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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 29/01/18 16:50, 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.

And FTR, the Spectre v2 series explicit sizes were only because I tried
fixing Xen's ability to do sensible things with nops in alternative, but
got far too bogged down and decided that the Spectre work was more
important.

I will be dusting the series off in due course and removing the explicit
sizes.

~Andrew

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