Re: [Xen-devel] arm64: incorrect pa setup cause dom0 boot failure

[Julien Grall <julien.grall@xxxxxxx>]

Hi,

On 11/05/17 19:24, Feng Kan wrote:
> Ian:
Ian left Citrix a year ago. I would recommend you to CC ARM maintainers 
(Stefano and I) if you want to get a prompt answer.
> Just a quick question regarding the table below in xen/arch/arm/p2m.c
> @@ -1479,7 +1487,7 @@ void __init setup_virt_paging(void)
>         [0] = { 32,      32/*32*/,  0,          1 },
>         [1] = { 36,      28/*28*/,  0,          1 },
>         [2] = { 40,      24/*24*/,  1,          1 },
> -        [3] = { 42,      24/*22*/,  1,          1 },
> +        [3] = { 42,      22/*22*/,  1,          1 },
> ^^^ Is this just a typo, or its there for a reason, as I see you made
> a comment regarding this later.
Which comment are you referring to? If it is the one in the commit 
message that introduce this code (1c92a2aaf8):
"Systems with 42-bits are an interesting case, since they only support 3 
levels of paging, implying that 8 pages are required at the root level. 
So far I am not aware of any systems with peripheral located so high up 
(the only 42-bit system I've seen has nothing above 40-bits), so such 
systems remain configured for 40-bit IPA with a pair of pages at the 
root of the p2m."
My understanding is Ian decided to configure guest with only 40 bits to 
avoid allocate a big root level (they have to be allocated at domain 
creation).
> We do have a system that PA is 42 bits, so I think that would make the
> statement below invalid?
>         [4] = { 44,      20/*20*/,  0,          2 },
>         [5] = { 48,      16/*16*/,  0,          2 },
>         [6] = { 0 }, /* Invalid */
>
>
> @@ -220,9 +222,11 @@ static lpae_t *p2m_get_root_pointer(struct p2m_domain *p2m,
>     root_table = gfn_x(gfn) >> (level_orders[P2M_ROOT_LEVEL - 1]);
>     root_table &= LPAE_ENTRY_MASK;
>
> -    if ( root_table >= P2M_ROOT_PAGES )
> -        return NULL;
> +    //if ( root_table >= P2M_ROOT_PAGES )
> +    //    return NULL;
> ^^^ at least for 42 bit case?
A guest would be free to pass any IPA via hypercall. So this is here for 
a sanity check, to avoid dereferencing non-existing page.
If you want to support 42 bits, you would have to update the root-order 
(3rd field in pa_range_info) to 3. So the code will allocate the correct 
amount of root pages when the domain is created.
Cheers,

--
Julien Grall

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