W dniu 3.06.2024 o 18:47, Leif Lindholm via groups.io pisze:
In 2020 we got version C of spec (and then D, E, F) which allows to
use more registers:
> Allow R4—R7 (SMC32/HVC32) to be used as result registers.
> Allow X8—X17 to be used as parameter registers in SMC64/HVC64.
> Allow X4—X17 to be used as result registers in SMC64/HVC64.
And I started to wonder how to update EDK2 to newer version of SMCCC
spec as one of in-progress QemuSbsa SMC calls may return more than 4
values.
Yes, definitely. This has been a wishlist item for some time, but in
reality we've only ever updated these when we needed new functionality.
ARM_SMC_ARGS in ArmSmcLib.h can be expanded to handle up to Arg17 in
an easy way and guarded by "#if defined(__aarch64__)" to not change it
on Arm32.
My example code for it (including comment update):
diff --git a/ArmPkg/Include/Library/ArmSmcLib.h
b/ArmPkg/Include/Library/ArmSmcLib.h
index beef0175c35c..af27781b72f6 100644
--- a/ArmPkg/Include/Library/ArmSmcLib.h
+++ b/ArmPkg/Include/Library/ArmSmcLib.h
@@ -23,14 +23,31 @@ typedef struct {
UINTN Arg5;
UINTN Arg6;
UINTN Arg7;
+#if defined(__aarch64__)
+ UINTN Arg8;
+ UINTN Arg9;
+ UINTN Arg10;
+ UINTN Arg11;
+ UINTN Arg12;
+ UINTN Arg13;
+ UINTN Arg14;
+ UINTN Arg15;
+ UINTN Arg16;
+ UINTN Arg17;
+#endif
} ARM_SMC_ARGS;
/**
Trigger an SMC call
- SMC calls can take up to 7 arguments and return up to 4 return values.
- Therefore, the 4 first fields in the ARM_SMC_ARGS structure are used
- for both input and output values.
+ on SMC32/HVC32
+ - R0 is a function identifier, R1-R7 are arguments
+ - R0-R7 are results, R4-R7 must be preserved unless they contain results
+
+
+ on SMC64/HVC64
+ - W0 is a function identifier, X1-X17 are arguments
+ - X0-X17 are results, X4-X17 must be preserved unless they contain results
**/
VOID
Then ArmCallSmc() in {AArch64,Arm}/ArmSmc.S needs changes. But here it
gets tricky.
On Arm we preserve r4-r8 and restore them after call like spec says.
Which we do not do on AArch64 as version B of spec did not required
that (and this changed in version C).
If we start handling more than 4 results then we need to know how many
results are expected and restore rest of r4-r7/x4-x17 registers:
Yeah, it feels like we may want to add a version field or/and number of
registers to save/restore to a new struct. And we should definitely
align Arm/AArch64 behaviour.
From what I saw in both edk2/ and edk2-platforms/ most of code uses
ArmCallSmc() function with ARM_SMC_ARGS structure populared with
arguments. ArmCallSmc[0-3]() are used by Smbios, Psci and QemuSbsa
code only.
The ArmCallSmc[0-3] helpers were only added as shorthand for most common
cases. I don't think we should worry about how those work for making any
design decisions. Everything they do can be described by the main
ArmCallSmc functions and a few lines of struct stuffing.
Now the question is: how to handle change?
It could be that it would be easiest to add a new call function, and
maybe even ra new egister struct, than trying to adapt the existing ones
in ways that doesn't break existing users?
That is if we care about existing users. We could always modify it to
guarantee breakage and expect users to update their code. Since this is
a library, and not a protocol, we *mostly* don't need to worry about
users mixing prebuilt binaries using old structs with new functions.
We could add ArmCallSmc[4-17] but that name only tells how many
arguments we pass to SMC call, not how many results we expect. Or
should we add NumberOfResults argument to ArmCallSmc() to know which
registers we should preserve and which are results? And how
complicated this assembly function will become?
I don't think the assembly function needs to be that complicated. It'd
be a bit tedious with a bunch of tests, but...
Note: I do not know aarch64 assembly. Did some changes to ArmCallSmc().
I moved to use x18 register (preserved on stack) instead of x9 one. And
changed code to handle 8 results (without preserving x4-x7).
diff --git a/ArmPkg/Library/ArmSmcLib/AArch64/ArmSmc.S
b/ArmPkg/Library/ArmSmcLib/AArch64/ArmSmc.S
index 4a8c2a8f59ea..0525a0a7887f 100644
--- a/ArmPkg/Library/ArmSmcLib/AArch64/ArmSmc.S
+++ b/ArmPkg/Library/ArmSmcLib/AArch64/ArmSmc.S
@@ -8,8 +8,10 @@
#include <AsmMacroIoLibV8.h>
ASM_FUNC(ArmCallSmc)
+ // preserve x18
+ str x18, [sp, #-16]!
// Push x0 on the stack - The stack must always be quad-word aligned
- str x0, [sp, #-16]!
+ str x0, [sp, #-32]!
// Load the SMC arguments values into the appropriate registers
ldp x6, x7, [x0, #48]
@@ -19,14 +21,18 @@ ASM_FUNC(ArmCallSmc)
smc #0
- // Pop the ARM_SMC_ARGS structure address from the stack into x9
- ldr x9, [sp], #16
+ // Pop the ARM_SMC_ARGS structure address from the stack into x18
+ ldr x18, [sp], #32
// Store the SMC returned values into the ARM_SMC_ARGS structure.
- // A SMC call can return up to 4 values - we do not need to store back x4-x7.
- stp x2, x3, [x9, #16]
- stp x0, x1, [x9, #0]
+ // A SMC call can return up to 18 values, let handle 8 for now
+ stp x6, x7, [x18, #48]
+ stp x4, x5, [x18, #32]
+ stp x2, x3, [x18, #16]
+ stp x0, x1, [x18, #0]
- mov x0, x9
+ mov x0, x18
+ // restore x18
+ ldr x18, [sp], #16
ret
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