Re: [Xen-devel] [PATCH] xen/balloon: don't alloc page while non-preemptible

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Wed, 25 Sep 2013, David Vrabel wrote:
> On 25/09/13 14:26, Stefano Stabellini wrote:
> > On Thu, 19 Sep 2013, David Vrabel wrote:
> >> 
> >> -  put_balloon_scratch_page();
> >> +  /* Ensure that ballooned highmem pages don't have kmaps. */
> >> +  kmap_flush_unused();
> >> +  flush_tlb_all();
> > 
> > The change with possible side effects in this patch is that
> > kmap_flush_unused and flush_tlb_all are now called after setting the p2m
> > and after calling balloon_append instead of before.
> > 
> > However they are still called before the XENMEM_decrease_reservation
> > hypercall, so I think it's OK.
> 
> Yes, the requirement is that the kmap cache is flushed before releasing
> the frame to Xen (in case it contains a kmap for a frame we're about to
> release).
> 
> I also wonder if the flush_tlb_all() is even necessary.  Surely Xen has
> all the appropriate TLB flushes in the decrease_reservation hypercall.
> It can't possibly rely on the guest doing the right thing.
 
That would only be the case for PV guests, but yes, Xen has certainly a
flush.

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