Re: [Xen-devel] [PATCH v3] arm: Fix asynchronous aborts (SError exceptions) due to bogus PTEs

[Shanker Donthineni <shankerd@xxxxxxxxxxxxxx>]

Hi Stefano/Julien,

Any other comments to be addressed? Please propose an alternative
solution to fix the problem if this patch changes are not appropriate.

On 03/28/2016 11:46 PM, Shanker Donthineni wrote:
> From: Vikram Sethi <vikrams@xxxxxxxxxxxxxx>
>
> ARMv8 architecture allows performing prefetch data/instructions
> from memory locations marked as normal memory. Prefetch does not
> mean that the data/instruction has to be used/executed in code
> flow. All PTEs that appear to be valid to MMU must contain valid
> physical address with proper attributes otherwise MMU table walk
> might cause imprecise asynchronous aborts.
>
> The way current XEN code is preparing page tables for frametable
> and xenheap memory can create bogus PTEs. This patch fixes the
> issue by clearing page table memory before populating EL2 L0/L1
> PTEs. Without this patch XEN crashes on Qualcomm Technologies
> server chips due to asynchronous aborts.
>
> The speculative/prefetch feature explanation is scattered everywhere
> in ARM specification but below two sections have useful information.
>
> E2.8 Memory types and attributes (ver DDI0487A_h)
> G4.12.6 External abort on a translation table walk (ver DDI0487A_h)
>
> Signed-off-by: Vikram Sethi <vikrams@xxxxxxxxxxxxxx>
> Signed-off-by: Shanker Donthineni <shankerd@xxxxxxxxxxxxxx>
> ---
> Changes since v2:
>     Include ARM spec version in commit text.
>
> Changes since v1:
>     Replace memset() with clear_page()
>     Edit commit description 
>
>  xen/arch/arm/mm.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
> index 81f9e2e..3fda8f3 100644
> --- a/xen/arch/arm/mm.c
> +++ b/xen/arch/arm/mm.c
> @@ -730,6 +730,8 @@ void __init setup_xenheap_mappings(unsigned long base_mfn,
>          else
>          {
>              unsigned long first_mfn = alloc_boot_pages(1, 1);
> +
> +            clear_page(mfn_to_virt(first_mfn));
>              pte = mfn_to_xen_entry(first_mfn, WRITEALLOC);
>              pte.pt.table = 1;
>              write_pte(p, pte);
> @@ -773,6 +775,7 @@ void __init setup_frametable_mappings(paddr_t ps, paddr_t 
> pe)
>      second = mfn_to_virt(second_base);
>      for ( i = 0; i < nr_second; i++ )
>      {
> +        clear_page(mfn_to_virt(second_base + i));
>          pte = mfn_to_xen_entry(second_base + i, WRITEALLOC);
>          pte.pt.table = 1;
>          write_pte(&xen_first[first_table_offset(FRAMETABLE_VIRT_START)+i], 
> pte);

-- 
Shanker Donthineni
Qualcomm Technologies, Inc. on behalf of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux 
Foundation Collaborative Project


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