42f91093b0
Thanks to address translation being performed out of order with respect to loads and stores, it is possible for a CPU to take a translation fault when accessing a page that was mapped by a different CPU. For example, in the case that one CPU maps a page and then sets a flag to tell another CPU: CPU 0 ----- MOV X0, <valid pte> STR X0, [Xptep] // Store new PTE to page table DSB ISHST ISB MOV X1, #1 STR X1, [Xflag] // Set the flag CPU 1 ----- loop: LDAR X0, [Xflag] // Poll flag with Acquire semantics CBZ X0, loop LDR X1, [X2] // Translates using the new PTE then the final load on CPU 1 can raise a translation fault because the translation can be performed speculatively before the read of the flag and marked as "faulting" by the CPU. This isn't quite as bad as it sounds since, in reality, code such as: CPU 0 CPU 1 ----- ----- spin_lock(&lock); spin_lock(&lock); *ptr = vmalloc(size); if (*ptr) spin_unlock(&lock); foo = **ptr; spin_unlock(&lock); will not trigger the fault because there is an address dependency on CPU 1 which prevents the speculative translation. However, more exotic code where the virtual address is known ahead of time, such as: CPU 0 CPU 1 ----- ----- spin_lock(&lock); spin_lock(&lock); set_fixmap(0, paddr, prot); if (mapped) mapped = true; foo = *fix_to_virt(0); spin_unlock(&lock); spin_unlock(&lock); could fault. This can be avoided by any of: * Introducing broadcast TLB maintenance on the map path * Adding a DSB;ISB sequence after checking a flag which indicates that a virtual address is now mapped * Handling the spurious fault Given that we have never observed a problem due to this under Linux and future revisions of the architecture are being tightened so that translation table walks are effectively ordered in the same way as explicit memory accesses, we no longer treat spurious kernel faults as fatal if an AT instruction indicates that the access does not trigger a translation fault. Reviewed-by: Mark Rutland <mark.rutland@arm.com> Signed-off-by: Will Deacon <will@kernel.org> |
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| include | ||
| kernel | ||
| kvm | ||
| lib | ||
| mm | ||
| net | ||
| xen | ||
| Kconfig | ||
| Kconfig.debug | ||
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| Makefile | ||