Re: [Xen-devel] [PATCH v6.5 11/26] x86: Support indirect thunks from assembly code

[David Woodhouse <dwmw2@xxxxxxxxxxxxx>]

On Thu, 2018-01-04 at 00:15 +0000, Andrew Cooper wrote:
> +         * We've got no usable stack so can't use a RETPOLINE thunk, and are
> +         * further than +- 2G from the high mappings so couldn't use 
> JUMP_THUNK
> +         * even if was a non-RETPOLINE thunk.  Futhermore, an LFENCE isn't
> +         * necesserily safe to use at this point.

I count three typos, pedantry about ± and GiB aside.
Late night? :)

> --- a/xen/arch/x86/extable.c
> +++ b/xen/arch/x86/extable.c
> @@ -158,7 +158,7 @@ static int __init stub_selftest(void)
>          memcpy(ptr, tests[i].opc, ARRAY_SIZE(tests[i].opc));
>          unmap_domain_page(ptr);
>  
> -        asm volatile ( "call *%[stb]\n"
> +        asm volatile ( "CALL_THUNK %[stb]\n"

If you make that %V[stb] then...

> +    .if CONFIG_INDIRECT_THUNK == 1
> +
> +        $done = 0
> +        .irp reg, rax, rbx, rcx, rdx, rsi, rdi, rbp, r8, r9, r10, r11, r12, 
> r13, r14, r15
> +        .ifeqs "\arg", "%\reg"
> +            \insn __x86.indirect_thunk.\reg
> +            $done = 1
> +           .exitm
> +        .endif
> +        .endr
> +        .if $done != 1
> +        .error "Bad register arg \arg"
> +        .endif
> +

... you don't need this.

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