Messages in this thread | | | Date | Fri, 20 Jun 2008 17:27:58 -0700 | From | "H. Peter Anvin" <> | Subject | Re: Various x86 syscall mechanisms |
| |
Jeremy Fitzhardinge wrote: > Hi Roland, > > As far as I can work out, an x86_32 kernel will use "int 0x80" and > "sysenter" for system calls. 64-bit kernel will use just "syscall" for > 64-bit processes (though you can use "int 0x80" to access the 32-bit > syscall interface from a 64-bit process), but will allow "sysenter", > "syscall" or "int 0x80" for 32-on-64 processes. > > Why does 32-on-64 implement 32-bit syscall when native 32-bit doesn't > seem to? Or am I overlooking something here? Does 32-bit also support > syscall?
The reason is that not all 64-bit processors (i.e. K8) support a 32-bit sysenter in long mode (i.e. with a 64-bit kernel.) sysenter is *always* entered from the vdso, since the return address is lost and this is also where a 64-bit kernel can put a syscall.
There is no reason we couldn't do syscall for 32-bit native, but the only processor that would benefit would be K7, and that's far enough in the past that I don't think anyone cares enough.
Note that long mode syscall is different from protected mode syscall, even in 32-bit compatibility mode. The long mode variant is a lot saner.
-hpa
| |