Re: [Xen-devel] [PATCH v3 5/5] x86: remove usage of .skip with non-absolute expressions

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 29.01.18 at 13:26, <roger.pau@xxxxxxxxxx> wrote:
> Clang assembler doesn't support using .skip with non-absolute
> expressions:
> 
> entry.S:109:15: error: expected absolute expression
>         .skip .Lcr4_alt_end - .Lcr4_alt, 0x90
>               ^
> 
> This usage of .skip was to fill code sections with NOPs in order for
> them to be patched at run time if required by the alternatives
> framework. Instead of using .skip use the appropriate number of NOPs
> to match the size of the alternative code.

NAK - I've voiced a number of times my opposition to Andrew adding
the various ASM_NOP<N> in his Spectre v2 series, and I'm planning
to eliminate them in due course (by using - you guess it - .skip). It
is pretty much unacceptable to me to have to encode the size of
certain instructions in a second place, risking them to go out of sync
with what they shadow.

If ugliness like this is needed to support clang's integrated assembler,
then I see no way other than avoiding its use.

Jan


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