Re: Intel P6 vs P7 system call performance

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

Jamie Lokier wrote:
> H. Peter Anvin wrote:
> 
>>Terje Eggestad wrote:
>>
>>>fd = open("/dev/vsyscall", );
>>>_vsyscall = mmap(NULL, getpagesize(),  PROT_READ|PROT_EXEC, MAP_SHARED,
>>>fd, 0);
>>
>>Very ugly -- then the application has to do indirect calls.
> 
> 
> No it doesn't.
> 
> The application, or library, would map the vsyscall page to an address
> in its own data section.  This means that position-independent code
> can do vsyscalls without any relocations, and hence without dirtying
> its own caller pages.
> 

Oh, I see... you don't really mean NULL in the first argument :)

This has one additional advantage: an application which wants to
override vsyscalls can simply map something instead of the kernel page,
and UML can present its own vsyscall page.

> In some ways this is better than the 0xfffe0000 address: _that_
> requires position-independent code to do indirect calls to the
> absolute address, or to dirty its caller pages.
> 
> That said, you always need the page at 0xfffe0000 mapped anyway, so
> that sysexit can jump to a fixed address (which is fastest).

That's a possiblity, or if the task_struct contains the desired return
address for a particular process that might also work -- it's just a GPR
after all.

	-hpa