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

[David Vrabel <david.vrabel@citrix.com>]

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.

David