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

To: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH] add support for XCHG instruction accessing APIC
From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Date: Wed, 5 Apr 2006 11:17:58 +0100
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, Boris Ostrovsky <bostrovsky@xxxxxxxxxxxxxxx>
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On 5 Apr 2006, at 10:50, Keir Fraser wrote:

32-bit SMP Linux uses xchg instruction to access APIC (see
apic_write_atomic()). Attached is patch that adds support for
this instruction.

The patch also fixes missed dependencies (in xen/Rules.mk) on
header files in include/asm-x86/hvm.
I wonder if it is worth adding the locking? xchg is only used to write 
to broken APICs -- the OS does not care about the returned value. The 
APIC is implemented entirely in Xen, so the lock isn't held across an 
ioreq round-trip to qemu-dm, right?
In fact I'm pretty sure the locking is not needed even if we did care 
about atomicity. You're only protecting guest accesses from other guest 
accesses, and each VCPU has its own local APIC model, so there cannot 
be multiple simultaneous guest accesses to a single local APIC.
 -- Keir


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