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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Jan 29, 2018 at 09:50:56AM -0700, Jan Beulich wrote:
> >>> 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.

Right, I agree this is ugly and prone to error. This seems to be fixed
in clang thunk, so I've asked for a backport of the fix to 6.0.

Roger.

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