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SubjectRe: Calling current() from interrupt context


"David S. Miller" wrote:
>
> Date: Tue, 10 Oct 2000 00:44:58 +0200
> From: "Andi Kleen" <ak@suse.de>
>
> On Mon, Oct 09, 2000 at 11:41:13PM +0100, Alan Cox wrote:
> > I dont actually know a CPU that doesnt have one in SMP mode. Its just often
> > behind an I/O interface that is slower than ideal
>
> Where would you put it for IA32 ? I don't know any free MSR that could
> be abused for that, and acessing MSRs is insanely slow anyways. Any
> other ideas ?
>
> The local APIC holds the hardware cpu number (which happens to equal
> the logical cpu number in the current x86 code). And I believe the
> local APIC has a 32-bit scratch register or 2 as well... but don't
> quote me on that one.

Accessing the CPUID from the local APIC is slower than mollasses. The
reason for this is due to the fact that the APIC address ranges are
treated as non-cacheable memory, and will always generate a
non-cacheable memory reference when you read from the APIC_ID register
and shift the ID.

A simpler solution is to use the CR2 register to store the CPU number,
and always reload the register after a page fault (since CR2 will hold a
faulting address). THis is even better than Linus' stack based method
for storing the number since you just are reading a register, which is
fast as hell. The other methods are slower.

hHis is how it's done in NetWare and MANOS. It works....
:-)

Jeff

>
> Later,
> David S. Miller
> davem@redhat.com
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