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SubjectRe: Intel P6 vs P7 system call performance
Linus Torvalds wrote:
>>Destroying %ecx is a lot less destructive than destroying %eip and %esp...
> Actually, as far as the kernel is concerned, they are about equally bad.

Right, but from a user-mode point of view it means at least one extra

> Destroying %eip is the _least_ bad register to destroy, since the kernel
> can control that part, and it is trivial to just have a single call site.

Trivial, perhaps, but it requires a call/ret pair in userspace, which is
a fairly expensive form of push/pop.

> The good news is that since both of them suck, it's easier to make the
> six-argument decision. Since six arguments are problematic for all major
> "fast" system calls, my executive decision is to just say that
> six-argument system calls will just have to continue using the old and
> slower system call interface. It's kind of a crock, but it's simply due to
> silly CPU designers.

Oh, so you're not going to do the "read from stack" thing? (Agreed, by
the way, on the CPU design -- both SYSENTER and SYSCALL suck. SYSCALL
was changed rather substantially in x86-64 for that reason.)


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