Re: [PATCH v2] ARM: io: avoid writeback addressing modes for __raw_ accessors

[Arnd Bergmann <arnd@xxxxxxxx>]

On Tuesday 21 August 2012, Will Deacon wrote:
> 
> Data aborts taken to hyp mode do not provide a valid instruction
> syndrome field in the HSR if the faulting instruction is a memory
> access using a writeback addressing mode.
> 
> For hypervisors emulating MMIO accesses to virtual peripherals, taking
> such an exception requires disassembling the faulting instruction in
> order to determine the behaviour of the access. Since this requires
> manually walking the two stages of translation, the world must be
> stopped to prevent races against page aging in the guest, where the
> first-stage translation is invalidated after the hypervisor has
> translated to an IPA and the physical page is reused for something else.
> 
> This patch avoids taking this heavy performance penalty when running
> Linux as a guest by ensuring that our I/O accessors do not make use of
> writeback addressing modes.
> 
> Cc: Marc Zyngier <marc.zyngier@xxxxxxx>
> Reviewed-by: Nicolas Pitre <nico@xxxxxxxxxx>
> Signed-off-by: Will Deacon <will.deacon@xxxxxxx>

Reviewed-by: Arnd Bergmann <arnd@xxxxxxxx>

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