Re: [PATCH 1/3] Implement emulator_write_phys()

[Avi Kivity <avi@qumranet.com>]

Anthony Liguori wrote:
> On Mon, 2007-08-27 at 18:45 +0300, Avi Kivity wrote:
>   
> > Anthony Liguori wrote:
> >     
> > > Since a hypercall may span two pages and is a gva, we need a function to write
> > > to a gva that may span multiple pages.  emulator_write_phys() seems like the
> > > logical choice for this.
> > >
> > > @@ -962,8 +962,35 @@ static int emulator_write_std(unsigned long addr,
> > >  			      unsigned int bytes,
> > >  			      struct kvm_vcpu *vcpu
> > >       
> > I think that emulator_write_emulated(), except for being awkwardly 
> > named, should do the job.  We have enough APIs.
> >
> > But!  We may not overwrite the hypercall instruction while a vcpu may be 
> > executing, since there's no atomicity guarantee for code fetch.  We have 
> > to to be out of guest mode while writing that insn.
> >     
>
>
> Hrm, good catch.
>
> How can we get out of guest mode given SMP guest support?
>
>   

kvm_flush_remote_tlbs() is something that can be generalized.  
Basically, you set a bit in each vcpu and send an IPI to take them out.

But that's deadlock prone and complex.  Maybe you can just take 
kvm->lock, zap the mmu and the flush tlbs, and patch the instruction at 
your leisure, as no vcpu will be able to map memory until the lock is 
released.

--
Any sufficiently difficult bug is indistinguishable from a feature.

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