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

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

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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