On Tue, Aug 15, 2023 at 9:20 AM Chao Li <lic...@loongson.cn> wrote:
>
> Hi Andrew,
>
> Yes, you are right, I also think that SMC is a bit flawed in terms of
> security, but can we use some security mechanism to protect the SMC, like
> encryption and decryption? Sorry, I'm not consider mature enough about SMC
> security.
There isn't any. Actual use cases in something like a kernel are
heavily vetted and read-protected as soon as possible.
>
> I can tell you real problem, there are some CSR instructions in LoongArch64
> that can only accept immediate value, for example: `csrrd $a0, 0x1`, the 0x1
> is the selection of CSR register number, it can't use the registers to
> select. This operation should be in the MdePkg base library.
>
> I know that .c or .h files in MdePkg shouldn't depend on a single compiler
> feature, so I can't use the GNU AT&T style inline ASM function(AT&T style
> inline supports input parameters being immedite value, use "i" option). In
> this case, I think using SMC can handle this, that is use register transfer
> the CSR registers selection, and dynamically modify CSR instructions during
> execution phase with reference to transfer register value, this way is depend
> on the .text section or target memory is executable and writable.
FYI, poking instructions willy-nilly is unsafe and unreliable (except
on x86 due to kludges, but then it's slow).
>
> The problem of immediate values can only be handled by preprocessing stage or
> using SMC, otherwise I can only write a lot of similar functions and use
> `switch case` to call them. This method will cause the program size to expand
> a lot.
>
> So, I think I have following choice:
>
> Choice 1:
>
> Use AT&T style inline function, and create a file named: CsrOperationGcc.c,
> and other future compiler feature-dependent files will be named:
> CsrOperationClang.c, CsrOperationXlang.c and so on.
If you're going to use inline assembly, just expose them directly? I
don't see the problem there, I don't expect loongarch to be picked up
by visual studio any time soon.
>
>
> Choice 2:
>
> Use SMC.
>
>
> Choice 3:
>
> Write a lot of similar CSR functions.
You /could/ use a GAS macro.
.macro csr_write csr
.global CsrWrite\csr
CsrWrite\csr:
csrw a0, \csr
ret
(this is riscv pseudo-asm but I know your arch is similar enough)
--
Pedro
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