Re: [PATCH v3 4/4] x86/syscall: use int everywhere for system call numbers

["H. Peter Anvin" <hpa@zytor.com>]

Answer: I don't think it is a good idea to have the system can table offset ... it seems like an unnecessary debugging headache.

On May 15, 2021 8:37:12 AM PDT, Andy Lutomirski <luto@kernel.org> wrote:
>On 5/14/21 6:10 PM, H. Peter Anvin wrote:
>> From: "H. Peter Anvin (Intel)" <hpa@zytor.com>
>> 
>> System call numbers are defined as int, so use int everywhere for
>> system call numbers. This patch is strictly a cleanup; it should not
>> change anything user visible; all ABI changes have been done in the
>> preceeding patches.
>> 
>> Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
>> ---
>>  arch/x86/entry/common.c        | 93
>++++++++++++++++++++++++----------
>>  arch/x86/include/asm/syscall.h |  2 +-
>>  2 files changed, 66 insertions(+), 29 deletions(-)
>> 
>> diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
>> index f51bc17262db..714804f0970c 100644
>> --- a/arch/x86/entry/common.c
>> +++ b/arch/x86/entry/common.c
>> @@ -36,49 +36,87 @@
>>  #include <asm/irq_stack.h>
>>  
>>  #ifdef CONFIG_X86_64
>> -__visible noinstr void do_syscall_64(struct pt_regs *regs, unsigned
>long nr)
>> +
>> +static __always_inline bool do_syscall_x64(struct pt_regs *regs, int
>nr)
>> +{
>> +	/*
>> +	 * Convert negative numbers to very high and thus out of range
>> +	 * numbers for comparisons. Use unsigned long to slightly
>> +	 * improve the array_index_nospec() generated code.
>> +	 */
>> +	unsigned long unr = nr;
>> +
>> +	if (likely(unr < NR_syscalls)) {
>> +		unr = array_index_nospec(unr, NR_syscalls);
>> +		regs->ax = sys_call_table[unr](regs);
>> +		return true;
>> +	}
>> +	return false;
>> +}
>
>How much do you like micro-optimization?  You could be silly^Wclever
>and
>add a new syscall handler:
>
>long skip_syscall(struct pt_regs *regs)
>{
>	return regs->ax;
>}
>
>and prepend this to the syscall tables -- it would be a sort-of-real
>syscall -1.  Then the call sequence becomes:
>
>int adjusted_nr = nr + 1 (or nr - x32bit + 1);
>
>if (likely(nr < NR_adjusted_syscalls)) {
>   unr = array_index_nospec...;
>   regs->ax = sys_call_table[unr](regs);  /* might be a no-op! */
>} else {
>    regs->ax = -ENOSYS;
>}
>
>which removes a branch from the fast path.

-- 
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