Messages in this thread |  | | | Date | Mon, 7 Mar 2011 15:55:37 +0000 | | From | Stefano Stabellini <> | | Subject | Re: "x86-64, mm: Put early page table high" causes crash on Xen |
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On Wed, 2 Mar 2011, Konrad Rzeszutek Wilk wrote: > On Wed, Mar 02, 2011 at 03:23:48PM +0000, Stefano Stabellini wrote: > > On Tue, 1 Mar 2011, Yinghai Lu wrote: > > > We do need map_low_page ( BTW, that name is totally misleading...) > > > > > > the reason is we put page_table high and at that time is not under max_pfn_mapped. (aka not mapped). > > > > > > So have to use > > > adr = early_memremap(phys & PAGE_MASK, PAGE_SIZE); > > > to early map it and Read/Write to it. > > > > I think I have figured out a way to update the logic of mask_rw_pte to > > account for the new way of allocating kernel pagetables. > > The appended patch fix the boot crash for me. > > > > --- > > > > xen: update mask_rw_pte after kernel page tables init changes > > > > Already existing kernel page table pages can now be mapped using > > early_ioremap too so we need to update mask_rw_pte to make sure these > > pages are still mapped RO. > > We do that by mapping RO all the pages mapped using early_ioremap apart > > from the last one that has been allocated because it is not a page table > > page yet (it has been hooked into the page tables yet). > ^- has not? > > ..this is b/c the initial_kernel_mapping and its family of calls update > the pgt_buf_end (used to be called e820_table_end), _before__ > it calls the Xen MMU to set the PTE entries and then after it is done > update the PMD?. Hence the simple check to see if the PFN is the _old_ > value of the pgt_buf_end and if so skip altering the mapping from RW to RO > and leave them be?
yes and yes
> On subsequent passes we fall within the first conditional > and update the PTE to RO? > When is that pass done?
kernel_physical_mapping_init doesn't go through the same pagetable pages twice so that is not the issue. The problem is that kernel_physical_mapping_init is now re-mapping all the initial pagetable pages using early_ioremap, so the usage of early_ioremap is not enough anymore to distinguish between freshly allocated page table pages and already hooked pagetable pages. I am investigating the possibility that the introduction of map_low_page is actually overkill for this reason.
> You also might want to mention the git commits that inspired this patch > and include the nice description you provided in the first email of this > thread.
sure
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