[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [Xen-devel] [PATCH 7/9] x86: skip check for spurious faults for non-present faults
If a fault on a kernel address is due to a non-present page, then it cannot be the result of stale TLB entry from a protection change (RO to RW or NX to X). Thus the pagetable walk in spurious_fault() can be skipped. This avoids spurious_fault() oopsing in some cases if the pagetables it attempts to walk are not accessible. This obscures the location of the original fault. This also fixes a crash with Xen PV guests when they access entries in the M2P corresponding to device MMIO regions. The M2P is mapped (read-only) by Xen into the kernel address space of the guest and this mapping may contains holes for non-RAM regions. Read faults will result in calls to spurious_fault(), but because the page tables for the M2P mappings are not accessible by the guest the pagetable walk would fault. This was not normally a problem as MMIO mappings would not normally result in a M2P lookup because of the use of the _PAGE_IOMAP bit the PTE. However, removing the _PAGE_IOMAP bit requires M2P lookups for MMIO mappings as well. Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx> Reported-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> Tested-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> --- x86 maintainers, this is a prerequisite for removing Xen's usage of _PAGE_IOMAP so I think this is best merged via the Xen tree. --- arch/x86/mm/fault.c | 6 ++++-- 1 files changed, 4 insertions(+), 2 deletions(-) diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c index 8e57229..c39e249 100644 --- a/arch/x86/mm/fault.c +++ b/arch/x86/mm/fault.c @@ -936,8 +936,10 @@ spurious_fault(unsigned long error_code, unsigned long address) pte_t *pte; int ret; - /* Reserved-bit violation or user access to kernel space? */ - if (error_code & (PF_USER | PF_RSVD)) + /* Only check for spurious faults on supervisor write or + instruction faults. */ + if (error_code != (PF_WRITE | PF_PROT) + && error_code != (PF_INSTR | PF_PROT)) return 0; pgd = init_mm.pgd + pgd_index(address); -- 1.7.2.5 _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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