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SubjectRe: 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


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