Re: [Xen-devel] [PATCH 2/5] xen: arm: Handle 4K aligned hypervisor load address.

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2014-07-16 at 18:49 +0100, Julien Grall wrote:
> On 16/07/14 17:53, Ian Campbell wrote:
> > On Wed, 2014-07-16 at 16:41 +0100, Julien Grall wrote:
> >>> +        /* Identity map clashes with boot_third, which we cannot handle 
> >>> yet */
> >>> +        PRINT("Unable to build boot page tables - virt and phys 
> >>> addresses clash.\r\n")
> >>> +        b     fail
> >>
> >> AFAIU, this can happen if the kernel is loaded around 2MB in the memory,
> >> right?
> >
> > Yes from 2MB up to (but not including) 4MB.
> >
> > It is an error (I think) that this patch bugs if Xen is loaded at
> > exactly 2MB, since then the virtual and identity-physical mappings are
> > the same.
> >
> >> Also what does prevent Xen to be shared between 2 third page table?
> >
> > This is the virtual mapping, which always starts at exactly 2MB, so that
> > can only happen if Xen is larger than 2MB, which we assume is not the
> > case both here and in various bits of the C code start of day
> > relocating/setup etc.
> 
> Sorry I was thinking that boot_third is used for the 1:1 mapping.
> 
> It looks like you are using a 2MB mapping for the identity mapping:
> 
> +        /* ... map of paddr(start) in boot_second */
> +        lsrs  r1, r9, #SECOND_SHIFT  /* Offset of base paddr in 
> boot_second */
> +        mov   r2, #0x0ff             /* r2 := LPAE entries mask */
> +        orr   r2, r2, #0x100
> +        and   r1, r1, r2
> +        cmp   r1, #1
> +        bne   2f                     /* It's not in slot 1, map it */
> 
> r9 contains the physical address of start, but the binary could cross 
> the 2MB boundary (because, for instance, the start address is at 
> 0xXX2FXXXXX). So the assembly code to enable the pagination may not be 
> on the same slot.

This is indeed a theoretical possibility which I hadn't considered.

What saves us in practice is that the code in head.S from _start to
paging is <4K and therefore given a 4K aligned load address cannot cross
a 4K boundary or a 2MB boundary, etc.

The easiest fix is probably a BUILD_BUG_ON of some sort I think.

> I think this very unlikely, but if it happens it will be hard to debug.
> Maybe you can add a sanity check or add a label before the pagination is 
> enabled and use it in the slot.
> 
> BTW I think you can use lsr instead of lsrs to get the offset.

True.

Ian.


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