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

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 5 Apr 2006, at 15:01, Boris Ostrovsky wrote:

> > 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?
Yes, also my second argument was that there is *no way* for two VCPUs 
to conflict on a local APIC access, since LAPIC accesses are always to 
the VCPU's own LAPIC. So there is no potential concurrency that needs 
to be serialised, regardless of the guest OS.
 -- Keir


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