Re: [PATCH 2/2] KVM: selftests: Fix ambiguous mov in KVM_ASM_SAFE()

[Sean Christopherson <seanjc@google.com>]

On Fri, Jul 22, 2022, David Matlack wrote:
> Change the mov in KVM_ASM_SAFE() that zeroes @vector to a movb to
> make it unambiguous.
> 
> This fixes a build failure with Clang since, unlike the GNU assembler,
> the LLVM integrated assembler rejects ambiguous X86 instructions that
> don't have suffixes:
> 
>   In file included from x86_64/hyperv_features.c:13:
>   include/x86_64/processor.h:825:9: error: ambiguous instructions require an explicit suffix (could be 'movb', 'movw', 'movl', or 'movq')
>           return kvm_asm_safe("wrmsr", "a"(val & -1u), "d"(val >> 32), "c"(msr));
>                  ^
>   include/x86_64/processor.h:802:15: note: expanded from macro 'kvm_asm_safe'
>           asm volatile(KVM_ASM_SAFE(insn)                 \
>                        ^
>   include/x86_64/processor.h:788:16: note: expanded from macro 'KVM_ASM_SAFE'
>           "1: " insn "\n\t"                                       \
>                         ^
>   <inline asm>:5:2: note: instantiated into assembly here
>           mov $0, 15(%rsp)
>           ^
> 
> It seems like this change could introduce undesirable behavior in the
> future, e.g. if someone used a type larger than a u8 for @vector, since
> KVM_ASM_SAFE() will only zero the bottom byte. I tried changing the type
> of @vector to an int to see what would happen. GCC failed to compile due
> to a size mismatch between `movb` and `%eax`. Clang succeeded in
> compiling, but the generated code looked correct, so perhaps it will not
> be an issue. That being said it seems like there could be a better
> solution to this issue that does not assume @vector is a u8.

Hrm, IIRC my intent was to not care about the size of "vector", but that may just
be revisionist thinking because the:

	"mov  %%r9b, %[vector]\n\t"				\

suggests it's nothing more than a screwed up.  A static assert on the size would
be nice, but I don't know how to make that work since the macros dump directly
into the asm.

> Fixes: 3b23054cd3f5 ("KVM: selftests: Add x86-64 support for exception fixup")
> Signed-off-by: David Matlack <dmatlack@google.com>

Reviewed-by: Sean Christopherson <seanjc@google.com>