[Xen-ia64-devel] Re: copying data to guest

[Jes Sorensen <jes@xxxxxxx>]

tgingold@xxxxxxx wrote:
> The basic answer is you can't do that reliably.
> 
> I don't have Xen code under my hand now, but there are a few examples in such
> direct write for some PAL calls and some EFI calls.
> 
> But they may fail: the TLB may not have the entry and there is currently no
> way to recover from such a miss.
> 
> The correct way is to use a xencomm descriptor.  Of course, this requires some
> modification in kernel code but much safer.

Hi Tristan,

I just don't follow this, if this is the case then we have a major
problem. There are multiple SAL calls that are defined to take a kernel
virtual address to receive a result or provide input data to the call.
We need to be able to do this for domU's if we want to provide full
virtualization so it's not really something we can avoid.

Given that Xen uses a 1:1 mapping for all physical memory, we ought to
be able to simply take the virtual kernel address from the SAL call,
which we should be able to reliably convert to a metaphysical address
by walking the page tables, or in the case of Linux, we know it uses a
1:1 mapping, then convert the metaphysical address to a physical address
and access that via the 1:1 mapping? Or am I missing something here?

Thanks,
Jes



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