[PATCH] KVM: arm64: nv: Work around lack of pauth support in old toolchains

[Arnd Bergmann]

On Tue, Apr 23, 2024, at 14:06, Marc Zyngier wrote:
> On Tue, 23 Apr 2024 13:00:55 +0100,
> "Aiqun Yu (Maria)" <quic_aiquny at quicinc.com> wrote:
>> On 4/23/2024 4:24 PM, Arnd Bergmann wrote:
>> > On Tue, Apr 23, 2024, at 00:48, Marc Zyngier wrote:
>> >> We still support GCC 8.x, and it appears that this toolchain
>> >> does not understand "pauth" as a valid architectural extension.
>> >> After all, it's only been 8 years since ARMv8.3 was released...
>> > 
>> > Just to clarify: I'm fairly sure that all supported toolchains
>> > support ARMv8.3 and PACGA, the problem with ".arch_extension pauth\n"
>> > seems to be that it was retroactively made an optional
>> > feature for earlier architecture versions a few years after
>> > ARMv8.3, so most binutils versions we support understand
>> > pacga as an armv8.3 feature but reject the pauth name for the
>> > extension.
>> Kind of agree with Arnd here.
>> Shall the fix just remove the ".arch_extension pauth"?
>> 
>> I've tried gcc 7 failed with the pauth name for the extension.
>> After I remove the ".arch_extension pauth" and use "pacga" instruction
>> directly pass the gcc 7 compilation.

It really depends on the binutils version, not gcc of course.

> And breaks with LLVM:
>
>   CC      arch/arm64/kvm/pauth.o
> arch/arm64/kvm/pauth.c:40:9: error: instruction requires: pauth
>                      "pacga %0, %1, %2" : "=r" (pac) : "r" (ptr), "r" (mod));
>                       ^
> <inline asm>:2:1: note: instantiated into assembly here
> pacga x19, x1, x9
> ^

It works when building with LLVM_IAS=0, which we obviously don't
want to mandate here. The variant below works for both clang+ias
(including all still supported versions) and gcc+binutils, but at
that point it gets obscure enough that your .inst version is easier
to understand.

    Arnd

--- a/arch/arm64/kvm/pauth.c
+++ b/arch/arm64/kvm/pauth.c
@@ -36,7 +36,12 @@ static u64 compute_pac(struct kvm_vcpu *vcpu, u64 ptr,
        __ptrauth_key_install_nosync(APGA, ikey);
        isb();
 
-       asm volatile(ARM64_ASM_PREAMBLE ".arch_extension pauth\n"
+       asm volatile(ARM64_ASM_PREAMBLE
+#ifdef CONFIG_AS_IS_LLVM
+                    ".arch_extension pauth\n"
+#else
+                    ".arch armv8.3-a\n"
+#endif
                     "pacga %0, %1, %2" : "=r" (pac) : "r" (ptr), "r" (mod));
        isb();