Werner Almesberger <wer...@almesberger.net> schrieb:
>Yann Sionneau wrote:
>> I think instead of "going into user mode upon eret" I should just "go
>> back into previous state upon eret".
>
>Yes, you could use the same mechanism as IE.IE / IE.EIE.
>
>You'll also need another state bit, namely an indication whether
>you're in kernel/supervisor mode or not. In kernel mode, things
>like WCSR and maybe RCSR would be allowed, while they'd be
>restricted outside kernel mode.
>
>It seems that the LM32 core doesn't come with such a distinction.
>
>For simplicity, you could just treat WCSR and RCSR as no-ops when
>restricted. Restrictions would mean:
>
>- no writes to IE, IM, EBA, and anything related to the TLB
>- you may also want to deny reads of TLB registers, since they
> could be used for side channel attacks,
>- likewise, reads of CC - if implemented - could be restricted to
> prevent side channel attacks.
>
>Reset, interrupts, and exceptions would
>
>- copy and clear the TLB enable bit(s), e.g.,
>
> X.EDTLB = X.DTLB;
> X.DTLB = 0;
> X.EITLB = X.ITLB;
> X.ITLB = 0;
>
>- copy and clear the user mode bit, e.g.,
>
> X.EUSR = X.USR;
> X.USR = 0;
>
>ERET would then restore the X.E* bits to X.*, without changing
>X.E*. E.g.,
>
> PC = gpr[ea]
> X.DTLB = X.EDTLB;
> X.ITLB = X.EITLB;
> X.USR = X.EUSR;
>
>"X" are one or more suitable registers. If you put everything into
>IE, then the logic outlined above would be reasonably
>backward-compatible with regular LM32.
this makes all sense to me. +1
>Thinking of compatibility, if using IE, you may also want to have
>a bit that controls changing X.DTLB and X.ITLB, so that you don't
>need to remember these bits when disabling and restoring
>interrupts.
>
>E.g.,
>
> WCSR to IE would work like this:
>
> if (gpr[rX].TLBCNG) {
> IE.DTLB = gpr[rX].DTLB;
> IE.ITLB = gpr[rX].ITLB;
> }
> X.IE = gpr[rX].IE;
>
> RCSR would work like this:
>
> gpr[rX] = IE;
> gpr[rX].TLBCNG = 0;
so we can't read back the i/dtlb bits? iirc the Linux kernel already saves the
complete ie register. because saving only one bit wastes just cycles by
bitoperations instructions.
although the above is handy to avoid read modify write, IMHO it should be
possible to read back the tlb settings.
>
> ERET would still work as above, i.e., it would always set DTLB
> and ITLB.
>
>Implementing nested interrupts and/or exceptions would then be up
>to the respective handlers.
>
>- Werner
--
Michael
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