Re: [PATCH v5 13/14] KVM: ARM: Handle I/O aborts

Marc Zyngier
2013-01-15T13:29:44Z

On 15/01/13 13:18, Gleb Natapov wrote:
> On Tue, Jan 08, 2013 at 01:40:05PM -0500, Christoffer Dall wrote:
>> When the guest accesses I/O memory this will create data abort
>> exceptions and they are handled by decoding the HSR information
>> (physical address, read/write, length, register) and forwarding reads
>> and writes to QEMU which performs the device emulation.
>>
>> Certain classes of load/store operations do not support the syndrome
>> information provided in the HSR and we therefore must be able to fetch
>> the offending instruction from guest memory and decode it manually.
>>
>> We only support instruction decoding for valid reasonable MMIO operations
>> where trapping them do not provide sufficient information in the HSR (no
>> 16-bit Thumb instructions provide register writeback that we care about).
>>
>> The following instruction types are NOT supported for MMIO operations
>> despite the HSR not containing decode info:
>> - any Load/Store multiple
>> - any load/store exclusive
>> - any load/store dual
>> - anything with the PC as the dest register
>>
>> This requires changing the general flow somewhat since new calls to run
>> the VCPU must check if there's a pending MMIO load and perform the write
>> after userspace has made the data available.
>>
>> Rusty Russell fixed a horrible race pointed out by Ben Herrenschmidt:
>> (1) Guest complicated mmio instruction traps.
>> (2) The hardware doesn't tell us enough, so we need to read the actual
>> instruction which was being exectuted.
>> (3) KVM maps the instruction virtual address to a physical address.
>> (4) The guest (SMP) swaps out that page, and fills it with something else.
>> (5) We read the physical address, but now that's the wrong thing.
> How can this happen?! The guest cannot reuse physical page before it
> flushes it from all vcpus tlb cache. For that it needs to send
> synchronous IPI to all vcpus and IPI will not be processed by a vcpu
> while it does emulation.

I don't know how this works on x86, but a KVM/ARM guest can definitely
handle an IPI.

Furthermore, TLB invalidation doesn't require an IPI on ARMv7 (unless
we're doing some set/way operation which is handled separately).

M.
--
Jazz is not dead. It just smells funny...