Re: [Xen-devel] [PATCH v3 26/28] xen/arm64: head: Use a page mapping for the 1:1 mapping in create_page_tables()

[Julien Grall <julien.grall@xxxxxxx>]

Hi Stefano,

On 8/24/19 2:16 AM, Stefano Stabellini wrote:
> On Mon, 12 Aug 2019, Julien Grall wrote:
> >           lsr   x2, x19, #THIRD_SHIFT  /* Base address for 4K mapping */
> >           lsl   x2, x2, #THIRD_SHIFT
> > @@ -674,21 +591,80 @@ create_page_tables:
> >           cmp   x1, #(LPAE_ENTRIES<<3) /* 512 entries per page */
> >           b.lt  1b
> Why can't we use create_mapping_entry here?
If we re-use create_mapping_entry, then we will compute everything at 
each loop.
At the moment, the loop execute 3 instructions (excluding the branch and 
comparison). We would end up to have an extra 5 instructions for arm64 
(9 instruction for arm32).
At the moment, we iterate 512 times the loop, so this adds 2560 
instructions (resp. 4608) in the boot code. Long-term plan, we will want 
to increase the Xen mapping to 4MB (so you double up the number iteration).
This is a less than ideal solution. I haven't come up with a clever 
solution so far, hence why it is left alone.
> > -        /* Defer fixmap and dtb mapping until after paging enabled, to
> > -         * avoid them clashing with the 1:1 mapping. */
> > +        /*
> > +         * If Xen is loaded at exactly XEN_VIRT_START then we don't
> > +         * need an additional 1:1 mapping, the virtual mapping will
> > +         * suffice.
> > +         */
> > +        cmp   x19, #XEN_VIRT_START
> > +        bne   1f
> > +        ret
> > +1:
> > +        /*
> > +         * Only the first page of Xen will be part of the 1:1 mapping.
> > +         * All the boot_*_id tables are linked together even if they may
> > +         * not be all used. They will then be linked to the boot page
> > +         * tables at the correct level.
> > +         */
> > +        create_table_entry boot_first_id, boot_second_id, x19, FIRST_SHIFT, 
> > x0, x1, x2
> > +        create_table_entry boot_second_id, boot_third_id, x19, SECOND_SHIFT, 
> > x0, x1, x2
> If I understood the code right, it is not actually required to link
> boot_first_id -> boot_second_id and/or boot_second_id -> boot_third_id:
> it depends on whether x19 clashes with XEN_FIRST_SLOT, XEN_SECOND_SLOT,
> etc. Do you think it would be possible without making the code complex
> to only call create_table_entry boot_first_id, boot_second_id and
> create_table_entry boot_second_id, boot_third_id when required?  So
> moving the calls below after the relevant checks? It looks like it could
> be done by adding those calls before "ret". I wouldn't mind if they are
> duplicated but we could avoid it by adding appropriate labels and having
> a single return path:
>
> out1:
>    create_table_entry boot_first_id, boot_second_id, x19, FIRST_SHIFT, x0, x1, 
> x2
> out2:
>    create_table_entry boot_second_id, boot_third_id, x19, SECOND_SHIFT, x0, x1, 
> x2
> out3:
>    ret
I have implemented something similar. I will send it as part of the next 
version in a bit.
Cheers,

--
Julien Grall

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel