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multitasking and ring0/ring3
Posted: Fri Nov 16, 2007 11:33 am
by sigler
Hello,
I've got my kernel with software multitasking, up to now all the tasks are running in ring0. So I tried to setup a task to run in ring3. I made two new entries in the gdt, one for code and one for data in ring3.
my gdt now looks like this
code ring0 // 8
data ring0 // 16
code ring3 // 24
data ring3 // 32
tss
Then I setup the task like this (the only difference I've done from ring0 tasks is change the cs and ds)
Code: Select all
// ring0
// int cs = 8;
// int ds = 16;
// ring3
int cs = 24;
int ds = 32;
// iret pops these
*(--esp) = EFLAGS | EFLAGS_IF; // eflags
*(--esp) = cs; // cs
*(--esp) = (dword)entrypoint; // eip
// the timer interrupt pops/pushes these
*(--esp) = 0; // ebp
*(--esp) = 0; // esp
*(--esp) = 0; // edi
*(--esp) = 0; // esi
*(--esp) = 0; // edx
*(--esp) = 0; // ecx
*(--esp) = 0; // ebx
*(--esp) = 0; // eax
*(--esp) = ds; // ds
*(--esp) = ds; // es
*(--esp) = ds; // fs
*(--esp) = ds; // gs
If I correctly understand what's happening, I get a general protection fault when this task is selected in the timer interrupt and iret is performed.
(again the only change I've made to what's working, is change the cs/ds in the tasks stack)
What am I missing?
Must I introduce ldt ? (I haven't yet understood what ldt is for)
thanks,
--
Sigurd Lerstad
Posted: Fri Nov 16, 2007 11:52 am
by XCHG
I remember I had a lot of difficulty when I was implementing software context switching in my kernel. Some of the most important ones were (Perhaps you can use them as a simple checklist):
1) I wasn't using the TSS correctly (with the SS:ESP of the kernel in TSS)
2) I was not setting EFLAGS correctly for the new task (Interrupt Flag and etc were not correctly set).
3) I was not setting the GS/DS/FS/ES of the kernel upon each Timer Interrupt. So when I was addressing a memory location with DS, I could be using an arbitrary memory address.
These were just a few. I think it would help if you gave us Bochs' debug output.
Re: multitasking and ring0/ring3
Posted: Fri Nov 16, 2007 2:04 pm
by egos
sigler wrote:What am I missing?
In user mode (ring3) the values of segment registers must include in self RPL=3, e.g. cs=24+3=27.
Must I introduce ldt?
No, but it is possible.
Posted: Fri Nov 16, 2007 3:04 pm
by AJ
Hi,
Something else to check:
If you are using paging, ensure that all code and data for your ring 3 task has the u/s bit of the PDE and PTE *set*. If not, you will get a GPF. This includes the stack, all variables, bss and all code.
Cheers,
Adam
Posted: Fri Nov 16, 2007 4:23 pm
by Combuster
since you IRET to a lower privilege, you must add an additional SS and ESP to the stack frame to give it all the data it needs
Posted: Sat Nov 17, 2007 2:41 pm
by sigler
Combuster wrote:since you IRET to a lower privilege, you must add an additional SS and ESP to the stack frame to give it all the data it needs
Okay, I now setup task like this:
Code: Select all
unsigned int cs = 24+3;
unsigned int ds = 32+3;
// iret pops these
*(--esp) = ds; // ss
*(--esp) = ustacktop; // esp
*(--esp) = EFLAGS | EFLAGS_IF; // eflags
*(--esp) = cs; // cs
*(--esp) = (dword)entrypoint; // eip
The bochs error message is:
00003694910p[CPU0 ] >>PANIC<< get_SS_ESP_from_TSS: TR.cache invalid
So is there something wrong in my tss? not setup correctly etc.
I have added a tss to the gdt. Do I have to do more to make it visible? (I think there's a task register in hardware multitasking, but in software multitasking ?)
Before calling the iret that crashes, I set the global tss.esp0 to the task's esp, and tss.ss0 to 32+3 (the task's stack segment), all other fields in tss are zero.
I'm not setting the NT flag in EFLAGS ever, should I ?
Anything else?
thanks,
--
Sigurd Lerstad
Posted: Sat Nov 17, 2007 6:00 pm
by pcmattman
When you do a privilege level switch from a lesser privilege to a higher one, the CPU reads SS0:ESP0 from the TSS and sets that as the ring0 ss:esp pair. You'll need to make sure that the TSS is loaded into the TR (even for software multitasking) and that a valid SS0:ESP0 pair is stored in it.
Posted: Sun Nov 18, 2007 8:22 am
by sigler
Thanks everyone,
It works now.
One more question:
When an interrupt occurs, to know the ring that the current task was running in, is the correct way to store a ring field in the task structure?
*edit*
I just thought of:
I could look at the lower two bits of cs segment that is on the stack?
--
Sigurd Lerstad
Posted: Mon Nov 19, 2007 3:10 am
by AJ
Yes - that's how to do it
I originally had a flags field in my task structure and stored it there - the only problem with this was that if you preempted a task when it had called a system service, you are in ring 0 even though it is a ring 3 task. As you say - look at the lower bits of CS.
Cheers,
Adam