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

[Boris Ostrovsky <bostrovsky@xxxxxxxxxxxxxxx>]

Keir Fraser wrote:

> 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?
Yes, that's right --- we never leave Xen to handle APIC access.

> 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.

My only argument in favor of using the lock would be for completeness of 
the emulation. You are
absolutely right in that for Linux there seems to be no need to hold the 
lock. My concern is that
other OSs  may treat this differently. And if we don't have sources, it 
may be somewhat difficult
to  figure out that the atomicity (or lack of it) was the cause of a 
problem.
If, however, there is a strong feeling that we don't need the lock, I am 
happy to drop it.
I guess you are mostly unhappy about adding a new field to hvm_domain, 
not about performance
impact?

-boris

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