[Xen-devel] Re: [PATCH 03 of 36] x86: add memory barriers to wrmsr

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

H. Peter Anvin wrote:
> Arjan van de Ven wrote:
> > > >   
> > > I suppose, though I would be inclined to put the barriers in the
> > > wrmsr macro itself to act as documentation. 
> >
> > yeah I meant like this:
> >
> > static inline void native_write_msr(unsigned int msr,
> >                      unsigned low, unsigned high)
> > {
> >     barrier();
> >     asm volatile("wrmsr" : : "c" (msr), "a"(low), "d" (high));
> >     barrier();
> > }
> >
> > or in the same in the thing that calls this.
> Actually, I believe the barrier(); before is actually incorrect, since 
> it would affect the wrmsr() register arguments rather than the wrmsr 
> instruction itself.
How so?  What kind of failure do think might occur?  Some effect on how 
the wrmsr arguments are evaluated?
barrier() is specifically a compiler optimisation barrier, so the 
barrier before would prevent the compiler from moving anything logically 
before the wrmsr to afterwards.
That said, making the wrmsr itself a memory clobber may be simpler 
understand with a comment, rather than separate barriers...
   J

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