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Re: [Xen-devel] [PATCH] x86/build: Use new .nops directive when available
- To: Jan Beulich <JBeulich@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Tue, 28 Aug 2018 18:58:15 +0100
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- Cc: Xen-devel <xen-devel@xxxxxxxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>
- Delivery-date: Tue, 28 Aug 2018 17:58:30 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 17/08/18 13:45, Jan Beulich wrote:
>>>> On 15.08.18 at 19:57, <andrew.cooper3@xxxxxxxxxx> wrote:
>> --- a/xen/arch/x86/Rules.mk
>> +++ b/xen/arch/x86/Rules.mk
>> @@ -29,6 +29,10 @@ $(call as-option-add,CFLAGS,CC,"invpcid
>> (%rax)$$(comma)%rax",-DHAVE_AS_INVPCID)
>> $(call as-option-add,CFLAGS,CC,\
>> ".if ((1 > 0) < 0); .error \"\";.endif",,-DHAVE_AS_NEGATIVE_TRUE)
>>
>> +# Check to see whether the assmbler supports the .nop directive.
>> +$(call as-option-add,CFLAGS,CC,\
>> + ".L1: .L2: .nops (.L2 - .L1)$$(comma)9",-DHAVE_AS_NOP_DIRECTIVE)
> Can this please be HAVE_AS_NOPS_DIRECTIVE?
Ok.
>
>> --- a/xen/arch/x86/alternative.c
>> +++ b/xen/arch/x86/alternative.c
>> @@ -84,6 +84,19 @@ static const unsigned char * const p6_nops[ASM_NOP_MAX+1]
>> init_or_livepatch_cons
>>
>> static const unsigned char * const *ideal_nops init_or_livepatch_data =
>> p6_nops;
>>
>> +#ifdef HAVE_AS_NOP_DIRECTIVE
>> +
>> +/* Nops in .init.rodata to compare against the runtime ideal nops. */
>> +asm ( ".pushsection .init.rodata, \"a\", @progbits\n\t"
>> + "toolchain_nops: .nops " __stringify(ASM_NOP_MAX) "\n\t"
>> + ".popsection\n\t");
> Any particular reason not to put them in .init.text?
Because its data, not executable code.
>
>> @@ -27,6 +26,14 @@ extern void add_nops(void *insns, unsigned int len);
>> extern void apply_alternatives(struct alt_instr *start, struct alt_instr
>> *end);
>> extern void alternative_instructions(void);
>>
>> +asm ( ".macro mknops nr_bytes\n\t"
>> +#ifdef HAVE_AS_NOP_DIRECTIVE
>> + ".nops \\nr_bytes, " __stringify(ASM_NOP_MAX) "\n\t"
>> +#else
>> + ".skip \\nr_bytes, 0x90\n\t"
>> +#endif
>> + ".endm\n\t" );
> Did you take a look at "x86/alternatives: allow using assembler macros
> in favor of C ones" yet? Perhaps this redundant macro definition could
> be avoided that way? I'm not going to exclude though that some more
> work might be needed to get there.
No - not looked at that yet. That does look like we can get a large net
cleanup, and can avoid using always_inline to work around the same problem.
>
> Anyway, I'm in principle fine with the change, irrespective of the other
> discussion we've had, so with the name change and possibly the other
> two remarks taken care of
> Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
Thanks,
~Andrew
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