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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2014-09-24 at 11:02 +0100, Stefano Stabellini wrote:
> On Wed, 24 Sep 2014, Ian Campbell wrote:
> > On Wed, 2014-09-24 at 00:35 +0100, Julien Grall wrote:
> > > 
> > > 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.
> > 
> > We will have to try harder then, requiring LPAE simply isn't acceptable
> > IMHO.
> 
> I agree but the solution is not simple.
> 
> With the current scheme we would need to find a way to map pages at
> 64bit physical addresses in Dom0 without CONFIG_ARM_LPAE. Not sure if
> that is possible.

I'm pretty certain it isn't...

> Otherwise we would need to come up with an entirely new scheme.

I fear this may end up being the case.

I've got a cold towel and a whiteboard waiting for you in the office...

Ian.


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