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<damo22>preserving a register during interrupts/traps is hairy

<damo22>youpi: if i need a value saved for determining where to return to at the end of an interrupt or trap, can i just put it on the stack after pusha and PUSH_SEGMENTS? or will eg R_CS(%rsp) be then at the wrong place?

<damo22>where can i put such a value

<damo22>s/where to return to/whether swapgs is needed or not

<damo22>can i store it in a segment register? :P

<youpi>you can as well just look at the pushed cs value

<youpi>ah, no, because gs might have been switched or not

<damo22>yeah i need to save it in the SWAPGS_ENTRY thing

<damo22>that is where it knows

<youpi>adding something on the stack is possible, yes, it just needs an update of the offsets

<damo22>where are these offsets

<damo22>why is there no r11:

<damo22>offset i386_saved_state r r10

<damo22>offset i386_saved_state r r12

<youpi>just nobody needed it

<youpi>you can just add it if you need

<damo22>does that mean r11 is spare throughout

<youpi>no

<damo22>but we are not saving it?

<youpi>we have to, otherwise it'd be a bug

<youpi>the pusha macro does it

<damo22>ah its just nobody referred to it in the code

<youpi>yes

<damo22>why cant i use something like r15 that isnt saved or restored in the ISR?

<damo22>and just do an extra save and restore for that?

<youpi>I'm not sure what you call ISR

<damo22>PUSH_REGS_ISR

<damo22>its seems only a subset are needed for the interrupt/traps?

<youpi>there is no free lunch :)

<youpi>all_intrs knows it doesn't have to save r15 because it knows that its callees will not change it

<youpi>but otherwise it has to keep it unchanged

<youpi>otherwise it'd thrash state

<damo22>ok

<damo22>what is %es for?

<youpi>similar to %ds

<youpi>just another segment register (not the default)

<youpi>only for USER32

<damo22>ok

<damo22>i need to rewrite the stack

<youpi>? really ?

<youpi>you can just add what you want in i386_interrupt_state

<youpi>there is no constraint there

<youpi>it just needs to match what you push/pop on interrupt

<damo22>ohh that makes more sense

<damo22>i was looking for that struct

<damo22>if i add r12 that isnt already there, does that mean i can use r12 throughout the interrupt context as long as i push and pop it ?

<damo22>if i set r12 near the beginning, will it then survive until the end even with all the stack switching etc and traps or do i need to push it to the stack again with a new value

<youpi>yes

<youpi>it won't be changed by the callees since it's callee-saved

<youpi>so you don't need to push it again

<damo22>that will save me a lot of headache

<youpi>you can just save the value from the interrupted context, and put what you want it in

<damo22>yeah

<youpi>it'll stay there, just like previously the value of the interrupted context was staying there

IRC channel logs

2026-01-16.log