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Re: [Xen-devel] [PATCH] add support for XCHG instruction accessing APIC

On 5 Apr 2006, at 15:26, Boris Ostrovsky wrote:

OK, that's fair. Here is updated patch with lock removed.


I don't think I then understand why Linux is using atomic accesses to
local APICs. It's interesting though that 64-bit code doesn't do it ---
they use vanilla apic_write().

The atomic accesses are to work around APIC bugs in the original Pentium processors, where an APIC write in some cases needs to be preceded by a read for the write to work correctly.

The XCHG trick isn't needed on x86_64 because only modern processors support 64-bit mode and none of them have a buggy LAPIC.

 -- Keir

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