Re: [Xen-devel] [linux-linus test] 30356: regressions - FAIL

[Julien Grall <julien.grall@xxxxxxxxxx>]

On 23/09/2014 19:40, Ian Campbell wrote:
> On Tue, 2014-09-23 at 18:31 +0100, Julien Grall wrote:
> > I guess we will have to select LPAE when XEN is enabled, right? If
> > it's the case that would mean the user won't be able to compile a
> > Linux guest with short page table and Xen.
> >
> > Any though?
> Things must work without in guest LPAE too, so something somewhere else
> will need fixing.
>
> Apart from restricting the user in an unwanted way requiring LPAE will
> mean that practically no distro installer will work in a Xen guest.
Xen does an identity mapping for the host physical address into DOM0
for the grant mapping. DOM0 will use a scratch page (see commit 340720b
"xen/arm: reimplement xen_dma_unmap_page & friends") and map and this 
physical address.
That means on platform with an address space higher than 32 bits, which 
is the case on Midway, we have to handle 64 bits physical address in DOM0.
With the current implementation in Linux we can only use LPAE when a 
guest is started. The distro installer will still be able to work with
short page table.

The drawback is we are requiring LPAE from DOM0 and a different kernel
in the guest if the user doesn't want to use LPAE.

As the code is already pushed in Linux 3.17, I don't find a simpler 
solution to fix Linux boot without requiring LPAE.
Regards,

--
Julien Grall

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