Re: [uml-devel] SYSCALL, ptrace and syscall restart breakages (Re: [RFC] weird crap with vdso on uml/i386)

H. Peter Anvin
2011-08-23T19:16:51Z

On 08/23/2011 09:22 AM, Borislav Petkov wrote:
> On Tue, Aug 23, 2011 at 12:11:43PM -0400, Andrew Lutomirski wrote:
>> In any case, this seems insanely overcomplicated. I'd be less afraid
>> of something like my approach (which, I think, makes all of the
>> SYSCALL weirdness pretty much transparent to ptrace users) or of just
>> removing SYSCALL entirely from 32-bit code.
>
> I don't think that removing SYSCALL from 32-bit code just so that UML
> trapped syscalls work is something we'd like since SYSCALL is much
> cheaper than INT $0x80:
>
> "As a result, SYSCALL and SYSRET can take fewer than one-fourth the
> number of internal clock cycles to complete than the legacy CALL and RET
> instructions."
>
> http://support.amd.com/us/Processor_TechDocs/24593.pdf, p. 152.
>
> I know, it is 32-bit syscall on 64-bit kernel which should be pretty
> rare but still...
>

Right, but if you had said the difference had disappeared on current AMD
silicon it would be much less of an issue. That's why I wanted to find
that bit out from you.

-hpa