Re: [Xen-devel] [PATCH] Allow programatic iomem permissions

[Keir Fraser <keir@xxxxxxxxxxxxx>]

On 13/7/07 12:05, "Kieran Mansley" <kmansley@xxxxxxxxxxxxxx> wrote:

> Any thoughts on the above?
> 
> I'm happy to do some work to rectify this situation but don't want to
> expend effort on something that won't get accepted.  Any background on
> the reasons for the restriction (i.e. what the TLB flushing issues are)
> would also be of help.

The issue is that the granter is informed that the grant is released before
stale grantee TLB entries are flushed. If the grantee is multi-vcpu then he
could theoretically still access a granted page via a stale TLB entry after
the granter has recycled the page. The window is extremely tiny though! The
correct fix is to reorder the unmap operation to be unmap-list-of-grants
then TLB-flush then update-grant-entries-to-indicate-release. Then the whole
problem disappears.

 -- Keir


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