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


  • To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxx>
  • From: David Woodhouse <dwmw2@xxxxxxxxxxxxx>
  • Date: Thu, 11 Jan 2018 13:03:15 +0000
  • Delivery-date: Thu, 11 Jan 2018 13:03:40 +0000
  • Face: 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
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

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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