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

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

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

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

> 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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