functions into QEMU (
hw/virtio/virtio.c).
I've pushed improvements on virtio library to allow constructing a descriptor chain. Can you try it out?
@yashrajkakkad Sure. Can you share your git branch for KASAN?
@yashrajkakkad I tried your code and it failed with general protection fault.
[kernel] WARN: Exception #13
[kernel] RIP = 0000000001005cc5 CS = 000000000000002b RFL = 0000000000000246
[kernel] SS = 0000000000000023 RSP = 000000000300ad70 RBP = 000000000300adb0
[kernel] RAX = 0a110ced0a110ced RBX = 0000000000000004 RCX = 0000000000000001
[kernel] RDX = 0000000004124120 RSI = 00000000000000aa RDI = f8f8f8f8f8f8f910
[kernel] R8 = 0000000000000003 R9 = 0000000000000000 R10 = 000000000300abd0
[kernel] R11 = 0000000000000246 R12 = f8f8f8f8f8f8f910 R13 = 0000000000000010
[kernel] R14 = 0000000000000020 R15 = f8f8f8f8f8f8f8f8 ERR = 0000000000000000
[vm] WARN: test: exception occurred, killing the task...
[kernel] destroying test...
QEMU 4.2.1 monitor - type 'help' for more information
(qemu) xp /i 0x1005cc5
0x01005cc5: 00 00 addb %al, (%rax)I think this is caused by accessing a non-canonical address in RAX by ADD instruction.
I'm not sure but I guess some part of the memory (e.g. return address in the stack) is accidentally overwritten because RIP = 0x1005cc5 seems to be incorrect:
$ objdump -d build/vm.debug.elf|egrep '1005cc[0-9a-z]'
1005930: 0f 85 8d 03 00 00 jne 1005cc3 <vaddr2paddr+0x3e3>
1005cc3: 48 bf c0 19 00 03 00 movabs $0x30019c0,%rdi
1005cca: 00 00 00 <-- instruction at 1005cc5 does not exist
1005ccd: 48 89 de mov %rbx,%rsiSadly, fixing your bug might take a time...
__shadow is too small and shadow_free accidentally destroyed other memory area. Thus it reproduces even if KASAN is disabled. Try the following patch: diff --git a/libs/resea/malloc.c b/libs/resea/malloc.c
index e8040b25..5f78190c 100644
--- a/libs/resea/malloc.c
+++ b/libs/resea/malloc.c
@@ -345,6 +345,14 @@ void shadow_free(struct malloc_chunk *chunk)
reladdr = ptr_cur - (paddr_t)__heap;
reladdr >>= 3;
DBG("%u", reladdr);
+
+ size_t len = chunk->capacity + MALLOC_REDZONE_LEN;
+ if ((vaddr_t) &__shadow[reladdr] + len >= (vaddr_t) __shadow_end) {
+ WARN_DBG("__shadow is too small: addr=%p, len=%d, reladdr=%p",
+ &__shadow[reladdr], len, reladdr);
+ return;
+ }
+
for(size_t i = 0; i < (chunk->capacity + MALLOC_REDZONE_LEN); i++)
{
__shadow[reladdr++] = SHADOW_FREED;Hi, @Manav-Kumar. We have already implemented basic features like the microkernel, FAT file system, TCP/IP, and some device drivers. We're working on some improvements and new features:
- A GUI server based on Wayland protocol (still in the very early stage)
- Rust support improvements: nuta/resea#49
Since I'm working on another OS project in Rust in very recent weeks, the progress is slow. That said, if you'd like to contribute to Resea, I can support you :)
Aside from those projects, we welcome your ideas and would like you to work on a project whatever you want. For example, integrating Linux Kernel Library would be an interesting topic.
Does resea supports multitasking features in current version. Can we plan to improve it in any way if it does.
Yes it supports. In a nutshell, Resea kernel implements a multi-tasking concept called task (so-called process). Each task has its own virtual memory space and can communicate with other tasks over message passing IPC. Threads are not supported intentionally because all applications in Resea are implemented in a single-threaded, event-driven way to make it easier to debug (we can still implement sort of threads in the userland by mapping same memory pages through vm_map syscalls btw).
Memory Management, vm, paging, interruptions, scheduling.. this might already be implemented in basic kernel features, are there any scopes of improvement.
We only have rudimental implementation so I think there's room for further improvements. Among the mentioned areas, user-level scheduling (nuta/resea#16) might be interesting to work on. A MINIX wiki page is informative on this topic.
task_schedule syscall) but the feature is unused at all.
- copy-on-write: Not implemented. Mapping zero-filled pages (e.g
.bsssection) using CoW might improve the startup time of a userland application. - zero filled on demand: I don't know what this feature does. BTW, preparing zero-filled pages in advance during an idle state would be good for performance.
- demand paging: Implemented. In fact, every page is mapped by the pager task on the first access.