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Re: [Xen-devel] [PATCH v2] arm: Fix asynchronous aborts (SError exceptions) due to bogus PTEs

(CC some ARM folks)

On 21/03/2016 23:18, Shanker Donthineni wrote:
Hi Julien,

Hello Shanker,

Sorry for the late answer.

Do you have any other comments to be addressed?

I have a question regarding the implication for what you wrote in the commit.

As far as I understand, any speculative table walk might cause an imprecise asynchronous abort. So if a guest is using page tables that contain garbage, it would be possible to receive an SError. Am I right?

On 03/16/2016 02:08 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
G4.12.6 External abort on a translation table walk

As said on an earlier version of this patch, please mention the version of the spec when you quote it.

Signed-off-by: Vikram Sethi <vikrams@xxxxxxxxxxxxxx>
Signed-off-by: Shanker Donthineni <shankerd@xxxxxxxxxxxxxx>
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,
              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 
      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;


Julien Grall

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